Simple DIY Induction Heater - RMCybernetics

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This great little project demonstrates the principles of high frequency magnetic induction. The circuit is very simple to build and only uses a few common components. With the induction coil shown here the circuit draws about 5A from a 15V supply when a screwdriver tip is heated. It takes approximately 30 second for the tip of the screwdriver to become red hot!

The control circuit uses a method known as ZVS (zero voltage switching) to activate the transistors which allows for an efficient transfer of power. In the circuit you see here, the transistors barely get warm due to the ZVS method. Another great thing about this device is that it is a self resonant system and will automatically run at the resonant frequency of the attached coil and capacitor.

How Does Induction Heating Work?

When a magnetic field changes near a metal or other conductive object, a flow of current (known as an eddy current) will be induced in the material and will generate heat. The heat generated is proportional to the current squared multiplied by the resistance of the material. The effects of induction are used in transformers for converting voltages in all sorts of appliances. Most transformers have a metallic core and will therefore have eddy currents induced into them when in use. Transformer designers use different techniques to prevent this as the heating is just wasted energy.In this project we will directly make use of this heating effect and try to maximise the heating effect produced by the eddy currents.

If we apply a continuously changing current to a coil of wire, we will have a continuously changing magnetic field within it. At higher frequencies the induction effect is quite strong and will tend to concentrate on the surface of the material being heated due to the skin effect. Typical induction heaters use frequencies from 10kHz to 1MHz.

DANGER: Very high temperatures can be generated with this device!

The Circuit

The circuit used is a type of collector resonance Royer oscillator which has the advantages of simplicity and self resonant operation. A very similar circuit is used in common inverter circuits used for powering fluorescent lighting such as LCD backlights. They drive a center tapped transformer which steps up the voltage to around 800V for powering the lights. In this DIY induction heater circuit the transformer consists of the work coil and the object to be heated.

The main disadvantage of this circuit is that a center tapped coil is needed which can be a little more tricky to wind than a common solenoid. The center tapped coil is needed so that we can create an AC field from a single DC supply and just two N-type transistors. The center of the coil is connected to the positive supply and then each end of the coil is alternately connected to ground by the transistors so that the current will flow back and forth in both directions.

The amount of current drawn from the supply will vary with the temperature and size of the object being heated.

From this schematic of the induction heater you can see how simple it really is. Just a few basic components are all that is needed for creating a working induction heater device.

R1 and R2 are standard 240 ohm, 0.6W resistors. The value of these resistors will determine how quickly the MOSFETs can turn on, and should be a reasonably low value. They should not be too small though, as the resistor will be pulled to ground via the diode when the opposite transistor switches on.

The diodes D1 and D2 are used to discharge the MOSFET gates. They should be diodes with a low forward voltage drop so that the gate will be well discharged and the MOSFET fully off when the other is on. Schottky diodes such as the 1N5819 are recommended as they have low voltage drop and high speed. The voltage rating of the diodes must be sufficient to withstand the the voltage rise in the resonant circuit. In this project the voltage rose to as much as 70V.

The transistors T1 and T2 are 100V 35A MOSFETs (STP30NF10). They were mounted on heatsinks for this project, but they barely got warm when running at the power levels shown here. These MOSFETs were chosen due to having a low drain-sorce resistance and fast response times.

The inductor L2 is used as a choke for keeping the high frequency oscillations out of the power supply, and to limit current to acceptable levels. The value of inductance should be quite large (ours was about 2mH), but also must be made with thick enough wire for carrying all the supply current. If there is no choke used, or it has too little inductance, the circuit might fail to oscillate. The exact inductance value needed will vary with the PSU used and your coil setup. You may need to experiment before you get a good result. The one shown here was made by winding about 8 turns of 2mm thick magnet wire on a toroidal ferrite core. As an alternative you can simply wind wire onto a large bolt but you will need many more turns of wire to get the same inductance as from a toroidal ferrite core. You can see an example of this in the photo on the left. In the bottom left corner you can see a bolt wrapped with many turns of equipment wire. This setup on the breadboard was used at low power for testing. For more power it was necessary to use thicker wiring and to solder everything together.

As there were so few components involved, we soldered all the connections directly and did not use a PCB. This was also useful for making the connections for the high current parts as thick wire could be directly soldered to the transistor terminals. In hindsight it might have been better to connect the induction coil by screwing it directly to the heatsinks on the MOSFETs. This is because the metal body of the transistors is also the collector terminal, and the heatsinks could help keep the coil cooler.

The capacitor C1 and inductor L1 form the resonant tank circuit of the induction heater. These must be able to withstand large currents and temperatures. We used some 330nF polypropylene capacitors. More detail on these components is shown below.

The Induction Coil and Capacitor

The coil must be made of thick wire or pipe as there will be large currents flowing in it. Copper pipe works well as the high frequency currents will mostly flow on the outer parts anyway. You can also pump cold water through the pipe to keep it cool.

A capacitor must be connected parallel to the work coil to create a resonant tank circuit. The combination of inductance and capacitance will have a specific resonant frequency at which the control circuit will automatically operate. The coil-capacitor combination used here resonated at around 200kHz.

It is important to use good quality capacitors that can withstand large currents and the heat dissipated within them otherwise they would soon fail and destroy your drive circuit. They must also be placed reasonably close to the work coil and using thick wire or pipe. Most of the current will be flowing between the coil and capacitor so this wire must be thickest. The wires linking to the circuit and power supply can be slightly thinner if desired.

This coil here was made from 2mm diameter brass pipe. It was simple to wind and easy to solder to, but it would soon start to deform due to excess heating. The turns would then touch, shorting out and making it less effective. Since the control circuit stayed relatively cool during use, it seemed that this could be made to work at higher power levels but it would be necessary to use thicker pipe or to water cool it. Next the setup was improved to tolerate a higher power level...

AVAILABLE PARTS: Resistors, Diodes, Capacitors,MOSFETs, Heatsinks, Ceramic support, 4mm Brass Pipe, 4mm Copper Pipe, Clear PVC Tubing, Large Bolt, 30A Cable, 12V Water Pump, 12V Regulator, Current Meter, Volt Meter, Power Supply

Low Cost International Shipping Available

Pushing it Further

The main limitation of the setup above was that the work coil would get very hot after a short time due to the large currents. In order to have larger currents for a longer time, we made another coil using thicker brass tubing so that water could be pumped through when it was running. The thicker pipe was harder to bend, especially at the center tapping point. It was necessary to fill the pipe with fine sand before bending it as this prevents it from pinching at the sharp bends. It was then cleared out using compressed air.

The induction coil was made in two halves as shown here. They were then soldered together and a small piece of pvc pipe was used to connect the central pipes so that water could flow through the whole coil.

Less turns were used in this coil so that it would have a lower impedance and therefore sustain higher currents. The capacitance was also increased so that the resonant frequency would be lower. A total of six 330nF capacitors were used to give a total capacitance of 1.98uF.

The cables connecting to the coil were just soldered onto the pipe near the ends, just leaving room for fitting some PVC pipe.

It is possible to cool this coil simply by feeding water through directly from the tap but it is better to use a pump and radiator to remove the heat. For this, an old fish tank pump was placed in a box of water and a pipe fitted the outlet nozzle. This pipe fed to a modified computer CPU cooler which used three heat-pipes to move the heat.

The cooler was converted into a radiator by cutting the ends off the heat pipes and then linking them with PCV pipes to the the water would flow through all 3 heatpipes before exiting and going back to the pump.

If you do cut some heatpipes yourself, make sure to do it in a well ventilated area, and not indoors as they contain volatile solvents that can be toxic to breathe. You should also wear protective gloves to prevent skin contact.

This modified CPU cooler was very effective as a radiator and allowed the water to remain quite cool.

Other modifications needed were to replace the the diodes D1 and D2 with ones rated for higher voltages. We used the common 1N4007 diodes. This was because with the increased current there was a larger voltage rise in the resonant circuit. You can see in the image here that the peak voltage was 90V (yellow scope trace) which is also very close to the 100V rating of the transistors.

The PSU used was set to 30V so it was also neccesary to feed the voltage to the transistor gates via a 12V voltage regulator. When no metal was inside the work coil, it would draw about 7A from the supply. When the bolt in the photo was added, this went up to 10A and then gradually dropped again as it heated up beyond curie temperature. It would certainly go over 10A with larger objects, but the PSU used has a 10A limit. You can find a suitable a 24V, 15A PSU in our online shop.

The bolt you can see glowing red hot in the photo took about 30 seconds to reach maximum temperature. The screwdriver in the first image could now be heated red hot in about 5 seconds.

In order to go to higher power than this, it would be necessary to use different capacitors or a larger array of them so that the current was more distributed between them. This is because the large currents flowing and high frequencies used would heat the capacitors significantly. After about 5 minutes of use at this power level the DIY induction heater needed to be switched off so that they could cool down. It would also be necessary to use a different pair of transistors so that they could withstand the larger voltage rises.

In all this project was quite satisfying as it produced a good result from just a simple and inexpensive circuit. As it is, it could be useful for hardening steel, or for soldering small parts. If you decide to make your own induction heater project, please post your photos below. Please read through the other comments before making your own as it could save you time later on.

If you wish to simulate this project for testing different inductance values or transistor choices, please download LTSpice and run this DIY Induction Heater Simulation(Right click, Save as)

Troubleshooting

If you have trouble getting this working, here are a few tips to help troubleshoot your home made induction heater project....

