onetesla tuning guide draft

8/10/2019 Onetesla Tuning Guide Draft

1/11

Tuning your oneTeslaDraft version 2013-11-19, Stefan Gustavson ([email protected]).

Pulis!e" un"er ## $%-S& license (see creativecommons.org for "etails).

The oneTesla kit was designed to be a manageable project for relatively inexperienced hobbyists. Ifassembled correctly, it should perform well and give you long, beautiful sparks without any extra

adjustments. However, the performance of a Tesla coil is very much dependent on its tuning, and

due to component tolerances and small details in your assembly of the kit, it is quite possible that

your particular coil is not as well tuned as it could be, and that it could perform even better. tuned

coil gives longer sparks and a louder sound, draws less power and runs cooler, thereby reducing the

risk of component failures. coil that is very badly out of tune will produce weak sparks and might

break down on higher power settings. This guide is meant to teach you how to detect tuning

problems in your oneTesla, and how to bring it into tune.

Some 'eo'le !ave !a" 'rolems it! secon"ar coils t!at ere acci"entall oun" it! t!e

incorrect numer of turns, !ic! results in more severe tuning 'rolems an" com'onent failures

even at mo"est 'oer settings. *!is gui"e is meant to !el' also in t!ose circumstances.

Tuning?

In any dual resonant Tesla coil, including the solid state variety of which the oneTesla is an

example, there are two !" resonant circuits involved in creating the high tension sparks# the

primary circuit and the secondary circuit. n !" resonant circuit consists of an inductor $!% and a

capacitor $"% connected either in series or in parallel. Its resonant frequency f is easily computed

from the inductance + and the capacitance # according to#

f= &

'(+#)(or a Tesla coil to be well tuned, the primary and secondary circuits should resonate at the same

frequency, or at least almost the same. This is paramount to performance.

How does it work?

In order to understand why tuning is so important, it is helpful to understand how a Tesla coil

works. (%ou "o not nee" to un"erstan" t!is section, or even rea" it, to "etect an" fi tuning

'rolems, ut e elieve t!at t!e est tos are also e"ucational.)

The oneTesla is an interrupted coil, which means that it is not creating a continuous spark, but rapid

successions of sparks in short bursts, controlled by the interrupter. This is what makes it able to emit

a tone of a specific pitch. )uring the *on+ time for each pulse from the interrupter, the output

transistors $insulated gate bipolar transistors, or I-Ts for short% are driving the primary circuit by

means of a feedback circuit that adapts to the primary resonance frequency. ith each cycle, more

electrical energy is *pumped+ into the primary circuit, gradually increasing its resonance amplitude.

This energy will then transfer to the secondary, but the magnetic coupling between the primary and

secondary coils is weak by design, unlike a regular transformer, were you want strong coupling.

This means that the energy transfer will not happen instantaneously, but over time across several

cycles. In this manner, the secondary gets pumped by the primary, and increases its resonance

amplitude $*rings up+%, while the primary is active. /ome power is lost to the spark when it breaks

out, and there is a maximum voltage you can reach both in the primary and the secondary, partly

because of thermal and magnetic losses, but also because a pumped0up secondary will starttransferring some of its stored energy back to the primary. However, the main limitation of spark

power during normal operation of the oneTesla is the time you allow the primary to ring up. If you


2/11

make that time too long, the device will actually suffer a catastrophic breakdown. The primary

circuit, and the I-T bridge in particular, was designed for an interrupted Tesla coil, and the

components are not rated to handle continuous operation at full mains voltage. If you were to try

continous operation, the current in the primary coil would become too high and fry the I-Ts

within milliseconds. "ontinuous operation is possible at a much reduced " supply voltage from a

variac, like 12 volts or so, or by feeding the bus capacitors directly from a )" power supply, but

then the output voltage from the secondary will be reduced accordingly, and the spark will be short,faint and not very loud.

The secondary oscillations will tend to follow the oscillations of the primary while it is being

pumped by it, but when left alone it will resonate at its own preferred frequency. If the two are

significantly different, the secondary will not respond well to being pumped by the primary

frequency, and the energy transfer will be inefficient. 3erfect tuning is definitely best, but rather

hard to accomplish, because the secondary resonance frequency varies quite significantly with the

spark length. slight detuning is 45 and will only make a small difference. ith stronger

detuning, the secondary will respond well to the first few cycles of the primary, but after a while,

when the secondary has had more energy transferred to it, the secondary self0resonance will drift

out of phase with the primary, and the energy transfer will stop. serious detuning will make the

secondary run out of phase sooner, making it fight against the primary oscillations for some part of

the cycle and load the primary circuit down instead of sucking up the energy from it.

