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A DIY Tesla Coil Tuner
By Terry Fritz
The Tesla Coil Tuner (TCT) is a simple and low cost signal source that can be used to find the resonant frequencies of the primary and secondary circuits of Tesla coils. It uses simple commonly available parts. It can be assembled in a few hours with minimal electronic skills. The cost of all the parts is about .
The TCT is simply a LMC555 IC square wave generator. An audio taper pot and a 2% polypropylene timing capacitor control the 50% duty cycle oscillator's frequency.
A bi-color LED in series with the output senses the current being drawn and a frequency dial indicates the frequency setting.
| Qty | Parts |
| 2 | Sets red / black alligator clips |
| 1 | Large control knob |
| 1 | Bi-Color LED |
| 2 | 10uF 16V tantalum capacitors |
| 1 | LMC555 CMOS IC timer chip |
| 1 | 8 pin IC socket |
| 2 | 470 ohm 1/2 watt resistors |
| 1 | 1.8nF 50V Polypropylene 2% capacitor |
| 1 | 10K Audio taper pot with switch |
| 1 | Plastic box |
| 1 | Battery Clip |
| 1 | Battery Holder |
| 1 | Prototyping Board |
Assembly:
There are many ways to put the TCT together and it will work fine. For those less familiar with assembling things like this, I will describe how I did it.
I selected the plastic cover and located two points on the cover. The first was two inches from the bottom and the other one inch from the top. I drilled a 1/4 inch hole at the bottom mark and enlarged it a bit to fit the 10k pot. I then snapped the little tab off the pot with pliers and mounted the pot with the 2.25 x 2.25 inch scale under the nut. I then installed the knob using the off position for alignment. I connected a 470 ohm resistor to the center leg of the pot. I drilled a 3/16 hole at the top mark and was able to force the bi-color led into it. I added a bit of epoxy to hold it in place. I also epoxied the battery holder in the bottom half of the box. I drilled two 9/64 inch holes for two 8 inch lengths of wire to act as test leads in the bottom of the box. I tied and epoxied the leads to the box and installed alligator clips to the ends.
I used two other alligator clips to make a 6 inch jumper to short the spark gap for primary testing.
Circuit: Snap the two circuit boards in half and solder the 8 pin socket in the center of one. Following the schematic, solder the components to the circuit boards noting that S1, R2, R3, LED1, and the battery are mounted off the board. Use hookup wire to make the needed connections and bridge the pads with solder where needed. I put leads on the board for parts off the board.
Finish wiring the top and bottom of the box together following the schematic. See the picture for how the pot leads and switch are wired.
Install the battery and assemble the box top with the four screws.
Calibration:
The provided scale will be fairly close. However, if you have a frequency counter or voltmeter with that function, you can calibrate your own scale
Operation:
The TCT is very easy to use for primary and secondary frequency measurements. Obviously, these test should be done will all power removed from the coil and all capacitors completely discharged! The procedures follow:
Insure all power is removed from the coil and all the capacitors are completely discharged!
Testing the TCT: To tests the TCT's operation, connect the two test leads together. The LED should light and remain lit through the entire frequency range. Replace the battery if the light is dim.
Secondary Fo: To test the secondary's fundamental frequency, simply connect the TCT between the ground and the base wire from the secondary as shown below. Slowly turn the frequency through the range until the brightest spot is found. The lowest and brightest frequency spot is the fundamental. You may see the dimmer 3rd harmonic at ~3 x Fo. It is probably best to test the secondary frequency on the coil in the actual configuration since the secondary frequency is sensitive to the surrounding objects.
Primary Fo: To test the primary circuit's frequency, simply connect the TCT across the primary cap and short the spark gap with the jumper. Slowly turn the frequency through the range until the dimmest spot is found and read the frequency on the dial. You may want to remove the secondary coil to prevent the secondary from affecting this test.