PSU (Power Supply)If your PSU is unable to deliver a large surge of current when the induction heater is powered on, then it will fail to oscillate. The voltage from the supply will drop during that moment (although the PSU may not display this) and this will prevent the transistors from switching correctly. To help with this problem, you can place severallarge electrolytic capacitors in parallel with the supply. When charged they will be able to deliver a large surge current to your circuit. A good powerful supply would be our 24V 15A DC PSU.

Choke (inductor L2)This limits the power to your induction heater. If yours is not oscillating, then you may need more inductance to prevent voltage drop in your PSU. You will need to experiment with how much inductance you need. Better to have too much, than too little as this will only limit the power of the heater. Too little may mean it wont work at all.

WiringKeep the connecting wires short to reduce stray inductance and interference. Long wires add unwanted resistance and inductance to the circuit and can result in unwanted oscillations or poor performance. Our 30A power cable is well suited to this.

ComponentsThe transistors chosen must have a low voltage drop / on-state resistance otherwise they will overheat, or even prevent the system from oscillating. IGBTs will proabbaly not work, but most MOSFETs with similar ratings should be OK. The capacitors must have a low ESR (resistance) and ESL (inductance) so they can tolerate the high current and temperatures. The diodes should also have a low forward voltage drop so that the transistors switch off correctly. They should also be fast enough to work at the resonant frequency of your induction heater.

Powering it upWhen switching it on, do not have metal within the heating coil. This can lead to larger current surges which could prevent the oscillation from starting as mentioned above. Also do not try to heat large amounts of metal. This project is only suitable for small induction heaters. If you want to control or gradually turn up the power, you can use one of our power pulse modulator circuits. See post 5108 below for details.

BrainYou will need a brain that functions reasonably well to make this project safely. If you are one of those people who is known by your friends as "a bit thick", then congratulations on being able to use a computer and navigate to this page, but unfortunately this project is not for you. It can be very dangerous to build an induction heater, so if you are new to electronics, you should get someone to help you make it. Approach things logically; If it is not working, check the components used are not faulty, check connections are correct, read this whole article and all the comments, search Google if you do not understand any of the terms, or read through our Learn Electronics section. Remember: Hot things will burn you and can set things on fire; Electricity can electrocute you and also cause fire. Put safety first.

Rusdi - Sunday, 30th October 2011 1:01am - #4646

I intend to buy 3sets of all the parts from u guys to build this, but I am a newbie in electronics, is there any special considerations to build this other than then current and voltage hazard? And whats with the osciloscope? anything special with the osciloscope, like it has to be grounded in some particular way or anything else? Or any other particular considerations?

Surastyo - Sunday, 30th October 2011 3:46am - #4648

I build this today and ended with burnt FET. I use FQP60N06. I use 6 turn and 4 x 470nF capacitors. How can it happen? Should I use 7812 to feed Gate? Can you show the drawing? Or wrong pin connection of FET? is it G, D, S if read from front side? Thanks

RMCybernetics - Sunday, 30th October 2011 11:01am - #4649

Rusdi,There is also the temperature hazard, and then potentially chemical hazards if components get burned. The scope is not needed, but was useful for seing the waveforms and frequency. The scope ground should be the same ground as your PSU. you can then measure at the MOSFET gate, or collector terminals. If you measure at the collector, remember that the voltage could be much higher than the supply voltage so set the scope accordingly.Surastyo.I can't seem to find the datasheet for that part number. What are its ratings? They are typically set as GDS, but you need to check the manufacturers datasheet. I also do not see any large inductor (L2) in your picture. I've attached a diagram showing the 7812 voltage regulator being used.

Garudadidada - Sunday, 30th October 2011 8:51pm - #4650

DELETED: No sending of secret messages in Indonesian or any other language.

Surastyo - Monday, 31st October 2011 3:20am - #4652

FQP60N06: I think its 60V, 18A.its chinnese and hardly to find the data sheet. The large inductor (L2)is behind the CAPs. see my new pic. Thanks for 7812 diagram. I'll try another other MOSFET I have, while waiting the the order from RMCybernetics arrived. BTW what is modification needed to make it a "Induction Furnace" to melt aluminum (700 deg. C)? Thanks

RMCybernetics - Monday, 31st October 2011 1:06pm - #4654

I think 60V is much too low of a voltage rating for the MOSFETs. THey are probably being destroyed by over voltage.

To get hotter temperatures, you need to use more powerful transistors and capacitors, and then use a higher supply voltage.

Rusdi - Monday, 31st October 2011 5:10pm - #4655

This is something else to use this as an induction furnace, but could you recommend stronger caps and transistors and how about the higher voltage power supply? I'd like to buy them from you, rmc Rusdi - Wednesday, 2nd November 2011 8:26am - #4656

Rmcybernetics, could u recomend some "higher" transistors and caps? I intend to buy some sets from u but I think ill go for the "higher" components if they exist in ur site and with ur recomendations.

RMCybernetics - Wednesday, 2nd November 2011 9:43am - #4657

We don't have anything else available at the moment. You would just have to look for components with larger voltage and current ratings.

Martin - Tuesday, 8th November 2011 3:42pm - #4659

I've tried building an induction heater based on your schematic and it works quite well. However, my heater won't heat up to 'glowing point' (maybe due to my power supply, it's only 30V, 5A) and, a bigger problem, my FETs tend to become pretty hot. When i look at my coil voltage with a scope it's way more noisy and irregular than what you are having. Do you have any clue about what might be going on here?

joco - Tuesday, 8th November 2011 4:33pm - #4660

FET's going warm....Amps...up like hell...what is going on? I used irfpb4615, 4 pcs of 330n capacitor...the rest the same as yours...oh...with a 25V supply.

RMCybernetics - Tuesday, 8th November 2011 6:24pm - #4661

Martin, If you are not getting nice sinusoidal oscillations, this could be due to having too little, or even too much inductance in your choke (L2). You may need to experiment with different choke designs to find something that works well with your other parts. Your work coil should ideally be only a little larger than the object to be heated as this will maximise the concentration of magnetic flux. If that does not help, add a photo of your scope showing the gate voltage, and drain voltage waveforms.

Joco, I can't find that part number. Did you make a typo?

Tohooloo - Tuesday, 8th November 2011 9:06pm - #4662

What is the maximum capacitance for C1.

joco - Wednesday, 9th November 2011 6:45am - #4663

sorry for part number. it's irfb4615

RMCybernetics - Wednesday, 9th November 2011 8:38am - #4664

Tohooloo,C1 can be as large as you like. The combination of capacitance of C1 and the inductance of L1 form a resonant circuit. A larger capacitance or inductance will result in a lower frequency.

Joco, That MOSFET seems ok. Try using a larger choke (L2).

deyan - Friday, 11th November 2011 11:35pm - #4666

To get faster heating do i need higher or lower frequency

RMCybernetics - Saturday, 12th November 2011 9:55am - #4667

Neither. You need higher power.

deyan - Sunday, 13th November 2011 5:59pm - #4668

i am usinga 12 volt 10 amp power suply my trnsistor are p50n06 R1 R2 the dides and L2 are the same as yours. Is my suply weak because i cant get i to gloing point and a nother thing my car are burning in about a minute or so they are reatetd for 250-275 volts what shuld i change to get better reasults? Thank you.

deyan - Monday, 14th November 2011 5:14pm - #4669

can i use a IGBT such as BUP203 as the transistors they are reated for 1000volts and if yes are there any difernces in the schematic?

Stephen - Thursday, 8th December 2011 8:35pm - #4680

What voltage rating are your capacitors?

RMCybernetics - Sunday, 11th December 2011 4:32pm - #4684

Deyan,It sounds like your capacitors have too much internal resistance (ESR). You need something that is better for high power, they are typically much larger in size for the same capacitance value. There are many types of transistors that may work, but it is up to you to test if the ones you have are ok. I suspect that a high voltage IGBT will have too large of a voltage drop between collector and emmiter which would mean that the opposite transistor would not switch off properly. I used these 100V, 35A MOSFETs which worked well.

Stephen,I used these 1000V, 330nF capacitors.

Rob - Monday, 12th December 2011 8:01am - #4686

I sent an email, but I think I should just ask here. Would these parts work for this project? : 240 OHM 5W 5% METAL OXIDE 1N4007 FDA69N25 N-CH MOSFET 250V 69A TO-3P FILM Capacitor 2UF 700VDC B32794D3205K Also, will this circuit be able to handle 55V at 10A (with upgrades)? -Cheers

RMCybernetics - Wednesday, 14th December 2011 11:59am - #4687

I can only suggest you follow the instructions given. If you use other components, they may or may not work. That is up to you to work out.

Surastyo - Friday, 16th December 2011 11:10am - #4688

Hi again I try new coil, new caps, and new Mosfet. I burtn 5 or 6 of mosfets, it's happen because I didn't realize that one diode is sorted so HV directly goes to G pin. Now I use 30A, 30V power supply, IRFP260, 6A diode, 7812, 7T copper pipe coil,etc.I am quiet happy with the result, but it seems take to long to heat a thing. I plant to melt aluminum with it. my question are: 1. is coil turn, coil diameter, capacitor size affect heating power, or it just shaping the wave? 2. my wave is slithly different from yours, why? 3. is it ok to increase the V+ to 50V or 90V to increase power (yes I'll count the mosfet and diode power rate) ? 4. how to trim this to be most efficient? is wave form can determine that? let say clear simetrical wave is most efficient? 5. can we create sinusoidal wave? 6. is sinusoidal wave the most efficient wave for induction heating? Ohh... I hope it's not to much.. Thanks Surastyo - Friday, 16th December 2011 11:12am - #4689

it makes things hot 100 or 200 deg C maybe.. here the other picture

RMCybernetics - Friday, 16th December 2011 11:39am - #4690

1. All those factors will vary the performance. The number of turns will determine the impedance of the coil and therefore how much current will flow for a given frequency. Less turns will allow for higher peak current, but too few and the transistors will blow.