"onversely, if the secondary is well tuned, the energy transfer from the primary will be efficient for

the entire duration of the primary oscillations, making the secondary ring up to a high voltage while

only putting a minimal load on the primary. The maximum output voltage from a well tuned Tesla

coil is much higher than what you could get from an ordinary, non0resonant transformer circuit with

the same ratio of turns between the primary and secondary windings.

Deliberate detuning

6ou generally don7t want to hit perfect tuning $exactly the same frequency for the primary and thesecondary% at lowest power, because then you will be out of tune at high power. This is because a

longer spark, or *streamer+ in coiler0speak, adds some to the top load capacitance and influences the

resonance frequency. This is called *streamer loading+. Therefore, after you have found the perfect

tuning on low power, you will need to retune the coil some more to perform its best at higher power.

8xperimental evidence from the oneTesla forum seems to indicate that the oneTesla, being a small

coil that throws big sparks, needs to have the primary frequency tuned significantly lower than the

secondary frequency for best performance. good rule of thumb seems to be that you should aim

for a primary frequency that is &9: to '2: lower than secondary frequency. 3opular online

resources for Tesla coils in general usually state this number as somewhere around 9: to &2:, but

the oneTesla is an unusually strong performer for its si;e, and needs a larger margin for the streamer

loading.

strong, full length streamer from the oneTesla seems to reduce the secondary resonance frequency

by up to '2: from the frequency at lowest power. t half power setting on the interrupter, the

difference is more like &2: from the low power state. Thus, if you want your coil to give good

results at full power, you will run it slightly out of tune at half power, and vice versa.


3/11

Measuring the resonance frequencies

There are many ways of measuring the resonance frequencies of your primary and secondary, some

direct and some indirect. The most convenient ways require you to have access to an oscilloscope.

6ou can manage without one, but let7s start by explaining how to do it with an oscillosope.

Using an oscilloscope and a running coil correctly built oneTesla will create a small spark from the breakout point even on the lowest

power setting. If the coil is not well tuned, the spark might be weak, almost invisible and not very

loud, but there should definitely be a spark. =sing an oscilloscope, you can measure the oscillating

electrical field from the top load by holding or hanging the probe somewhere in the air a few meters

from the running coil. /et the time base to '2 us or &2 us>div, and adjust the sensitivity of the scope

to have a trace that looks similar to the image below.

(image: a typical trace of a oneTesla with some detuning issues)

4n this trace, you can notice detuning problems in several ways, the most direct being a difference

in frequency between the first and the second part of the waveform. healthy, tuned coil will show

the same frequency across the entire trace.(image: a trace from a oneTesla with good tuning)

hen you have a trace like the ones shown above, you can count and measure the pulses in the

leading part and the trailing part of the waveform, and calculate the frequency for each part. ?ost

digital oscilloscopes have built0in tools in the user interface to measure a time interval. /et the

cursors to coincide with two peaks or two ;ero crossings in the ringing0up or ringing0down part of

the trace, respectively, read the time interval, and divide by the number of cycles between the peaks

to get the period * . 6our corresponding resonance frequency is &/* .

If you have an analog oscilloscope, you can get a very reliable measurement by snapping a straight

head0on, high resolution picture of the trace with a digital camera and count pixel distances in an

image editing program, as shown in the image below. ?easuring this directly on the display is alsopossible, but it gives you a less accurate reading.


4/11

Using an oscilloscope and a signal generator without running the coil

6ou can also measure the primary and the secondary resonance without powering up your oneTesla,

but then you need both an oscilloscope and a signal generator capable of outputting a signal with a

variable frequency up to & ?H;, or at least in the range '22 kH; to 192 kH;. The signal does not

have to be sinus shaped @ a square wave from the digital output of a simple timer circuit will do just

fine.

TO DO: include or link to instructions on how to build a 555 timer circuit to test resonance.

If your signal generator does not have a built0in accurate reading of its currently set frequency, a

frequency counter or an oscilloscope with built0in frequency measurement is a big help. 6ou can

count cycles and measure distances on an analog scope as shown in the previous section, but it7s not

nearly as convenient.

Primary

To measure the primary resonance, disconnect your oneTesla from all power sources. !ocate the

primary lead that is farthest from the tank capacitor and disconnect it from its screw terminal.

