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ATX to Lab Bench Power Supply Conversionby mat_the_w on December 8, 2005

Table of Contents

intro:   ATX to Lab Bench Power Supply Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

step 1:   Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

step 2:   Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

step 3:   Drilling holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

step 4:   Connecting front-panel items . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

step 5:   Power Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

step 6:   Finished Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

intro:  ATX to Lab Bench Power Supply ConversionIn my sophomore year of college at the University of Minnesota, I started into my main electronics classes, and needed a good power supply for working on lab projectsat home. My roommate Adam told me about somebody online who had converted an ATX computer power supply into a lab bench power supply, so I decided to do thesame thing. You can also check out this link for a very similar guide by their user Abizarl. I have also documented this project on my website athttp://www.mbeckler.org/powersupply.html if you are interested.

Warning! There are several large capacitors in ATX power supplies, that will store a dangerous charge for a long time. Please let your power supply discharge,completely unplugged from the wall outlet, for a few days before opening it up. You can probably be seriously hurt, so please be very careful. I am not a lawyer, but Ihereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.

step 1: BackgroundFirst, a bit of background on a typical ATX power supply:

Computer power supplies are Switch Mode Power Supplies (SMPS), which use high-frequency switching circuit elements to provide a high-quality output voltage, withgood energy efficiency. One side effect of this technology is the minimum load requirement that each power supply has. In order to function properly, the power supplyneeds at least a very small electrical load connected to it. In other words, ATX power supplies will only work if you have something connected to it. We will be using apower resistor to provide this minimum load.

Also, modern power supplies do not simply have an OFF/ON switch, they have what is known as a "soft" power switch. This normally makes no difference to the user, asthe computer behaves the same, but when you shutdown your computer, the motherboard can turn off the power supply when it has finished shutting down. This requiresus to add our own power switch to the power supply chassis.

To protect our circuit from accidental (and careless!) short circuits, we will install a fuse-holder and fuse, which will disconnect the circuit if too much current flows throughit. The size of the fuse is up to you, but a 1 amp fuse will work just fine for most circuits. If you are planning to use more than one output voltage at the same time, say, ifyou wanted to use both the +5v and +12v supplies at the same time, then it would be a good idea to add fuses to all the voltage lines, and not just the ground line.

step 2: PlanningPlanning is the most important step of any successful project. To plan this project, I created a few images. I am going to be using four binding posts, a power switch, afuse holder, a power resistor, and two light emitting diodes (LED's) with current-limiting resistors. The first image details the circuit connections inside the power supply,where everything will be connected

When the power supply is connected to the wall socket, but not yet turned on, it provides a +5v standby signal, that can be used by the motherboard for things like wake-on-LAN functionality. We use this signal line to indicate when the power supply is plugged in with a red LED and a 330 ohm resistor. On my power supply, this signal linehas a purple wire, and is labeled "+5VSB" on the circuit board.

When the power supply is first turned on, it must go through a start-up sequence, to ensure that everything is working, and that it is able to provide stable power to thecomputer. When the start-up sequence has completed, it signals the motherboard by providing +5v on the "Power Good/Steady" signal line. We will use another red LEDand 330 ohm resistor to indicate when the power supply is running. On my power supply, this signal line has a gray wire, and is labeled "PGS" on the circuit board.

The power resistor is a 10 ohm, 10 Watt resistor, commonly called a "sandbar", because they are usually coated with a material that feels like sand. Most power suppliesneed a minimum load to keep them running, so this sandbar resistor provides a constant minimum load between the +5V rail and Ground. I've heard that newer powersupplies also need a load on the 3.3v rail, your mileage may vary.

In the second image, you can see the diagram for the front of the power supply. Here I have marked where the components will go, including the LED's, the bindingposts, the fuse holder, and the switch.

The third image is what the power supply looks like without any modifications. You can see the various voltages I am going to use along the front edge.

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

Image Notes1. Circuit board connection points

step 3: Drilling holesHere, I planned out and drilled the holes in the case. My power supply was a smaller form factor, (It was from a mini-tower case), so there wasn't a lot of space to workwith.