The coil diameter should ideally be just slightly larger than the workpiece that is to be heated. This maximises the field coupling and is more efficient.

The capacitor size will also determine peak current. larger is better, but not too large or the resonant frequency will become too low. Physically larger capacitors will have low internal resistance too so they will perform much better. The bank of caps in your picture look way to small for the amount of metal you are trying to heat.

2. You should get a nice half sine waveform. You need to experiment witht the inductor (L2). Try different numbers of turns until you see some improvement. You could also add 15V Zener diodes between the gate and source pins of each transistor to help protect them from voltage spikes.

3. V+ can be as high as you like, just make sure your components are sufficioent to handle the voltage rise in the coil. You will also need to supply the gates from a stable 12V source.

4,5,6. See other answers.

Nicollas - Tuesday, 20th December 2011 12:10am - #4691

Hello I wonder if VDC power supplies used to feed the coil L2 and Gates/MOSFETS are two power suplies separates.

RMCybernetics - Tuesday, 20th December 2011 11:08am - #4692

You can use a single power supply, or seperate ones. If using a single supply it should be at most 15V, or 30V if you use the regulator as shown in another post.

Nicollas - Wednesday, 21st December 2011 8:21pm - #4696

HELLO, MY HILL LOOP, BUT WHEN PUT IN POWER SUPPLY CIRCUIT, He behaves like a short circuit and the voltage drops too, IF I DISCONNECT THE CENTER OF TENSION COIL Normalize, BUT NOT THE NOTHING HAPPENS

RMCybernetics - Friday, 23rd December 2011 10:50am - #4697

Your power supply is not able to deliver enough current. You need a bigger supply, or many more turns on the work coil or choke.

Rob - Sunday, 25th December 2011 8:40am - #4698

Will this circuit work properly with a power supply of 12v at 30A?

RMCybernetics - Wednesday, 28th December 2011 2:25pm - #4699

Yes

Nicollas - Thursday, 29th December 2011 8:55pm - #4701

I would like to know a way to reduce current consumption, because my source is 10 amps.

RMCybernetics - Wednesday, 4th January 2012 11:15am - #4705

The current will depend on the voltage you apply, and the load impedance. More turns on your work coil, smaller capacitors, and larger choke inductance will reduce current.

Rob - Thursday, 12th January 2012 5:01am - #4707

Just posting to share that I've successfully made my own heater using the info here. Some details of what I used: - ~.3uH work coil -12v 30A power supply -IRFB59N10DPBF mosfets -4.2uf capacitor bank (100A pulse) -2mH choke I posted a Video Response to the original video: http://www.youtube.com/watch?v=AT_lRjYjzUg -Thanks for sharing this information.

RMCybernetics - Thursday, 12th January 2012 2:33pm - #4708

Nice job. Thanks for sharing Rob.

james - Friday, 13th January 2012 11:00pm - #4709

Hello, Would it be possible for you to show a bit clearer picture of the physical connections of the transistors themselves and to also indicate which pins are what for the electronically challengened among us ?

Chanil - Saturday, 14th January 2012 12:56am - #4710

Hi, is it possible to run this on 15V at 1A? For my application I only need it to achive 200 degrees celcius. But it has to run for days or even weeks at a time(it's for worlds largest 3d ABS printer). I'm currently using every component that is recommended, but all of them are getting hot... I have already roasted a capacitor(used a wrong one, would it explain the rise in temperture of the mosfets?)In my tests I've powered them with a 19V 5A psu(Im using a 12v regulator for the gates). I'm planning to use multiple heatsinks with a small fan to provide cooling. So to conclude will it reach the required temperture if only using 15V at 1A? Will the components stay cool if it would work? And if not any suggestions? RMCybernetics - Sunday, 15th January 2012 12:08pm - #4713

James,If you Google the part numbers you will find the manufacturer pdf datasheets. These will tell you which pins provide a particular function. I think it is best you try working this out yourself as you will gain a better knowledge and understanding. You can also check out the section on the site for helping to learn electronics as it will explain a lot about the components and the physics of the electronics involved.

Chanil,Yes, if you want to limit the current to 1A, then you must increase the impedance of the circuit. Using many more turns in your work coil and the choke will limit the current.

Chanil - Friday, 27th January 2012 7:21pm - #4719

Check out my induction heater ^^. http://www.youtube.com/watch?v=iTIHvc6AohI I had a board made for it. This is one of the tests for it. Note: the cam can't read higher temps than 270 degrees celcius. And for my application only 205-210 is required. It reaches about 400-500 degrees celcius in the video.

RMCybernetics - Sunday, 29th January 2012 9:38pm - #4722

Chanil's video...

akram - Tuesday, 31st January 2012 9:26am - #4729

I used power supply 110 volt 60 A i want to design a circuit i want to know the kind of mosfet transistor and diode and resistors and capacitors so we want a quotation for the material justin - Sunday, 5th February 2012 11:16pm - #4731

hi guys i built this thing and had a little truble it turned out to be the diodes i was useing (4007) and changed them to some smaller glass ones (don't know what they are) and with 1.8uf worth of caps it works great!! it was a lot of fun and i built two more to give away all the parts i had got from a broken flat screen tv. they run around 125k and i used IGBT. thanks for the project and a good rundown on it!!!!!!

ad - Monday, 6th February 2012 4:27am - #4732

RMCybernetics, I really like your coil design! I'm working on a project where I need to have a tank that resonates at about 1 MHz. Do you have suggestions for modifying C1, L1, and L2 for that purpose? As long as the FETS can switch fast enough the rest of the circuit can likely remain the same; but I'm not sure whether I'm better of decreasing C or L to get the right resonance. if you had any references for choosing those values that would also be great! thanks, a.d.

Eric - Friday, 10th February 2012 10:22pm - #4737

I've noticed some interesting behavior with this circuit and I am curious what you think. I used all the same components (The coil is similar and the caps are similar, same mosfet, diodes and resistors (I did use a 124 ohm resistors as well, with similar behavior)). Using a 30V 30A supply the circuit pulls a lot of Amps (15 to 30A) at a very low voltage and the supply simply can't increase the voltage beyond a few volts. Using a 25V .5A supply the circuit draws power at a low voltage, but I can increase the voltage. At about 5V (according to the analog meter on the supply, could be different given the load) the resonant circuit kicks in and we get a beautiful sine wave on the resonant circuit (but minimal heating). Using this I went back to the large supply. I connected a switch between the supply and the circuit and set the supply to 12V. Flipping the switch (providing a near instantaneous 12V) drove the sine wave just as before (now pulling a bit more current as the supply could handle it). So there seems to be some issue at low voltages. It may explain why @joco was getting 'amps like hell' at low voltages.

RMCybernetics - Monday, 13th February 2012 4:57pm - #4740

ad,You can reduce C, but you will need t make sure that the remaining capacitors you have are able to take all the current. The advantage of using lots of capacitors is that the current is shared between them.

Eric,What you describes just sounds like it is due to the limitations of your PSU. Remember that voltage and current are directly related and will be affected by the impedance of your load. If your PSU shows low volts, and high amps, this indicates that the impedance of your circuit is low and your PSU is dropping the volts to keep the current within its ratings.

Eric - Thursday, 16th February 2012 1:24am - #4744

RMC, What is odd is that the circuit has very low impedance at low voltage (1-2 volts) and normal impedance at 12 volts. This is using the same circuit and supply in both cases. The only difference is that in the 12 volt case I immediately apply the full 12 volts via a switch instead of hooking up the supply and turning up the voltage. Using my low amperage supply I was able to see that the harmonic circuit wasn't kicking in until about 5V. My guess is that at low voltages the harmonic L-C system isn't being driven and the DC load is passing through a single mosfet through the coil. In this case the impedance would be very low. I have observed just a single hot mosfet in this odd low voltage scenario.

RMCybernetics - Saturday, 18th February 2012 3:52pm - #4748

Yes, it would seem that you just have DC flowing at low voltages

javier - Wednesday, 14th March 2012 4:01am - #4759

DISCULPA YO HE CONSTRUIDO UN TU CIRCUITO PERO NECESITO MANEJAR 110V Y 25A, PERO CUANDO CONECTO EL CIRCUITO A LA FUENTE LOS MOSFET EXPLOTAN, LOS MOSFET QUE USO SON DE 600V Y 40A. AYUDAME POR FAVOR ES MUY URGENTE SORRY I HAVE BUILT A CIRCUIT BUT I NEED YOUR DRIVER 110V and 25A, BUT WHEN THE CIRCUIT CONNECTED TO SOURCE THE EXPLOIT MOSFET, MOSFET THE USE THAT ARE 600V and 40A. PLEASE HELP ME IS URGENT

RMCybernetics - Wednesday, 14th March 2012 5:53pm - #4762

Your MOSFET needs to be able to withtsand the peak current. Measure the DC current your coil draws from your supply whehn you apply 120V. I suspect you need more turns on your work coil and choke.

david - Saturday, 17th March 2012 4:10pm - #4769

can you use 12 volts for this project?

Hampus - Thursday, 29th March 2012 12:42pm - #4771

Which one of the diods are you actualy using? The 40V or the 100V? RMCybernetics - Friday, 6th April 2012 9:04am - #4777

David, yes.

Hampus, Both. The first version uses the 40V ones, later when more power is used, the 1000V ones are used.