The information provided here can not be guaranteed as accurate or correct. Always check with an alternate source before following any suggestions made here.
This capacitor is charged and discharged by the 555 timer to produce the output frequencies. If you replace it with a larger capacitance then your output frequencies will be lower. Replace it with a smaller capacitance and the frequency will be higher.
Also... 2 questions:
1) How do I increase the frequency range? Do I make the capacitor smaller. I measured that mine had a range of 20 kHz to 415 kHz. I would like it to be more in the range of 100 kHz to 1.5 MHz or so. I have been working with rather small coils and they resonate at higher frequencies.
2) Is this an Astable 555 Circuit? If it is why is the discharge pin not hooked up?
Thanks in advance,
Chris F.
The discharge pin is not hooked up as the circuit is configured to respond to the inductive or LC nature of the coils. The discharging of the timing capacitor depends upon the nature of the curcuit you are 'tuning' rather than it being directly discharged by the 555 timer.
Will increasing the capacitor by a factor of 10 reduce the frequencies by a factor of 10?
If you used just diodes there would be no indicator showing tuning.
Increasing the capacitance by a factor of ten will not exactly reduce the frequencies by a factor of 10 but it may be close enough. This is because the rate at which the capacitor is charged and discharges depends on other factors too.
This device was designed by Terry Fritz of hot-streamer.com. His contact email listed on his site is "terry[at]hot-streamer[dot]com"
The tuner works very well.My coil is tuned correctly for the first time..Spark length was Maximum 17 Inches. The tuner proved that my primary coil was far from right, it would not tune any where near the secondary. Rebuilt primary, tuned, Powered up and Max Spark is now 25 Inches..A very useful tool.
A frequency counter is a very good partner for the tuner,quite a few Digital Volt Meters now have frequency counters.
Also useful for checking Bat Detector Frequencies, sonic ears etc..
( 22mm dia 740mm long, 0.27 mm enamelled wire, dia . approx 260 turns.
I calculate =377 micro Henry about = 223 kHz
1.344 pF ) I only get bright LED's all the time, they only dim at about 30khz which is near the end of the pot travel.
Any ideal what I am doing wrong... this is testing just the secondary coil with out any cap connected
Secondary coil and topload - brightes spot is resonant.
Primary coil and cap - dimmest spot is resonant.
I'm not sure that there is a 'standard' copper wire. This circuit uses very low currents so quite thin wires will be fine. I would guess that the standard you refer to is something around the thickness of the individual wires in a network or telephone cable. This thickness is fine.
Yes, the simplest way to use this is to measure the f of the secondary coil and then try to adjust your secondary turns or capacitance until it also resonates at this frequency.
If the resistance is really 471 than this should make no real difference but I think you are reading the codes wrong. 471 ohms is not a standard value. The colours should read Yellow(4), Purple(7), Brown(x10).
Dear RMC,I have three small and medium size BIPOLAR TCs with equal toploads on each end of the secondary.
They are functioning OK. However, I want to find resonant frequencies by means of DIV TC Tuner. Could you please advise how to connect the Tuner to test bipolar TC. Thank you.
If the applied fequency is not correct then the LED will not light. 555 timers come in a few flavours even thuogh they are all called a 555. Some work up to just 0.1 MHz, while others will go to 1 MHz and some even go to 2 MHz. Check the manufacturers datasheet for the 555 you have to see if it it goes high enough.
It could also be that your coil/topload combo is too small for the TCT. It will work well for large coils but smaller coils can resonate well above 1 MHz which would mean that the LED would not light up.
The brightness of the LED's indicates how much current is being drawn by the load (your LC circuit). If a lot of reactance is produced by the load then the LED's will be dimmer.