"onnect a short and straight wire from the tank capacitor lead that is closest to the corner of theboard to the primary lead you just detached. Then, connect your oscilloscope across the tank

capacitor. "onnect the signal generator to the same points, but through a &k resistor. schematic is

presented below.

/tart the signal generator at around '92 kH; and adjust the oscilloscope until you see a clear trace.

Then, adjust the frequency until you find the maximum amplitude for the trace. This peak amplitude

happens at the resonance frequency. The signal is not very strong, and the peak is not very

pronounced, but with some care, you should be able to determine the resonance frequency with

good accuracy. good digital oscilloscope with amplitude and frequency readout helps a lot here,

but an analog scope will do.


5/11

Secondary

There are several ways to measure the secondary resonance, but we recommend the following

method for its accuracy. )isconnect the primary coil from its terminals, and connect your signal

generator to the primary coil, possibly through a small resistor of about &22 ohms if your signal

generator is not comfortable with a short circuit across its output. "onnect the bottom of the

secondary coil to a good, low impedance ground or to a counterpoise $a sheet of metal, like

aluminium foil, about & m square%, and place the coil in the same kind of environment as during

operation, keeping it reasonably far away from the ceiling, walls and other objects. Hang the

oscilloscope probe still in a position a few feet away from the top load, and set the amplitude scale

of the oscilloscope to somewhere in the mA>div range to detect weak signals. /tart your signal

generator and adjust its frequency. hen you hit the secondary resonance frequency, you should see

a strong increase in the amplitude of the field that is emitted from the top load through the air to

your oscilloscope probe. This peak is a lot more pronounced than the peak you detected from the

primary circuit.

Measuring without an oscilloscope

Link to the online schematics and instructions for the circuit with the 555 timer and the LD

indicator. Does it work! " ha#e no e$perience with it. %eed to build it and test it& or insert a

disclaimer here saying 'untested'. econdary measurement should work. rimary is more

doubtful.

"s it possible to measure the peak with a multimeter capable of measuring *+ #oltage! The *+#oltages in#ol#ed are small& but at least for the secondary measurement it might be enough.

,uild an instrument amplifier with a simple op-amp to measure small *+ #oltages! ould be

ine$pensi#e& but not dead simple.

Hand tuning

If you do not have access to an oscilloscope and do not feel like assembling the home0brew signal

generator mentioned in the section above, you can tune your oneTesla using nothing but your eyes

and ears and some experimentation. 6ou will not be running blind, but you will be running with a

sort of tunnel vision, so an oscilloscope really makes things a lot more convenient. However, it is

perfectly possible to tune a Tesla coil without any extra equipment.-ad tuning manifests itself as poor performance of the coil# the sound is not as loud as expected,

and the sparks are shorter than expected. =se the gallery at the end of this document for reference.


6/11

8xactly how to tune your coil is presented in the next section. -elow, we just give you a hint on

how to tune without access to measuring equipment. The adjustments and modifications you need

to make are the same as described in the section *etting into tune+.

ithout any frequency measurements for reference, it is impossible to know which way to tune

your coil, but try a small adjustment in one direction and see if the situation improves. If it doesn7t,

try the other way. If things do not improve nor get any worse either way, try a somewhat larger

adjustment. "ontinue making small adjustments until you reach the optimal performance. It is wiseto do this on low power, around one0third setting on the power knob, or else you risk destroying

something. 6ou can also try it on minimum power to be absolutely sure that nothing bad happens.

(or reference, you should know that when the oneTesla is well tuned at lowest power $a situation

you generally don7t want, see the section *)eliberate detuning+ above%, the sound is a little too loud

for comfort without ear protection when you are standing close to it.

Tuning with an AC current meter

s strange as it sounds, you can also tune a Tesla coil by hooking up an " current meter to the

mains power line and adjust the tuning until the current draw is as small as possible for a certain

power setting on the interrupter. It may seem a bit counter0intuitive that a stronger spark is producedwhen the power input is smaller, but that is how it works. The power losses to heat when the tuning

is bad have a stronger influence on the total mains current than the extra power that gets pushed into

the spark when you hit perfect tuning.