Benjamin - Monday, 9th April 2012 12:59am - #4780

RMC, I have managed to reproduce your induction heater using the same components and circuit, but mine isn't working. I powered it with a 19V 4,5Amps laptop power supply. Actually, all the voltage is shared between the two resistors and it's like no current flows in the control circuit. Do you have any idea about where the problem could come from ?

Chris - Monday, 16th April 2012 7:54am - #4782

Hi there. I built a smaller induction heater following as closely as I could to yours, and after a few unfortunate accidents, i got it working and loved it. I am now upsizing to a 110 volt version from wall power rectified through a bridge and a transformer with rectifier for 12 volts to the gates. My question is can I solely use the choke to limit current so that I could have a one turn work coil if I wanted, and if so, do I add that impedance to half or all of the work coil as it is center tapped. Thank you for your great work

RMCybernetics - Monday, 16th April 2012 9:47am - #4784

Benjamin,Maybe it is the 19V supply. Make sure you use a regulator for the gates as shown in a previous post.

Chris,Yes the choke will limit the current, but only as long as it is oscillating. You should work based on the impedance of half the work coil.

MrLeeh - Wednesday, 18th April 2012 12:13pm - #4787

Dear RMCybernetics, dear users thanks for this smart and very simple way of inductional heating. I tried it and eventually it worked very well. We can heat things up to 350C and higher. At first we had problems with the MOSFETS getting hot and also switching through at a certain power voltage level (around 8V). So we couldn' t get any more then 40W of heating power. Then we discoverred that the wires we used to connect the SOURCE to the ground were simply to long and at the high frequency they produced lot' s of noise. So we decided to use only one thick wire as connection to the ground of the source and part it short-wired to the two MOSFETs. Now we still got some noise on the GATE but it works very well (15V, 10A). Greetings MrLeeh

nathan - Sunday, 22nd April 2012 5:44pm - #4792

amazing project. cant wait to try it out. I've seen some posts about upping the power with larger caps and higher rated mosfets with larger power supply but is it possible to use more of the same mosfets just hooked in parallel with a larger power supply and caps or would that cause issues with the gates. I've seen it done in certain types of sound amps but I'm a little unfamiliar with high voltage mosfets and want to make sure before trying RMCybernetics - Tuesday, 24th April 2012 12:27pm - #4794

It might be possible to parallel some MOSFETs, but you would need to make short neat connections otherwise they may suffer from interference problems.

Deven - Tuesday, 1st May 2012 4:21pm - #4795

which type of FET used in 30v 10A Supply in your second case, and give the dimention copper tube and redious of turn. RMCybernetics - Tuesday, 8th May 2012 12:10pm - #4796

The STP30NF10 is used in both versions. We used 4mm brass tube, with a coil radius of 25mm.

Gilles - Wednesday, 6th June 2012 3:34pm - #4804

Hi, Is it possible and what would it take to have a dimmer switch controlling the intensity of heat? RMCybernetics - Tuesday, 12th June 2012 2:39pm - #4805

You would need some type of interrupter circuit and then vary the pulse width of the interrupter. For example; You could use one of our Power Pulse Modulators set to a low frequency to supply the power to the circuit. Adjusting the pulse width setting would proportionally adjust the power in the heater.

Hans - Tuesday, 17th July 2012 8:46pm - #4807

Hi Would it be possible to use a welding AC "buzz box" whith a suitable bridge rectifier as a power source ? ( no load voltage 57 V and 25 V at 100 A) Or maybe a inverter welder? I wan't some type of transformer etc since I'm not to fond of hooking up the mains 240 V to a barbed wire !!! Hans

whisk - Thursday, 19th July 2012 7:08pm - #4808

I am trying to build a small version of this using 12v power supply but the solder used to connect the coil to the capacitors keeps melting. Any suggestions. I believe the circuit is working but I can't be sure since it only stays on a few seconds before the solder melts. Perhaps something else is going wrong. Should the coil be getting this hot?

RMCybernetics - Thursday, 19th July 2012 10:21pm - #4809

Hans,You would also need a very large smoothing capacitor, but it could work.

whisk,If your solder is melting so quickly it is probably because your wire is too thin. You need to make the coil from very thick wire or copper pipe.

wellington - Sunday, 22nd July 2012 1:22am - #4810

thanks for the info my question is if I can use this circuit to 24 volts from a battery or 110 volts i also used to reduce a source is 24v 0 30v please ci there is a change in the circuit please give me ce can also use more parallel trancistores

wellington - Sunday, 22nd July 2012 1:34am - #4811

irf150n MOSFET can be used, or income tax or 54n 64n tanbien the iprf250n for use in parallel bariums

RMCybernetics - Friday, 27th July 2012 12:33pm - #4814

wellinton, I do not understand you. If you are using an electronic translator, make sure to use good punctuation and grammar for an accurate translation. Aamer - Friday, 27th July 2012 8:42pm - #4815

I am planning to make this simple Induction Heater for heating Catheter Tipping Molds. Please see the attached picture of the mold for your reference. I plan to heat two molds simultaneously. Mold diameter 1/2" and distance between the molds would be 4". I have 2 questions: a)Is it important to have 1000v capacitors? What minimum voltage rating would be OK? b)The second picture shows 4 designs of work coils. Which of them would be more efficient to heat both the molds.

Aamer - Friday, 27th July 2012 8:46pm - #4816

Here are the four designs of work coils. Ah yes, another question: Does a solid copper wire coil would do the job?

kevin - Saturday, 28th July 2012 12:20am - #4817

Hey Guys, Ran some calculations on both of your heaters. In the first one, at about 200kHz, the inductor should be about 1 uH. Realize that the peak voltage that is ringing in the tank circuit is going to be pi* supply voltage, in this case about 47v. This yields a peak current of about 39 amps! (27.5 RMS amps) Given that 2mm brass tubing was used, this give the cross sectional area of about 3mm^2, and since it was tubing, most likely about 1mm^2 for current to flow, the approx. size of an 18 AWG wire. Passing 27 amps through 18 gauge wire would heat the wire up to well over 90C if it was pure copper, so being brass would increase the losses and it shows in the photo where unloaded the circuit is dissipating over 40 watts! no wonder the coild was starting to soften. The second circuit is a little better. It is using 1.98 uF of capacitance at 133 kHz. This gives us a coil inductance of about 0.723 uH. This yeilds a current in a tank of about 105 RMS A. Even though the tubing is larger and cooled, the losses before inserting the load was already at 210W. That being said, the current ringing in the work coil needs to be reduced. That can be done by one of two means, increase the inductance or reduce the capacitance or both. i(t)=v(t) sqrt (C/L). This is derived by setting the reactive power formula equal to one another and solving. KVAR = V^2/Xc = I^2*Xl. I would redesign to limit the resonant current in the work coil to be much lower and then see what quiescent losses are. Then put in your load. I bet the time to temperature would be greatly reduce because more power is going into the load and not be wasted in circulating current loses. Kev

RMCybernetics - Monday, 30th July 2012 6:40pm - #4818

Aamer, a) The capacitors voltage rating just needs to be larger than the voltage they might be exposed to. Remember to account for the resonant voltage rise. b) A would not be very good due to low coupling, but the others will be ok. Copper wire is only going to be ok at low power levels. Litz wire would be better, but water cooled pipe will be best.

RMCybernetics - Monday, 30th July 2012 6:41pm - #4819

Thanks for your comments Kev.

grant - Wednesday, 1st August 2012 3:58am - #4820

I would like a to make one that uses no more than 12V and 10A what size of capacitors and resisters etc. do I need. I need it to vaporize gasoline or diesel fuel in a metal pipe thats about 1/4 in ID max. (Goal temps around 140-200 F) I'm very green at this I need a lot of help with this science project

RMCybernetics - Friday, 3rd August 2012 2:23pm - #4821

Grant, You need to work out the values for yourself, we can not do your project for you.

john - Sunday, 5th August 2012 7:04am - #4822

Thank you for your time to answer this question...Is it possible to make an induction heater that would heat pieces of iron that are welded to a rotating drum as the individual pieces pass over the induction coil with say, 10 pieces of iron distributed around the circumference of this drum, and be able to heat the drum by this method quickly to around 460 degrees, with a wattage of in excess of 3000 watts? Again thank you for the time to tell me if I am barking up the wrong tree. John

RMCybernetics - Monday, 6th August 2012 1:20pm - #4823

It depends on how fast it is rotating, and how large the metal parts are.

Aamer - Thursday, 9th August 2012 10:10pm - #4824

Comparing my last proposal of work coils - B, C, and D, what do say about this modification?

RMCybernetics - Friday, 10th August 2012 6:40pm - #4825

It looks OK. Is there any reason the tips cant be closer together, or even placed end to end? Separating the two halves of the coil means that the field in each coil will oscillating in strength, rather than alternating polarity as you would have when the coils are together. You would really be better off having two separate circuits and coils as it would be most effective.

Hans - Friday, 10th August 2012 8:17pm - #4827

Hi I was thinking about Kevins post. I'm not an EE and can be totaly wrong but : In a self oscillating circuit like this the freqency tend to be where the capacitive and inductive reactance equals out eatch other ie: the voltage and current is in phase and the power factor is close to 1 . If this is true, then the losses must come from resistive load ie the resistance in the coil and copper would probably be a much better material for the coil ? Since it is a self resonant circuit, changing cap or coil would alter freqency : more inductance or less capacitance = higher freqency and less inductance or more capacitance = lower frequency. Now to my question : If I am right, what benefits would there be if I used higher (or lower) freqency? Would I get higher effeciency ? If I am wrong, where did I go wrong? Hans

Justin - Saturday, 11th August 2012 7:57pm - #4828

Is there a risk of shock with this circuit? -risk of shock by touching the element that is being heated in the coil with another metal rod? (or is it electrically isolated?) -risk of shock/spark/dangerous short by touching the element to be heated to the coil? -risk of shock by touching the brass coil? -anything else i might not be thinking of?

