I'm trying to build a cool tesla coil that I found online. However, before I design the primary coil, I need to figure out the resonant frequency of the coil with/without the top-load so that I can calculate the total LC impedance (which must be a little above 6 ohms) so that the IGBT's don't get destroyed, and also the sparks will not be weak. I need to use a frequency generator to determine the resonant frequency, but I don't have one. I do have an oscilloscope.

I made a secondary coil that has between 1,025 and 1,050 turns (approximately), is 11.5 inches long (approximately), 4.5 inches wide ( I used an acrylic pipe with an outer diameter of 4.5 inches), and with what I think is 30 AWG wire. The resonant frequency should be below 320kH
(I used online calculators).

My electronics knowledge is approximately intermediate.

So my questions are this...

No.1 - Do I need a frequency generator to find the resonant frequency?

No.2 - If so, does it have to be a $300 or more frequency generator, or can I just buy cheap $30 one?

No.3 - Also, does it need to be a DC signal, an AC signal, or can it be either?



Here is the link for the guide on it------ Building the Ultimate Solid State Tesla Coil (Music-Capable!) | a ...
If the link didn't work, I want to say this is the first time I've tried to upload a link on a computer...

 

the total LC impedance (which must be a little above 6 ohms) so that the IGBT's don't get destroyed

Do not worry, measure resistance of your coil by DMM.
It should be much more than 6 Ω.

Do I need a frequency generator to find the resonant frequency?

Assemble simple test circuit with 2N2222, using your coil.
It will oscillate on resonant frequency, which you can measure by oscilloscope.

Also you can calculate L and f_resonant with help of program Coil32.
https://coil32.net/

 

Thank you for your response.

 

You could even use a 555 pulse output to find the resonance frequency. But you will need to use the scope to ready the frequency. The oscillator would work, but getting the feedback correct might be difficult. That is why I suggested a 555: No feedback required.

 

Thank you, I'll try that!

 

Now that I see the circuit I need to give a warning, which is that this is either the same circuit, or very similar to a circuit that another thread starter had been struggling with for a long time. The reason that the circuit has problems is that it tries to get feedback for the oscillator from a capacitive pickup from the coil.
THAT IS A VERY POOR CHOICE TO EVEN CONSIDER.
An actual TESLA COIL that uses an actual oscillator uses a second coil to get the feedback signal. If you look at the circuit in the link, it uses a goofy antenna system to feed a clamping circuit to feed a driver IC to drive the two transistors. That is not much better than the so-named "Slayer"circuit that provides a lot of beginners with a lot of frustration.
Thank You for providing the link to that manual. Consider that the circuit that you have uses a 555 pulse generator to attempt to get the operation started, and then it tries to use that clamped antenna pickup scheme to move it to the resonant frequency.
It will work much better to use a feedback coil that will provide the drive at the resonant frequency . You may need to provide a way to control the feedback signal amplitude, though.

 

Thank you for pointing that out. I will keep that in mind.

 

Test No.1
I tried a 555 timer circuit to find the resonant frequency. However, when I use a nine volt battery I get a resonant frequency of approximately 280 kilohertz ( with the top-load ) or a resonant frequency of approximately 337 kilohertz ( without the top-load ).
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Test No.2
Then I powered the circuit with my variable power supply and gave the circuit 15 volts instead of nine volts and got a resonant frequency of approximately 222 kilohertz ( with the top-load ) or a resonant frequency of approximately 285 kilohertz
( without the top-load ). I also grounded the metal part of the potentiometer with an alligator jumper to the ground connection of my power supply during every test.
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Test No.3
Then I finally tried nine volts from my variable power supply and got a resonant frequency of approximately 283 kilohertz ( without the top-load ) or a resonant frequency of approximately 224 kilohertz ( with the top-load ).
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Which test should I trust?

Here is the link to the video I got the schematic from....

1:20
555 Tesla Coil "Tuner" + Schematics

 

OK, and very interesting. Now, while it does not show the method of driving the coil, that is certainly how I would suggest determining it on a scope. Thank you for the demonstration.

So can you describe how the signal is applied to the coil? That is what is unknown at this point. If it is just using the coil as an antenna, and not as a resonant element, that would explain the variations. If the drive signal is not inductively driving it, by means of another coil acting like a transformer primary, then it is not really an adequate resonance test.

 

I connect one end of the coil to the other end of the two LEDs ( one in reverse and in parallel to the other LED ) so that the two LEDs are in between the output of the circuit. Then I just point the oscilloscope probe to the coil at some distance.

Yes I think you are right. I think it does use the coil as an antenna so that at the correct frequency it will transmit energy most efficiently. Thank you for taking the time to answer me.

 

YES!!! YOU ARE MOST ABSOLUTELY, POSITIVELY, CORRECT!!!! The giant secondary coil I'm going to use was picking up the frequencies coming from my variable power supply whenever I used it.

Thank you very much.

 

Can you, instead, make a simple coil of a few turns of wire in a loop that will easily fit loosely around the base of that big coil? That should have the coils working like a Tesla coil, but maybe low power.

 

So I would use the 555 timer circuit to drive the "primary coil" instead?

 

That may work. Or whatever you were using that was connected to the end of the coil. Certainly try what yiu had been using first. You know what frequencies it can supply. and it is already on hand.

 

Yesterday I got the final part for the circuit. So I went ahead and built the tesla coil only to find out that the secondary coil ended up arcing to the primary coil and damaged the secondary coil, so now I need to make a new coil.

How do I protect the secondary coil from arcing to the primary coil? I've got some spray-on lacquer. Would that help?

 

Typically on many Tesla Coils the primary coil is much closer to the grounded end of the main coil. And also, usually, there is a significant air gap between the primary and the secondary.

Is your coil system wired like the one in the link in post #1?? I see one serious flaw, which is that no information is provided about the secondary relative to the primary. Mostly, one side of the primary would be tied to the same ground as the end of the secondary , and the primary would be positioned far toward the grounded end of the secondary. That will remove the large voltage difference that caused the arcing.

So please describe the relative positions of the two coils.
AND let us know if you used that circuit from the link, or some other circuit.

 

The primary coil was about half an inch to three-quarters of an inch in diameter larger than the secondary coil.
I made a new primary coil that is approximately 7 inches.
I did try to copy the circuit in the video.
I tried to run it with the new primary setup, but after discharging the voltage doubler it now keeps tripping the breaker whenever I try to use it.

 

I also was able to salvage the secondary coil.

 

I repaired the circuit and tried to power it up, and it tripped the breaker again...
I think it's the gate drive transformer, or maybe the primary coil is too big.

 

If the circuit breaker is tripping then the current is excessive for that circuit breaker. So the solution is to reduce the current, which can be done by reducing the voltage supply feeding thru the circuit breaker. Since I have not looked at the circuit I do not know the voltage. But reducing the voltage will reduce the current.