7/11

Getting into tune

?easure your resonance frequencies on minimum power, or on an unpowered coil using a signal

generator and an oscilloscope. If you are lucky, your primary frequency is about &9: lower than

your secondary frequency. In that case, you7re basically fine. 6our coil does not need any strong

tuning adjustments. The reason for having different frequencies for the primary and secondary is

that the secondary frequency will shift down somewhat during operation when there is a long spark

present. That spark is a conductor that will add to the capacitance of the top load and reduce the

resonance frequency by about &9:. /ee the section *)eliberate detuning+ above.

If you want absolutely top performance from your coil, you should measure the frequencies during

operation at full power, and aim for perfect tuning when there is a strong spark present. To measure

the resonance frequency during operation at higher power, hang the oscilloscope probe somewhere

a few meters from your coil. round the ground lead but keep the probe tip unconnected, and you

should be able to get a trace of the radiated electrical field, similar to what you saw earlier in this

guide for measuring the secondary resonance, only you will see a much stronger signal.

4n longer pulse widths $higher power settings on the interrupter%, you will see slight detuning

issues as an uneven amplitude in the *on+ part of the trace. This happens because the primary andsecondary circuits drift in and out of phase with each other, and when they are out of phase, some of

the energy in the secondary is transferred back into the primary. -y seasoned coilers, this is referred

to as *bucking+. s long as the bucking is small and only creates small variations in amplitude, like

in the plot below, it7s not a big issue, but if it gets stronger, it can make the primary coil run hot to

the point of smoking, and in extreme cases it will destroy the I-Ts.

(image: * trace where bucking is clearly #isible& but not strong enough to be a big problem)

(possible additional image: * trace of a long pulse without any bucking 0 if " can make one)

(possible additional image: * trace where bucking is strong enough to be a big problem)

relevant measure of your amount of detuning is the secondary frequency divided by the primary

frequency. 6ou want this ratio to be somewhere in the range &.& to &.'. If it is greater than &.' or

smaller than &.&, you are out of the *comfort ;one+ and should consider improving the performance

and stability of your coil by tuning it. If the ratio is as high as &.'9 or &.1 or more, you will destroy

your coil if you run it at full power, and in that case you must perform a tuning before you crank up

the power setting on your interrupter. If the ratio is lower than &.&, you will see good performance at

lower power, but a bad performance at higher power, and even in this case there is a risk of

destroying the coil by pushing it to higher power.

Adjusting the primary resonance

(rom above, recall the formula for the resonance frequency of an !" circuit#

f=&

'(+#)

The primary resonance circuit can be tuned in two ways# change the capacitance, or change the

inductance. In the oneTesla design, the value of the tank capacitor cannot be varied easily and

freely. If you happen to have access to a large stash of such capacitors, you could probably fix slight

detuning issues by using a capacitance meter to find one that has just the right value, but that is out

of reach for most people. The primary coil, however, is relatively easy to change. If your primary

has a too low resonance frequency, you should decrease the primary inductance, and if the

frequency is too high, you should increase the inductance. Increasing the inductance can be

performed by adding more turns to the default six turns. The inductance of a short coil like the

oneTesla primary is proportional to the number of turns squared, and inversely proportional toa weighted sum of its length l and diameter " #


8/11

+

'

l+2.B "

There is also some stray inductance in the connecting wires leading up to the coil, but that is not a

big contribution to the overall inductance of the primary.

The inductance of the standard C turn primary with the wire wound tightly $minimum length of the

coil, about &D mm% is approximately E FH. /preading the windings out to twice that length reduces

the inductance to about 9 FH.

Geducing the coil to 9 turns over &' mm length gives an inductance of about D. FH, and spreading

that 9 turn coil out to 12 mm length reduces the inductance further to around 1.9 FH.

"onversely, adding one turn and packing the E turns tightly together increases the inductance to B

FH, and spreading the E turns out over 12 mm reduces it to about C FH.

-ecause the resonance frequency is inversely proportional to the square root of the inductance, the

nominal resonance frequency for the C turn tight coil of around '1C kH; can be increased to 112

kH; for a widely spread 9 turn coil, and decreased to '22 kH; for a tightly wound E turn coil. This

should be a more than adequate tuning range to match even a very problematic secondary. The table

below summari;es this. reen cells represent a tightly wound coil. Ged cells represent coils that

would require thinner wire than the default to fit across the specified length. 6ellow cells represent a

more or less spread0out winding.

The process of changing the resonance frequency by expanding and contracting the winding is

called *scrunch tuning+. This is not quite as convenient as a commonly used method on larger Tesla

coils, where a movable tap on an uninsulated primary makes it possible to tap the primary coil at,

say 9.E turns, but the insulated primary coil on the oneTesla is a simpler and more safe design, and

it makes the coil less likely to create arcs between the secondary and the primary.