RMCybernetics - Saturday, 11th August 2012 8:45pm - #4829

Hans, Most losses will come from resistance in the components. Mostly in the coil in this case, but losses in the capacitor are not insignificant. It is important to use capacitors with a low ESR. You have things a little mixed regards to altering frequency. Increasing either inductance or capacitance will lower the frequency. At higher frequency there will be more losses in the capacitor dielectric, and also in the coil due to the skin effect. The skin effect would also mean that at high frequencies the heating would be concentrated in thinner layers on the surface of material sample. Justin, Yes, the voltage rises as described in the article would mean that exposed terminals or the coil could have high voltages present. You should avoid letting the heated object touch the coil as it could short circuit and damage your transistors.

wellington - Thursday, 16th August 2012 9:40pm - #4830

thanks for the design I did 12 volt and perfect design I am interested in a dipocitibo de24volts acer 110 volts and 220 volts mofet which I can use or would have k k using thyristors and the condenser capacities thanks for the design is very cool I worked for one time and I'm interested in Majorcan efficient voltage for faster heating

Nicollas - Friday, 17th August 2012 11:12pm - #4831

I set up a circuit like this. but he is not working. one of the mosfets is very warming and the current is very high. put 12V but the current one seems Curco circuit. aumeitei as the number of windings L1 and L2 no more works. nothing changes that. I'm using mosfet IRFP250N what might be happening? my source is 12V/20AMPS. RMCybernetics - Saturday, 18th August 2012 11:30am - #4832

wellington, I have no idea what you are talking about. Nicollas, I do not really understand all you wrote either, but it sounds like your circuit is not oscilating. If you are also using different diodes, it could be because they have too much voltage drop which prevents the transistors from being switched.

Mike - Friday, 24th August 2012 4:38am - #4835

Can stainless steel be used for the work coil?

RMCybernetics - Friday, 24th August 2012 9:04am - #4836

No. The resistance of the coil would be too high. The losses in the coil would cause it to get very hot and waste a lot of power.Copper is the best material to use. Brass was only used here because we had the pipe in our workshop and it was small enough for making this simple demonstration.

Nicollas - Saturday, 25th August 2012 12:13am - #4837

swapped for new 1N4007 diodes and still does not work, the voltage drops to 12 volts to 6 volts, and does not oscillate. 'm using 220ohm resistors, it interfere in the functioning of the circuit?

Mike - Monday, 27th August 2012 5:33am - #4838

Ok, great,one more question. I'm using your same circuit and I'm using a 12v 16.7A switching supply is this going to be a problem? I hooked it up and seemed to fry my diodes. I'm using a nte2396 FET with enhancement mode. I ordered the ones your using to see if that's the problem. it also makes my supply go into protection mode, almost like the supply saw the curcuit as a dead short, I'm hoping it's the transistors, or perhaps I need a bigger work coil, any help?

RMCybernetics - Monday, 27th August 2012 8:06pm - #4839

Nicollas, Mike; Use the diagram in post 4649 that includes a voltage regulator. Use a bigger choke, and place a large capacitor parallel to the PSU to help it cope with current surges.

Mike - Wednesday, 29th August 2012 4:22am - #4840

That's good avice, A: is there a calculation that I can use to determine the inductance for L2 based on the inductance/resistance of L1? Im using 3/8 inch copper pipe. B: do I stll need a 12V regulator in the circuit if I'm using a single 12V supply to drive the whole thing? Andreas - Thursday, 30th August 2012 12:33am - #4841

Hello. In advance, sorry about my poor English! We bougth a inductive heater at work a last week it cost about 1500 dollars, and i said that i was wondering about building one myself, so all the guys i work with laught at me and said i could not do it... So i desided that i had to do it to shut them up by building one! ;) but there is a few problems... I`m really not a expert on electronics but i have a mild understanding of it so i think i understand most of the shematic you have drew, exept on the right of the shematic there is someting called "L2". Can you maybe try to explain it to me? And the second question: I have a 22V power supply, If make the inductor just as you have shown in the first part of your DIY, exept i use My 22V supply instead of 15V, the Diodes that you used on the "phushing it further" part of the DIY, the thicer brass tubing and maybe 4 capacitors (or do i need 6 on 22V allso?) Would this work??? I thing its great that i can buy all the parts from your DIY on your page! Best regards!

Andreas - Thursday, 30th August 2012 10:28pm - #4842

Hello, I am going to use a 22V power supply, will i need the 12V voltage regulator mentioned in the diy? where do i conect it? Are the Mosfet`s marked so i know witch leg on the transistor goes where? wellington - Sunday, 2nd September 2012 10:21pm - #4843

is okey 12 volts 110volts ?mosfet? wat number plees

RMCybernetics - Monday, 3rd September 2012 11:55am - #4844

Mike, Andreas, The inductor L2 serves as a choke to prevent high frequencies reaching your power supply, and also as a ballast to limit current. The calulations will depend on your operation frequency and desired current. 22V is fine, but you will need to feed the gates via a regulator as shown in a previous post. You should probably use more than four capacitors.

Andreas - Monday, 3rd September 2012 9:50pm - #4845

Hello again. I have ordered 8 capasitors from you, is it best to use 6 like you have on your setup or will it be best to use them all?

reg - Friday, 7th September 2012 3:03pm - #4849

i make your schematic and it works very good! thanks just one question, my mosfets are very hot why ? thank you for answer

RMCybernetics - Friday, 7th September 2012 3:47pm - #4851

Andreas, 6 or 8 will work OK. Using 8 may share the current a little more and therefore not heat up so much. Reg, the temperature of the MOSFETs will depend on the current flowing and the frequency. If the frequency and current is quite high, it is normal for them to heat up. Try using a larger heatsink, or MOSFETs with a lower drain-source resistance.

reg - Friday, 7th September 2012 3:58pm - #4852

thanks for your answer i use mosfet irf 540, use 36V power supply and separate 12V supply for vgs 10 + 10 turns work coil with 680 nF capacitor 1000V , L2 2O turns it is good?

RMCybernetics - Friday, 7th September 2012 4:50pm - #4853

See the calculations posted by kevin in post 4817. With a 36V input and IRF540 transistor you are likely to blow the transistors. As shown in the article our 30V input was causing it to push close to the limits of the transistors used.

reg - Friday, 7th September 2012 6:06pm - #4854

ok ,i will try 25 volts power supply and greater heatsink , thank you for your fast answer bye...

reg - Tuesday, 11th September 2012 5:18pm - #4856

hello , do you know what it happen if the gates becomes less than 12V? Nicollas - Wednesday, 12th September 2012 7:20pm - #4857

I am very pleased with the outcome of my induction heater. after many failed attempts I managed to solve the problem and now it works fine. Thank you all for the help.

Waheed - Thursday, 13th September 2012 12:11pm - #4858

Thanks for useful information. My circuit consists of IRF3710, 5x330nF/100V caps, 4 Turns of 3mm OD tubing at 12V. Draws 4 Amps. Tried it on higher voltage using 7812 but didnt work - no oscillation. Removed 7812 but with a large object in coil the current shoots up uncontrollably, burning FETs. What could be the reason. Thanks

Waheed - Thursday, 13th September 2012 12:13pm - #4859

Just wanted to show the waveforms.

RMCybernetics - Thursday, 13th September 2012 2:00pm - #4860

reg, if the gates are not switched on with a high enough voltage, the transistors will heat up significantly. Waheed, adding a large object in the coil will increase current demand a lot. if you want to limit the current, use a bigger choke (L2).

Aamer - Friday, 14th September 2012 6:17pm - #4861

For the choke L2, can a straight ferrite bar be used instead of toroidal core? For the capacitor bank, is it OK to use 28 x 68nF (400V)? Aamer - Monday, 17th September 2012 2:47pm - #4862

Here is the work coil which I have designed for my application. I would like to know how shall I join the two ends encircled in red color for continuous flow of cooling water? Do I need to braze them with brass or simply connect them with PVC tube?

RMCybernetics - Tuesday, 18th September 2012 6:00pm - #4864

A straight bar is ok, but will need more turns compared to a toroid. Those caps should be ok as long as they are capable of withstanding large currents and high temperatures. You can just link the ends using some PVC pipe as long as the copper is connected together too.

Daniel - Wednesday, 19th September 2012 4:09pm - #4870

Hi I would like to make one that is about 500W Parts I have used: IRFP4768 transistor 1N4007 diodes 240 ohm resistor 12 turn work coil 7x0,15uF (total of 1,07uF) low ESR capacitors. About 2mH choke The powersupply is a 10A 30V lab supply. I use quite a big capacitor bank on the output of the powersupply to help it cope with the current surges. The problem is that when I crank up the voltage the transistor gets worm and shorts out (drain/source) I suspect it has something to do with the hairy coil since wave (blue trace)? I cannot figure out how to get rid of that noice, so i would really appreciate any help I can get on the subject. I tried mounting 12V zener diodes to protect the gate but haven't dared turning up the voltage again while the hairy sine wave is still there. I am on my last set of transistors. Thank you for sharing sutch a good project with us, and helping us all out.

Daniel - Wednesday, 19th September 2012 4:10pm - #4871

Here is a picture of the setup, I did not have my good camera here, so I can take a better picture later if that helps.