To make the reading more accurate you could use a comparator. This could be set up to compare a reference voltage to the voltage you measured on the LED's. For example; if measuring an LC series circuit the comparator could be set so that it lights another LED only when the measured voltage is high enough.
i decided to do some studying instead of asking dumb questions lol but im confused about this one equation for universal capacitance and hoped you could help
c[f]=r[m] divided by 9x10 power of 9
i know how to do the equation but how is this relevant to the seconadry windings, it only meassures the capacitance ,do you know any equations that would be relevant to both the secondary and the toroidal capacitance?
by this i mean from the seconadry equations how do you know what surface area to use as a toroid. i hope my question isnt confusing thanks
You can find info on toroid capacitance here
primary LC = to seconadry / toroid in frequency wise?
Yes, electrolyitis are fine
justin,
Not sure what you are talking about. I assume N1 and N2 refer to numbers of turns, but without any reference for context the formula is useless. If you want a good answer your question needs to contain more detail.
n1 = primary number of turns
L1 = primary inducatnace
n2 = secondary turns
L2 = secondary inductance
when you divide n1 by l1 it should = n2 ,l2
And one more problem: when I don't connect the test leads to anything, the LEDs (I'm using two, facing opposite directions) go out. I assume this is right. But when I connect it to the secondary coil to test its frequency, the LEDs stay out throughout the entire testing range. Is this right?
thanks for your help, as always!
Der Strom.
Yes, the LED's will light up proportionally to the current flowing between the test leads. If it is not lighting at all, it indicates that to little current is flowing. You could try increasing the frequency by using a smaller capacitor for C3.
Sorry, I'm a little confused.
Thanks again!
-Der Strom
thanks again!
Sincerely,
Der Strom
From what you describe, I would think it is not resonating in the range of frequencies on your TCT. Try just the second or 3rd turn (closest to secondary) as this would have a significantly higher frequency than the outer turns.
Also make sure your spark gap is shorted, the secondary is removed, and you may need to disconnect your transformer.
I am working with a water fuel cell that works most effeciently when pulsed at resonant freq (ReF). High voltage and low amperage are the most effecient ingrediants, though it should work with what i am using ie, a PWM @ 12V and up to 20amps.The ReF changes as the water level changes. Would this tuner be useful in identifying the ReF if so How? It is a design by Stan Meyer. I have attached a circuit diagram in case you are unfamiliar with the device.
It should work with pulsed frequency on the primary and secondary frequency by the way transformer is connected have a double pulse thus is twice primary f. the secondary is connected in parallel or series with the water capacitor which have a low resistance. Would it resonate even with the losses of the capacitor? Does the losses on the capacitor affect the Q factor?
And I would like to know if your lc resonator would be useful for this application too.
I would have a question about impedance transformation too. Which impedance is considered in the transformation? 1 the internal resistance of the driver circuit? or 1 + 2 the reactance of primary or 1+2+3 it depends on the load???? My circuit is a car battery and a mosfet irfz44n wich have 17.5 mOhm Rds(on). What is my internal resistance?
Thank you very much
You could possibly make the measurement across your capacitive load as it would have some resonant frequency when combined with the inductors in the circuit. Remember though, this circuit wont give you a frequency, you would still need an oscilloscope to measure that.
Fabio R,
The losses in the capacitor greatly effects the Q factor. The resistance could mean that when you use the tuner, you just see the LEDs lit all the time.
would it work if it was 1.8nf at 100v
How can I tell when the whole circuit is perfectly tuned?
-thanks
The LED's will not light unless enough current flows between the test leads. If you connect he test leads together, the LED's should illuminate.
thankyou.
Alternatively you can add a lower value pot in series with the other (R3). For example; if you add a 1k pot, you can use it to make the fine adjustments with 10 times more precision than the other pot.
There will be AC becasue of the coil ringing. This is why the two opposite LED's are used. You could use a DC meter in series with a diode, then another diode or LED parallel with those. This way the meter would measure the current in one direction while it would just flow through the diode in the other direction.
Yes. C1 allows the AC ringing in the coil.
Yes. Pretty much any capacitor within ratings will be OK