Adjusting the secondary resonance

The secondary coil of a oneTesla kit comes pre0manufactured and is cumbersome to rewind, but it7s

easier to change the capacitance of the top load. /light adjustments can be made by making thebreakout rod a little shorter or longer. This will change the tuning only slightly, but it is a good way

to do fine tuning for absolutely maximum performance.

(or stronger re0tuning, you should consider adjusting your primary. However, it is possible to adjust

the secondary by using a smaller or larger toroid for the top load, or adding a second toroid on top

of the first. The stamped toroid that comes with the oneTesla kit is a large part of its good looks, so

replacing it is not the recommended way of tuning a oneTesla, but it can be done. ny custom top

load you make should have a smooth, softly rounded surface, or else you will have sparks breaking

out all over the place. It should also be symmetrical about the central axis of the secondary.

toroidal shape is best, for various reasons, but a sphere, or something reasonably close to a spherical

shape, will work too.

The capacitance of a toroid can be computed with this excellent online resource#

http#>>deepfriedneon.com>teslafcalctoroid.html

10 12 14 16 18 20 24 28 32 40

5 4.97 4.80 4.64 4.49 4.35 4.21 3.97 3.76 3.56 3.23

6 6.91 6.68 6.46 6.26 6.07 5.72 5.41 5.13 4.65

7 9.09 8.80 8.52 8.26 7.78 7.36 6.98 6.33

278.6 283.4 288.1 292.7 297.3 306.3 314.9 323.4 339.7kHz

236.1 240.1 243.9 247.8 255.2 262.5 269.5 283.1

205.8 209.1 212.4 218.8 225.0 231.0 242.6

Coil lengt !mm"

#$m%er o& t$rn'

$H

(re)$ency *it 68 n(

5 t$rn'

6 t$rn'

7 t$rn'
http://deepfriedneon.com/tesla_f_calctoroid.htmlhttp://deepfriedneon.com/tesla_f_calctoroid.html


9/11

The formula is a little bit hairy, even though it7s only an approximation, but it goes like this#

#=&(&.'E&D'

D&)

' '(D&D')D'

'

D$p(%

where D&

and D'

are the major and minor diameters of the toroid, respectively. .&=E.&&

if D& and D' are measured in centimeters, and &='. if measurements are in inches. Thestamped aluminum toroid has )&J'22 mm $K% and )'JD9 mm $&.K%, which gives a capacitance of

.C p(.

The capacitance of a sphere is a lot more simple to compute# #=.'/ $p(%, where / is the

radius of the sphere, and .'=&.&& if / is in centimeters, and '=2.D1 if / is in inches.

combination of toroids and spheres is also fine. If you want to be creative, go ahead.


10/11

A gallery of sparks

!ast, we present a visual gallery of sparks from a well tuned oneTesla coil. This is the kind of

performance you should expect from a device that is properly tuned. Tesla coils are individuals, and

your coil might not perform exactly like the one we used to make this gallery, but if the results from

your coil are significantly smaller than what you see below, if your coil draws an abnormal power

from " mains, or if either the heatsink or the primary coil becomes hot to the touch during

operation, you need to tune your coil.

bove is the kind of image you get if you just point and shoot in dim ambient lighting using

automatic exposure mode with a typical camera# a long exposure, showing how the spark dances

around over the course of several seconds. This is not what it looks like in real life, but it7s a nice

image. In the close0up, you can see that this is an interrupted Tesla coil. To the naked eye, it looks

like a continuous streamer licking the air, but a spark is actually forming and extinguishing in a

cycle that repeats hundreds of times per second.

These are shorter $&>12 second% exposures from the same run, showing the spark fro;en in time.

This spark is '9012 cm long $&20&' inches% and was obtained in fixed frequency mode, at medium

frequency and 92: power setting on the interrupter. The current drawn from '12A " mains was

&.9 $G?/%, and the primary coil stayed cool to the touch even when the coil was left running inthis mode for several minutes. This coil was not in perfect tune $the ratio between secondary and

primary was &.&%, so the spark length did not increase significantly when the power was increased


11/11

from 92: to &22:. It got brighter and louder, but not longer, and the coil ran hot.

coil that is in perfect tune can throw some really impressive sparks up to '2 inches long at full

power, as can be seen from the images below.

onetesla tuning guide draft

Documents