RMCybernetics - Thursday, 20th September 2012 10:17pm - #4873

Try using a much much bigger choke, and regulating the voltage used to feed the gates. Make sure the supply to the gates is well regulated and filtered.

grant - Sunday, 23rd September 2012 10:41pm - #4879

where did you get the radiator from?

Steve - Monday, 24th September 2012 3:05pm - #4880

What voltage should the 2uf capacitor be rated for? 15 V or 90 V?

RMCybernetics - Monday, 24th September 2012 10:15pm - #4881

Grant, The radiator is a modified CPU cooler from a computer. I think this one came from an old DELL PC.

Steve, The voltage rise in the circuit will be about 3.14 times your input voltage. But your capacitors voltage rating should be significantly larger so that they are not being pushed to their limits. I would suggest using one rated for at least 6 times your input voltage.

Andreas - Monday, 24th September 2012 11:14pm - #4882

Hi. I buildt a Induction heater. I used the same components as you, so you know the specs off the transistors, capasitors, voltage regulator, resistors and diodes. My induction heater will not heat any thing and the transistors heat up really fast? They are mountet on heatsinks... what can be wrong here? My coil is maked from brake lines, i have allso tried whit mutch thiner wiers... Would it help if i uploaded a picture?

RMCybernetics - Tuesday, 25th September 2012 12:16am - #4883

Are brake lines stainless steel? That has a high resistance, as do thin wires. You need to use thick cable or pipe that is highly conductive.

Andreas - Tuesday, 25th September 2012 11:46pm - #4885

The wire i used vas about 1,5 mm2... do you think that`s the problem?

Aamer - Friday, 28th September 2012 10:08am - #4886

Just to have everything in perfect order, here is the choke which I made with 2mm, SWG no. 14 copper enameled wire. Is it OK? The 28 x 68nF capacitors which I have mounted at the end of the work coil are MYLAR capacitors rated 400V but they are small size about 12mm x 8mm x 3mm. I wonder if they would do the job or should I replace them.

RMCybernetics - Friday, 28th September 2012 1:40pm - #4887

Looks ok. You can try the caps you have but they may overheat quite easily.

Andreas - Friday, 28th September 2012 11:21pm - #4888

hi, can anyone see any thing wrong whit this setup? this is my second inductor, but i am having the exact same problem... mosfets are getting real hot real fast? any ideas?

RMCybernetics - Saturday, 29th September 2012 1:09pm - #4889

Looks ok, but dont let your heatsinks touch together or you will just be causing a short circuit. Fit a capacitor to the output of your regulator too, that will help smooth out any noisy power line. Something like 330uF or more.

Daniel - Saturday, 29th September 2012 2:01pm - #4890

Hi I have tried different size inductors for L2 and it dosn't really seem to do any difference for the high frequency that is superimposed on the sinewawe. The biggest inductor I tried was about 40mH, The voltage in the LC circuit went down, but other than that there was little difference. I also have problems with the voltage spike in the beginning of the sine wawe being about double the voltage of the top of the sine.

Andreas - Monday, 1st October 2012 8:59pm - #4891

I cant get this to work... the non of the conections or heatsinks are touching. Here is a picture the wiering, are the diodes the right way, have i solderes them to the right legs on the transistors?

RMCybernetics - Monday, 1st October 2012 9:59pm - #4892

Daniel, Andreas,

Make sure the supply to the resistors is regulated and smoothed with a capacitor.Check that your supply is capable of providing the peak current demanded by your coil.Decouple the supply to the gates by placing a capacitor from 12V to GND while keeping the connections as short as possible.Place a 12 to 15V zener diode between the gate and source of each MOSFET (cathode to the gate). This can help reduce noise and protects the transistors.

Aamer - Thursday, 4th October 2012 7:18pm - #4894

I have assembled the parts listed below as shown in the photo: MOSFET - STP30NF10 - 2 Pcs with Heatsinks DIODES - 1N4007 - 2 Pcs CAPACITORS - 330n x 6 (1250v) RESISTORS - 240 Ohms CHOKE L2 - SWG 14 (2 mm) Copper Wire 8 turns PSU - 12V 30A regulated, smoothed I switched ON the PSU for a few seconds, the MOSFET M1 in the photo started getting hot so, I switched OFF. Need your further assistance, please. One thing, the PSU is connected thru 50cm wires. RMCybernetics - Friday, 5th October 2012 11:34am - #4895

Try replacing your choke with something with huge inductance so that power is limited significantly. You can use an ordinary mains transformer for this. Just use an output winding as the choke. If it still wont oscillate, make sure you don't have a dud transistor or diode somewhere.

Aamer - Friday, 5th October 2012 3:52pm - #4896

Before replacing the choke I checked the MOSFETs and found that my all the pins of my M1 are short. For M2, while connecting the negative probe of multimeter (on diode checking mode) to the center pin (DRAIN) and the positive probe to the left pin (GATE) the multimeter beeps for continuity. And touching the positive probe to the right pin (SOURCE) the multimeter shows 490. And if connecting the pins vice versa then no continuity. I can understand that M1 is bad but not sure about about M2. As for replacing the choke with an ordinary transformer, do you mean I can use secondary winding of any 220->3v, 6v, 9v, 12v transformer (less than 1 Ampere)?

RMCybernetics - Friday, 5th October 2012 4:39pm - #4897

You should just replace both MOSFETs in your induction heater to be sure. Yes, pretty much any low power transformer will have enough inductance to limit the current to a couple of amps or less. It will be useless for significant heating, but allows you to test if it is oscilating without blowing components. You may also want to add zener protection to the transistor gates.

Frank - Saturday, 6th October 2012 12:59am - #4901

Couple questions. 1. Obviously if you push the voltage up, the Mosfet gate must be protected with a reg, but where are you putting that because it appears you are shorting the drain and gate?? 2. You mention using alt caps for this. Would I need the lowest ESR caps possible? IE poly or film/foil caps? 3. There was a question above regarding tubing choice. It would seem that copper might be better for this. Is that accurate? Why did you select brass?

RMCybernetics - Saturday, 6th October 2012 9:20am - #4902

See post#4649 for a diagram including a regulator. Yes, the capacitors in your induction heater must be low ESR otherwise resistance losses will soon heat them to destruction. Yes, copper is better due to lower resistance. Brass was used simply becasue we had some available at the time. I've since built one with copper which I will post later, but I did not see any noticable performance difference and the heater coil still needed to be water cooled.

Paul - Saturday, 6th October 2012 11:06pm - #4903

Hi, Just an experimenter in electronics but think I will have to try this! Two questions 1) I see people generally heating iron or steel. Are there other common materials which might heat even more efffectively and is there a simple answer to predict which materials will heat well (I am guessing high permeativity, high hysteresis?) 2) Is the heating effect uniform throughout the interior of the coil? Or is it concentrated more towards the center or periphery? Lets say you were trying to heat the water in the copper tubing, would a hollow metal tube fit to the diameter of the coil work? Well I guess that is more than two questions, In any event thanks this looks like a great project. RMCybernetics - Sunday, 7th October 2012 12:25pm - #4904

Iron based metals heat well because of hysteresis losses and conduction losses. Metals like copper and aluminium can be more difficult to heat partly due to higher conductivity.

The magnetic field is strongest at the inside edge of the coil. The skin effect also forces heating to concentrate on the surface of a material.

Aamer - Monday, 8th October 2012 3:02pm - #4905

I have replaced the MOSFETs. And also have replaced the choke with a low power transformer. I do not have an oscilliscope so how do I know if the circuit is oscillating or not.

RMCybernetics - Monday, 8th October 2012 3:19pm - #4906

Place something metal in the coil and see if it gets any warmer.

Frank - Monday, 8th October 2012 5:47pm - #4907

RM, Thanks for the reply regarding the voltage reg. I did notice the diagram with the 7812 but thought it was imcomplete because the drains of each mosfet still feed back to the gate of the other after the voltage reg. I was wondering if this should be done with two regs, both right at the gate and after the diodes? Just not sure how this configuration will work? Aamer - Monday, 8th October 2012 9:32pm - #4908

Okay, I have made the test replacing the choke L2 with a low power transformer. Initially, the work coil became warmer and warmer so I circulated water in it. Finally I was able to notice that the iron rod inside the coil has become warm. However, when I replaced the transformer with the choke L2, the MOSFETs became hot quickly. And nothing happened to the iron rod inside the work coil. By the way, I haven't yet introduced zener protection for the transistor gate.

RMCybernetics - Tuesday, 9th October 2012 7:46pm - #4909

Frank, The diagram is ok. You should only use one regulator. The way the connections switch on and off the gates is described in the article.

Aamer, Show us your work coil and iron rod.

Aamer - Tuesday, 9th October 2012 8:58pm - #4910

You can see the work coil at post no. 4862. You can see the circuit at post no. 4894. The iron rod was not a particular one. I tested on an iron nail, an 8mm iron rod and 5mm rod.....all three got warm.

RMCybernetics - Wednesday, 10th October 2012 5:05pm - #4911

I still think you need a bigger choke. The huge inductance of the transformer winding is allowing it to oscillate, so you just need to find the right iinductance value for your setup. When powering up the induction heater, I would suggest that you do so without any metal within the coil, then add the metal after.

Aamer - Wednesday, 10th October 2012 6:22pm - #4912

Do you mean I should increase the number of turns in my choke shown in post no. 4886? How many more turns do you suggest? How do we find out the matching coil size other than actual trails? By the way, does every new circuit we build (with same parts) needs different choke setting (inductance)?