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

step 4: Connecting front-panel itemsHere, I am connecting the appropriate wires to the binding posts, power switch, fuse holder, and LED's.

In an ATX power supply, there should be a wire that is used to turn on the power supply. You can see this wire (It's green) in the second picture; it is the green wire in themiddle, where it says "ON/OFF" on the PCB. I connected this to the switch, and the other pole of the switch went to ground. The +5, +12, and -12 are connected right totheir wires on the PCB. The ground wire is connected through the fuse holder before the binding post.

Initially, I was going to use green LED's, but I realized I had many more red LED's than green LED's, so I switched them over to reds. In the first picture, you can see theholders I installed into the front. I connected the LED's through a common resistor to ground. The LED on the left (from the front view) is a standby LED. It is lit wheneverI have the power supply plugged into the wall. It is connected to the +5V standby wire on the PCB. In my PS, it's purple. The other LED is the "Power On" LED, and it is litwhen I have the power supply turned on. It's connected to the "Power OK" signal wire, which goes to +5V when the power supply detects that it has stabilized thevoltages. In my PS, it's the gray wire.

Image Notes1. LED Holders

Image Notes1. Green wire - You can see "On/Off" written on the circuit board.

Image Notes1. As noted, these LED's were exchanged for red LED's.

Image Notes1. Connect "Power On" Led to gray wire.2. Connect "Standby" LED to purple wire.

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

step 5: Power ResistorMost modern ATX power supplies require a small load to stay in the ON mode. I added a 10 ohm, 10 watt resistor between +5V and ground to provide this small load. Itis strapped to the back wall of the power supply, where it should get plenty of air flow. It gets quite warm while running. I don't have any wireties in my dorm room, so Ihad to tie the resistor to the wall with regular wire, but I plan to upgrade to wireties when I go home for holiday break.

Image Notes1. Power Resistor - 10ohm 10 Watt

step 6: Finished ProjectHere you can see the finished project, both with and without the cover. If you have any questions, please leave a comment and I will try to check back often to answerthem. Thanks for looking, and good luck!

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

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Comments50 comments Add Comment view all 104 comments

 gull says:  Nov 27, 2008. 3:32 PM  REPLYI just completed converting an ATX power supply to use as a 12V supply for powering some radio gear. I found it was also necessary to tie the brown (+3.3VSENS) and orange (+3.3V) wires together on my supply or it wouldn't run. They're normally connected together at the motherboard connector, I think tocompensate for voltage drop in the wiring.

 mat_the_w says:  Feb 3, 2007. 12:20 AM  REPLYPlease note that there is no fuse protecting short circuits between any of the non-ground lines. This means that if you short +12 to -12, then there is noprotection from that. This is an oversight from the original design, but I don't have room in my power supply to add more fuses. A better option would be tofuse the AC side of things, but I'm not able to do that right now. Please be careful.

 gull says:  Nov 18, 2008. 2:23 PM  REPLYAn output fuse isn't really necessary to protect the power supply itself -- switching supplies like this one will go into shutdown automatically if they detecta short circuit. However, you may want one anyway to protect your project. In particular the +5V line provides more than enough current to smoke tracesright off a PC board and heat thin wires red hot.

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

 thermoelectric says:  Oct 31, 2008. 4:27 AM  REPLYUhhhhmm, I know ur probably not a technician, But after i unsoldered some of the output wires, But now when i connect the signal wire to the groung, Itstarts then immediately turns off, Any suggestions on what i have done?

 mat_the_w says:  Nov 3, 2008. 5:42 PM  REPLYI'd guess that the load isn't connected anymore, or there is a short between one of the power lines and ground.

 thermoelectric says:  Nov 3, 2008. 9:17 PM  REPLYOh, I'll check that this afternoon.....

 thermoelectric says:  Oct 30, 2008. 2:24 AM  REPLYHey, i just strapped my 100ohm 10watt power resister to the case in the corner cause it's too hard to get it where the airflow is, will it be alright, i strapped itdown so two of the sides are contacting the metal instead of one

 mat_the_w says:  Oct 30, 2008. 6:27 AM  REPLYIt should be just fine. I've never noticed the power resistor on mine getting warm at all.