RMCybernetics - Wednesday, 10th October 2012 8:01pm - #4913

Yes, I think you will need to experiment with what works best with your specific PSU and heating coil. You could also try using a Power Pulse Modulator between your PSU and the induction heater circuit. This would allow you to vary the power to the heater. You woild set the frequency settng to a low value like 20 Hz, then you can adjust the duty setting from 0 to 100% to give you a 0 to 100% adjustment in heating power.

paul - Sunday, 14th October 2012 10:12am - #4917

Hi. I did exactly as instructed here and used all the same components but mine doesnt seem to work. Only one mosfet gets really warm. The PSu which was initially set to 15v and 5 amps drops to below 5 volts once the control circuit is connect. Any idea on what seems to be the problem? The PSU is rated at 30V and 10A. Thanks.

RMCybernetics - Sunday, 14th October 2012 2:59pm - #4918

Your PSU is not able to meet the peak current demaned by your setup. When you first connect power, there will be a surge of current which is mostly limited by your choke (L2). Use a bigger choke, and add some large capacitors to your PSU.

Aamer - Monday, 15th October 2012 7:07pm - #4919

I increased the size of choke L2 from 8 to 9 turns. It increased the amperes and blew off 30A fuse of my PSU. Now, as per your suggestion to introduce Power Pulse Modulator, I have just finished the "SQUARE WAVE SIGNAL GENERATOR WITH PULSE MODULATION" for my Induction Heater. I am confused how would it withstand heavy amperes. Please guide me how should I use it so that I can tune my Induction Heater.

RMCybernetics - Tuesday, 16th October 2012 12:05pm - #4921

See the troubleshooting section I've added to the end of the article. You don't tune your induction heater. It automatically works at the resonant frequency. The link I gave you is to a product which will allow you to limit and control the input power so that you do not blow your transistors while testing it. I don't think adding another DIY circuit is going to help you, as it just increases complexity. Adding 1 turn to your choke is going to make little difference. Add something like 20 turns. I really do not think there is anything else left to suggest to you. If you are following the article and all the advice, but it is still not working, then you must have faulty parts, wrong connections, or a PSU that is not up to the job.

Allen - Saturday, 20th October 2012 11:42am - #4922

Since you guys have a bit more experience then myself with induction heating devices, I wanted to ask how well might an MTY100N10E N ch. Mosfet work for an induction heating application as far as the drive transistors? This transistor is rated for 100A @ 100v the on-resistance is quite low as well @ 0.011ohms, the only thing I see that is quite different from the recommended transistor pair, is the switching times (rise / fall) are quite a bit higher, but I suspect that to be normal for such high currents possibly? I'd like to get some feedback from someone with a bit more experience to see if I can use these transistors for my induction heating tests. I'm interested in heating a bit larger objects and a bit faster. Thanks -Allen

RMCybernetics - Saturday, 20th October 2012 12:10pm - #4923

While I can't comment on every individual transistor model anyone might use, I can suggest that if you use one with longer rise/fall times, you could compensate by using a larger capacitance to lower the resonant frequency of the system.

Allen - Saturday, 20th October 2012 7:41pm - #4924

Is there a ratio of rise / fall times to capacitance values? I'm curious to know the mathematics behind the circuit.

RMCybernetics - Saturday, 20th October 2012 8:44pm - #4925

It only becomes an issue if the rise/fall times are a significant value compared to the period of the operating frequency of the circuit. The transistors are switching when the voltage (between source and drain) is almost zero. Just work out what the switching time is as a percentage of the period. By minimising this value, the switching losses are reduced. In this circuit you would probably be more concened with conduction losses due to high currents unless you need it to operate at some very high frequency for some reason.

Joe - Monday, 22nd October 2012 9:49am - #4927

Hi...I have been looking for a circuit like this wit a straight forward explanation..Kudos RMCybernetics. Will the caps (pictured) work with this project. .003uF 4Kv (302K4M) I am seeing in my calculations that my project will resonate near 400Khz. And if I am right, not an engineer, would mean I can heat larger objects. RMCybernetics - Monday, 22nd October 2012 1:42pm - #4928

No they wont. That looks like a polystyrene capacitor and judging by the thickness of the wires, it is not meant for high currents. You need to use polypropylene capacitors or an equivalent that is specifically made for high currents. For larger objects you need more power.

Joe - Monday, 22nd October 2012 7:02pm - #4930

Thank you, I had guessed they werent going to work..wanted to ask before I released the magic smoke out of them

Joe - Tuesday, 23rd October 2012 8:07pm - #4931

Last question: What is, if any, the difference between polypropylene and mylar caps?

RMCybernetics - Wednesday, 24th October 2012 4:05pm - #4932

This link should answer that for you.Aamer - Thursday, 25th October 2012 10:07am - #4933

Good day Richard.....Appreciate your devotion to guide others. Just for reference, I posted my Work Coil design at #4862 and the choke at #4886. I had to replace one faulty STP40NF10 with IRFP150. Now my circuit is oscillating with STP40NF10 + IRFP150 but the trial load (4mm iron nail) only gets warm. As per your advice, I re-designed my choke with 20 turns of 2mm wire and then reduced gradually 18, 16, 14, 12, 11, 10, 9, 8, 7, 6, 5 turns but no significant change. However, at 9-8-7-6 turns the load got warmer + the STP40NF10 dispersed more heat. By the way, I inserted the load for 5-10 seconds.

Aamer - Thursday, 25th October 2012 10:16am - #4934

Another question: I want to understand the behavior of this circuit and hence plan to simulate it in LTspice. How should I check if my simulation is correct......I mean to say, at what points I should check for the waveforms and what type of waveforms?

RMCybernetics - Thursday, 25th October 2012 1:43pm - #4935

You should allow more time for your load to heat up. I've added a spice simulation to the article just above the troubleshooting section. TheSTP40NF10was not in the library so I just chose something else in the library. Run the simulation then check out the collector waveforms and inductor current. The simulation does not include any load, but you can add this if you wish. I'm not here to answer questions on how to use LTSpice though.

Aamer - Thursday, 25th October 2012 2:31pm - #4936

As per my results on various turns of the choke, the intensity of heat (as I felt) was almost the same. Of course, keeping the load for more time will make it hot. But, pardon me, my trial load is not thicker than the screwdriver shown in your article, which turned red hot in just 5 seconds. RMCybernetics - Thursday, 25th October 2012 3:34pm - #4937

You are using different components and some kind of DIY power supply. You will of course not get the same results.

Aamer - Thursday, 25th October 2012 8:34pm - #4938

Thanks for your swift response. Okay, I shall change the STP40NF10 to IRFP150 and then re-try with my existing PSU which is functioning quite OK. But, I shall definitely not ignore your tip to change the PSU. By the way, what do say about the 12V 30A SMPS, will it be OK?? Your netlist could not be simulated because LTspice is giving an error "Multiple instances of Flag". Would you please check it. Thanks for your attention.

RMCybernetics - Thursday, 25th October 2012 11:22pm - #4939

Lol, the spice file is fine, I really don't know what you must be doing. You will need a higher input voltage than 12V to get heating like shown in the video (I used 30V). Have you also considered that your capacitors may be damaged, or just not up to the job?

Aamer - Friday, 26th October 2012 12:08pm - #4940

I have limited knowledge/experience of electronics. I am simply following the instructions as well the experience of other fellows. I knew you used 30v 10A. But you also confirmed in post #4698 that the circuit will work with 12v 30A PSU, and Bob was successful (post # 4894). Anyway, I can increase the voltage to 24V at 30A, will it be OK? As far as the capacitors are concerned, frankly, when the circuit is oscillating, I understood that the caps are working fine. I will try again with 24v, if unsuccessful then will check the caps. I renamed the spice file extension to "*.cir" and opened it in LTspice and then clicked RUN icon. If you think I am wrong, can you provide the "*.asc" file??? Lastly, thanks for your guidance.

Nicollas - Sunday, 28th October 2012 1:07am - #4941

This is the result obtained by me. was very good and oscillates at frequency of 75kHz. Thanks for the help RMcybernetics and friends.

Charles - Sunday, 28th October 2012 2:12am - #4942

> > > > two questions < < < < 1. In a water cooled installation would one still need brass induction coils instead of plain copper tubing? 2. I see where the coil configuration can be modified. So for tempering knives one could have an egg shaped coil allowing 1/4" clearances on all 4 sides. Correct? . . .thank you chas. ps. great site.

RMCybernetics - Sunday, 28th October 2012 2:52pm - #4943

Aamer, It is already an asc file. I don't know why you renamed it. Yes 12V will work but of course less powerful than 30V. As mentioned multiple times, the capacitors need to be quality polypropylene or equivalent. It may oscillate when using other caps, but the performance will be poor. If they are damaged by voltage spikes, that will also prevent proper operation.

Nicollas, Thanks for sharing. Nice build quality.

Charles, please read through the other comments. As mentioned already, brass was only used because it was available at the time. Copper pipe would be better. Yes you can use whatever coil shape you like. However the heating effect may not exactly match the shape as the magnetic interactions and shape of the object will dominate the effect.

ad - Tuesday, 30th October 2012 10:20am - #4944

Hello i build you heater, and it works ok....Now i want more power....but i hav one question... If i use bigger FET's / IGBT,up to 600v and 150A (MG150Q2YS40).. Can is use the same diodes to the gate, i think the power over the gates will be more than 20 volts....

RMCybernetics - Tuesday, 30th October 2012 11:13am - #4945

You can not just replace the transistors with power IGBTs. The saturation voltage of the transistors would be too high, preventing proper switching as explained in the article. A different and more complex circuit would be needed.

Aamer - Tuesday, 30th October 2012 2:14pm - #4946

Hi Richard. (1) The link for simulation opens NETLIST in another window which deceived me that you are providing netlist instead of asc file. Anyway, I have opened your asc file and compared with mine which wasn't simulating. The main difference was that you used 2 power supplies. Now my simulation results are same as yours. (2) In the simulation, I noticed that the circuit (except the capacitors and inductors) is drawing less than 1 Amp. So, I physically connected 12v-2A supply to the MOSFETs and 12V-30A to the choke. No oscillation, one MOSFET burnt. Maybe no oscillation because MOSFET burnt. Am I wrong somewhere? I plan to use 24v-30A for the choke and 12v-2A for the MOSFETs. (3) Mosfets IRFP150 + 6 x 330n Polypropylene Caps still oscillating at 12v-30A (but not red hot). Shall I use 24v-30A supply and add 7812 for the MOSFETs?

RMCybernetics - Tuesday, 30th October 2012 2:46pm - #4947

The link is directly to an asc file. You can click to view it, or right click and save it to your computer.

The simulation does not include any load and is not accounting for the losses in the system which is why it only draws a low current. You should observe the voltage on the 30V supply. During the first moments it will drop very low which could prevent oscilation in a single supply system if not accounted for. You should also consider the initial voltage spike as this could blow any of your components. Try this simulation. It will not oscilate becasue the PSU has too much internal resistance. Run it, you will only see DC, then alter the resistance to something lower and you will see it begins to oscilate.

Yes, add the regulator and zeners as suggested.

Aamer - Wednesday, 31st October 2012 7:30am - #4951

Thanks for your swift response. (1) Okay, I shall change to 24V 30A + 7812 but for the sake of learning, I want to understand why my circuit did not worked with 2 power supplies? (2) The simulation helped me a lot. Thanks for the newer simulation with single supply. For load, I coupled another inductor L3 (1uH) to L1/L1b and connected a load in parallel to L3. The maximum readings (during the initial spike) I got with 200 Ohms......V at drain 200V, I at drain 24A, V at gate 14V, I at gate < 1A. So how about changing the mosfets to 300V/35A to ensure protection against the initial spike?

RMCybernetics - Wednesday, 31st October 2012 9:34am - #4953

Yes, use a single 24V supply and regulator for the gates. When using two supplies, did you create a common ground? You could change the MOSFETs if you like, just choose good ones with low resistance and high speed.

ad - Wednesday, 31st October 2012 9:47am - #4954

Okay thank for the reply..

ad - Wednesday, 31st October 2012 9:50am - #4955

If i use a fets who has a voltage of 600 and 50 Amps , can i use it from the net after reverse it from 220 volt to DC, the IN4007 and caps are up to 1000 volt...BUT what wil happen to the gate power of the fets...? Aamer - Wednesday, 31st October 2012 11:23am - #4956

Thanks for your usual prompt response. (1) Yes, I had a common ground when I used 2 supplies. (2) So changing the MOSFETs to higher rated voltage can withstand the initial spike?

RMCybernetics - Wednesday, 31st October 2012 1:48pm - #4957

Well theSTP30NF10used here does fine even with 30V input if you have a regulator and zener protection on the gates, but you can try other transistors if you want.

Aamer - Wednesday, 31st October 2012 3:05pm - #4958

(1) Noted. (2) Any comments --- why didn't my circuit worked with two supplies?

RMCybernetics - Wednesday, 31st October 2012 3:42pm - #4959

There's nothing else I can suggest.

ad - Wednesday, 31st October 2012 7:09pm - #4960

I used HFR3205, and it works good.....what kind of regulator and zener you maen or think is possible.....can you make a schematic.... thanks

ad - Thursday, 1st November 2012 11:03am - #4961

this is mine.....now want more power !!

RMCybernetics - Thursday, 1st November 2012 11:10am - #4962

Looks nice. DId you etch your own PCB? You may want to reinforce the PCB traces from the caps though as they will be carrying a lot of current. You can see the regulator diagram in post 4649. The zeners would just go between the gate and source with the cathode to the gate

Charles - Thursday, 1st November 2012 11:20am - #4963

I own a 12 volt, 5.5 amp dc power supply. I would like an induction furnace that could heat 25mm round or square stock. Is this power supply sufficient to accomplish this. . . 62 years ago I fell off the 'Wheatstone Bridge' in college..hence my need for assistance. thank you.

RMCybernetics - Thursday, 1st November 2012 11:30am - #4964

It would heat it, but I would not call it a furnace. You may even struggle to get it red hot. Once such a large load is placed in the coil, the current draw will rise significantly, and quite probably need more than 5.5A.We will however post a new project in future which allows for higher power levels.

ad - Thursday, 1st November 2012 12:44pm - #4965

the other side.....used a lot of solder tin :-)

ad - Thursday, 1st November 2012 1:03pm - #4966

one problem if i rectify 230 volt straight from the net (its 230 x 1,44)it will be 330 volt DC a simple regulator (7812) will go to max 30 volts...and how much volt wil there be on the diodes...?

RMCybernetics - Thursday, 1st November 2012 1:11pm - #4967

You can't use this circuit at those voltages. You would need a different, more complex setup which is beyond the scope of this article.

Mike - Friday, 2nd November 2012 11:32am - #4968

@#4966: Why do you want to use 230Vac? It is all about amperes for induction heating and high frequencies to stimulate hysteresis in the workpiece. When Xc and Xl (reactance of capacitance and solenoid) are the same you are in resonance and the circuit acts as a short (for this parallel circuit it does). So if you would have 0.4 Ohm resistance on the copper wires, you would have 330 / 0.4 = 825 Amperes with 250Vac rectified. Why not taking a 12 volt power supply instead? Then you would have 30 amps. That is quiete alot. Or take a 24v power supply and you would have ~60 Amps!!

Charles - Friday, 2nd November 2012 12:24pm - #4969

Just obtained 24vdc, 15 amp power supply. . . .Please furnish me with the type and number of torroid coils (L2)you recommend and sell to max this power supply. And the size, number, voltage and type of caps I will need, and the two transistors (T1 & T2), you sell. . . .I will want 4 Zener Diodes, plus 6 resistors (R-1 & R-2). . . .I have the heat sinks for the transistors and for the caps. I assume the Voltage Reg (7802?) is readily available locally. . . .I would appreciate the material costs and the shipping to zip code 59711, USA. I would best like an email in responce to any necessary questions regarding this request. . . .Thank you, charlie.

RMCybernetics - Saturday, 3rd November 2012 6:30pm - #4970

Links to all the available parts are in the article. If you add them to your basket, you can get a shipping price calculated automatically.

Nicollas - Saturday, 3rd November 2012 7:29pm - #4971

I wonder if this circuit can operate with a voltage of 220 volts? I know I'll have to change the components, but I wonder if the circuit is capable switching at higher voltages. I need heat a billet steel 15mm in diameter.

Aamer - Monday, 12th November 2012 6:18pm - #4975

Good day! (1) After many failures, I decided to start all over again. I made a new work coil with 2mm solid copper wire 5+5 turns. For choke I used 1mm wire, 16-18 turns. For MOSFETs 12v-2A supply and for CHOKE 19v-4A (laptop) supply. The combination worked nicely, the sample got very hot in just 3-5 seconds (not red hot because of only 4A supply). (2) The results encouraged me, I made another W/coil from 4.7mm outer dia/2.7mm inner dia copper tubing. I started with 3+3 turns and coil shape round unlike the one shown in post no. 4862. Power supplies remained as above. The load remained cool and the w/coil became warm without water circulation. Modified the turns of 1mm wire choke --- No improvements. Changed the choke to 2mm wire 7-8 turns ---- No improvements, additionally the MOSFETS are becoming pretty warm.

Aamer - Monday, 12th November 2012 9:00pm - #4976

(1) Failure of my oval shaped work coil leads me to make 2 circuits which I have to use simultaneously. Do I have to arrange separate PSUs for chokes in both circuit? (2) Will there be some sort of electrolysis effect if the cooling water from one work coil flows into the other? (3) Is there any way which can indicate resonance.... something like an LED or a neon bulb?

Mii Yeti - Saturday, 17th November 2012 10:15pm - #4978

Hi I would like to know if it is possible to use two 2n3055 transistors Thanks Mii Yeti

RMCybernetics - Monday, 19th November 2012 10:40am - #4980

As long as the water is not contaminated with significant amounts of electrolyte (like in salt water), then the length of tubing should make the total resistance so high that any current flowing in the water would be neglegable.

2n3055 is not suitable. Use the MOSFETs mentioned in the article.

Mii - Tuesday, 20th November 2012 4:23am - #4981

Thank you for your help and time.

Mike - Sunday, 25th November 2012 10:11am - #4983

How is it possible that the applied 12 Vgs on the MOSFETs gates cause a 90 Vpk-pk on the scope? I mean Vds will never be higher than Vgs, right? But (90Vpk-pk) / 2 = 45Vmax. 45 Vmax / SQRT(2) = 31.8 Vrms. Where 31.8 Vrms equals ~ the input voltage of 30V from the PSU. How can you ever reach a Vrms higher than the applied Vgs on the MOSFETs. Please do explain as I wanted to use a power supply of 45Vrms to crank up the power! RMCybernetics - Sunday, 25th November 2012 11:57am - #4985

Vgs does not determine Vds when the gate is saturated (such as above 10V). The voltage applied to the gate here is 12V, but the voltage from the supply for the rest of the circuit is 30V. When a 'Tank Circuit' (inductor and capacitor in parallel) is pulsed, it will resonate at its fundamental frequency. If the Q factor of the circuit is good enough (there are not significant losses through resistance etc), then there will be a resonant voltage