 thermoelectric says:  Oct 30, 2008. 1:59 PM  REPLYSo, they aren't supposed to get hot on the 5v line?

 mat_the_w says:  Oct 30, 2008. 6:51 PM  REPLYWell, ideally they would only be drawing a very small amount of power, and wouldn't be wasting much energy in the form of heat. They certainlyshouldn't be getting hot, as there's only 1/4 W being burned:

I=V/R = 5v / 100ohm = 0.05 A = 50 mAP=VI = 5v*50mA = .25 W

 thermoelectric says:  Oct 30, 2008. 10:21 PM  REPLYThanks for your help, 5 Star rating from me

 thermoelectric says:  Oct 30, 2008. 10:20 PM  REPLYOh, Now I'll go put it back together.....

 helijet says:  Oct 27, 2008. 3:31 PM  REPLYHi mat_the_w, I'm having a hard time getting my ATX power supply to work with a toggle switch that came with a built-in led light. Somehow, I can only geteither the fan to turn on or the led light. I want to get both the fan and led light to turn on at the same time. The toggle switch has three posts: POWER, ACC,GROUND. Can you help me with this? Thanks

 DBLinuxLover says:  Oct 7, 2008. 1:18 PM  REPLYOut of curiosity, what would happen if you only used 1 wire, instead of bundling them all together, then attaching them to the binding post?

 mat_the_w says:  Oct 7, 2008. 6:23 PM  REPLYI probably didn't mention it in the write up, but I only connected one of each color wire to the binding posts. I cut the others as short as I could to thecircuit board, and left a lone wire to connect to the binding posts. It's worked just fine for me the past three years.

 DBLinuxLover says:  Oct 6, 2008. 2:34 PM  REPLYHey good instructable. But, I noticed a problem, in the second step, when you are referring to the resistor as a sandbar, you have "The power resistor is a 10ohm, 10 Watt resistor, commonly called a "sandbar", because they are usually coated with a material that feels like sand. Most power supplies need aminimum of". Just letting you know, you may want to fix that last sentance. but otherwise, nice job.

 mat_the_w says:  Oct 6, 2008. 3:23 PM  REPLYThanks for the heads up. I recently went back and re-did some of the drawings to try and get them to look better on the website. I must have messed upthe text of step 2.

Glad you liked the instructable,Matthew

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

 DBLinuxLover says:  Oct 6, 2008. 5:43 PM  REPLYYeah no problem. In the process of building one of these at the moment. Trying to find binding posts that dont cost an arm and a leg. Gonna have toget them online. They cost like 4 bucks for a pack of 2 at radioshack.

 mat_the_w says:  Oct 6, 2008. 6:35 PM  REPLYMake sure to get the right kind of binding posts. Some don't have plastic insulation around the metal post when going through the wall of theenclosure. The first pack I got were electrically connected to the case, which is no good here, with a metal case. It seems like the full-size bindingposts, that are about 2-3 cm tall, are usually insulated in the proper way. Good luck!

 DBLinuxLover says:  Oct 6, 2008. 7:19 PM  REPLYYeah. Need to find a decent set for not much money. Thanks for the heads up. I know radioshack sells them, but they're too much.

 pcludeg says:  Aug 16, 2008. 3:23 AM  REPLYwaw Sweet this projectFinally I have a use for my ever increasing stock pile of smps PSU's! keep up the great work and keep the ideas flowin'!

Cheers! and vary thanku

 kishida says:  May 22, 2007. 3:46 PM  REPLYNice workmanship.Can you substitute the load with a dc fan or a light bulb?

 xsmurf says:  Sep 24, 2007. 10:41 AM  REPLYI tried this... I moved the built in fan to the 5v, thinking that on top of adding load it would reduce noise... sadly it doesn't seem to be enough of a load...the voltages are still under their ratings.

 Estwald says:  Aug 3, 2008. 7:17 AM  REPLYTo get the voltages up see:The Fix, http://web2.murraystate.edu/andy.batts/ps/TheFIX_java_table.htm

 mat_the_w says:  Jun 2, 2007. 7:59 PM  REPLYIt would probably work. I do not know offhand what the minimum required load current on the 5v line is, and it probably changes for different powersupplies. Let us know if you have any success with a fan or bulb!

 phugedaboudet says:  Jul 30, 2008. 2:58 PM  REPLYvery nice. I've been needing a good, cheap AC120 to DC12 power supply for my RC LiPO charger-it's designed to connect to a car battery for field charging.This lets me use some older power supplies to feed my charger.

Plus I can buy a more expensive higher wattage power supply and use it to trickle charge motorcycle batteries.

thanks for the writeup!

 rp181 says:  Feb 20, 2008. 6:57 PM  REPLYNice Work! Im trying to do the same with my power supply, but cant get it to work. I have a supply shown here:http://www.hcditrading.com/Shop/Control/Product/fp/vpid/879633/vpcsid/0/SFV/29664

I tried connecting the green to the black with a 10ohm 10W wire-wound resistor from radio shack connecting the 5V and GND lines as a minimum load, butstill nothing. A "squeaking" noise comes every ~1sec when the green is connected to the black.

Any help would be appreciated

 spikeychops says:  Jul 29, 2008. 12:28 PM  REPLYisn't it meant to be a 330Ohm resistor connected across the black, and the 10ohm 10w power resistor wired across a red (5v) and a black?

spikey

 theyikes says:  Jun 4, 2008. 3:40 PM  REPLYSweet!Finally I have a use for my ever increasing stock pile of PSU's! keep up the great work and keep the ideas flowin'!

Cheers!

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

 freelancemissions says:  Apr 28, 2008. 10:45 AM  REPLYGreat work. A couple of things, the 10 resistor could be placed on a small plate of aluminium and mounted near the power supply on rubber feet. This couldkeep your coffee warm. If one is thinking that the 10 ohm resistor is wasting heat, you could install a 6Volt lamp as a power on indicator as well. Also yourwarning is correct on the power capacitators in the high voltage side. These caps store a charge over 300 volts at times, so if you do not want to do the "lookat me jumping like a fool because I touched something I should not have", please be careful.

 mat_the_w says:  Jan 18, 2007. 11:12 PM  REPLYNote: Some folks were discussing this project on a forum (I don't remember which), and someone mentioned that by fusing only ground, this design is onlyprotected from shorts between any of the other voltages and ground. It does not protect the supply from shorts between, say, -12 and +12. They suggestedfusing the AC line, which I don't really know anything about. You could probably also fuse each line individually, which would get tiresome, but would besafer.

 marxmarv says:  Apr 16, 2008. 2:54 PM  REPLYThe power supply probably has some current limiting already, at least on its main output. An open ground could conceivably damage circuits by reversepolarizing some components.

 n0ukf says:  Mar 22, 2008. 1:05 PM  REPLYI agree that you should fuse the DC outputs instead of the ground. The AC side of the PSU should already be fused somewhere on the board or perhapseven in the IEC connector coming into the case.

Another addition to the project if you have room could be to wire LEDs across the fuses (don't forget the limiting resistors) for each output. This can giveyou an indication that a fuse has blown if your project has shorted out.

 nerdzilla says:  Mar 23, 2008. 9:32 AM  REPLYIs it possible to have mini "breakers" instead of fuse(s)?

 gull says:  Nov 18, 2008. 2:27 PM  REPLYIf you use breakers make sure they're rated for DC. It's much harder to interrupt DC than AC, so DC breakers are "beefier" for the same currentlevel.

 nerdzilla says:  Nov 18, 2008. 3:10 PM  REPLYI know eh? Hey, I found a digital oscilloscope AND waveform generator for $200.

 n0ukf says:  Mar 23, 2008. 12:16 PM  REPLYAnything's possible if you can find a parts source for it.

 Gunsul says:  Jan 22, 2007. 4:07 AM  REPLYI'm wanting to power a car amp (don't ask) that has a 12V 40A peak rating. Can you see any issues with hooking two supplies in parallel to double thepower?

 n3ldan says:  Apr 3, 2008. 8:45 PM  REPLYum, I know this is a year old comment, but in case anyone else wants to do it you should just use diodes to ensure current would only flow out of thepower supplies

 navaburo says:  May 10, 2007. 10:55 AM  REPLYcareful! any difference in power supply voltages can result in current flowing t ough the smaller supply in reverse, which may have unexpected results!

the safe thing to do is get a dedicated supply of the correct rating.

 mat_the_w says:  May 10, 2007. 1:10 PM  REPLYAh, good point!

 thermoelectric says:  Oct 29, 2008. 11:43 PM  REPLYI put two supplies in series, trying to get 24v and the mains fuse in the smaller psu blew.....

http://www.instructables.com/id/ATX--%3e-Lab-Bench-Power-Supply-Conversion/

 mat_the_w says:  Jan 31, 2007. 10:01 PM  REPLYThe only think that I can think of is to ensure that the power supplies have the same ground connection, so you don't have any grounding loop issues.IANAEE (I am not an Electrical Engineer (yet :-) )), so be careful. You might want to look into a more dedicated power supply, for that much current.These are really more of a generic purpose, lighter-weight power supply that can be recycled from old computers.

 shaunak says:  Feb 18, 2008. 6:40 AM  REPLYNice,Its exactly what I needed!5v's for those pesky TTL IC's and +/-12 for Op-Amps!

 Mak5 says:  Feb 5, 2008. 9:18 PM  REPLYWorking on this project. I can only find one wire with each 3.3V 5V and 12V should there be more of each or just one? Very cool prject by the way! thanks

 mat_the_w says:  Feb 7, 2008. 7:32 AM  REPLYThis was a smaller power supply, with only a few connectors of each type. That being said, there were probably about 6 of each of the major wires (5, 12,GND) overall. Each cable running to a hard drive or floppy drive will have both 5 and 12 volt lines.

 Mak5 says:  Feb 7, 2008. 5:39 PM  REPLYwhy use such a large resistor as your load? I may be using ohms law incorrectly but I'v got a 220 ohm resistor across the 5v which should be pulling.113 amps. the supply seems to be staying on and I dont have a ton of excess heat. Please sir do explain.

 mat_the_w says:  Feb 7, 2008. 11:07 PM  REPLY"such a large resistor"? I'm only using a 10 ohm, 10 watt resistor to provide a constant load. Perhaps you mean "such a small resistor, resulting ina large current". As noted before, 10 ohms is what I had on hand, and I have no idea the minimum current required to keep the PS in the "on"mode. It probably varies between power supplies.

As for your calculations of current, if my V=IR doesn't fail me, we also know that I = V/R, so I = 5/220 = 0.0227 amps.

In conclusion, if you can get your PS to work properly with a larger resistor (smaller load current) then that's awesome. Just be sure that you don'texceed the power rating of your resistor (V*I). For 5v with a 220 ohm resistor, you've got 0.113 watts, which is definitely smaller than a 1/8th wattresistor can handle, so you should be ok, as 1/8=0.125.

 taylorasdfjkl says:  Aug 28, 2007. 11:41 PM  REPLYI tried this and it works great!I just have one question.. Is it normal for the metal casing of the power supply to be connected to ground, or did I short something? I have verified this with acontinuity tester, and I'm going to hold off on using the psu until I find out if this is normal.

 drakoes says:  Dec 30, 2007. 10:09 PM  REPLYI think this is normal. The reason being this. If for some reason you had the hot wire of the AC somehow achieve connection with the power suppliesmetal casing you could be killed by touching it if the box wasn't grounded. Thus by having it ground if something were to go wrong it would just blow theground fault protection circuit on the main of where ever you have the sucker plugged in.

Also, computer power supplies are grounded for an aditional reason, static discharge. By having the metal case of the psu grounded when you put it intoyour computer your computer case is now grounded and this will keep it from possibly gathering any static that could cause problems.

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