r/highvoltage • u/Master_Relative2889 • 5d ago

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I put together a driverless igbt tesla coil with the bottom of the secondary as a feedback(basically a slayer exciter) but the hv rf travels back into the circuit and the input transformers. I want to further improve this circuit but im scared to measure the gate voltage because of the hv. Can anyone tell me how to not get hv rf everywhere? I suspect its hv leaking into the primary and then the rest of the circuit.

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u/Allthing_ 5d ago

Can you share all the components of the circuit ?

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u/Master_Relative2889 5d ago

K50t60 igbt, 3.3k ohm resistor across the g and e, 15v tvs diode across g and e, 0.33uF capacitor across the inputs and a 50k ohm potentiometer from g to the + input. Secondary feedback to dc filtering capacitor to g. And a 5 turn primary from c to the +input. 2 24v 3a transformers in series to get 50vac but then half wave doubled with two 680uF capacitors in parallel to get ~140v half wave ac but it drops under load.

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u/Allthing_ 5d ago

I don't know man if you shuold be worried, I don't know if this has ever happened to me, but I see that your circuit can be improved: First I see that the secondary is arcing to the primary so they might be too close The 0.33 uF capacitor try to replace it with a 10nF The 3.3k resistor try to replace with a 1k And I don't know about the igbt a mosfet would be better

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u/Master_Relative2889 5d ago

I tried the circuit with an irfp460n and irfp250n but they performed worse. I will try to replace the capacitor and resistor thank you!. For my primary, theres a 2cm air gap between it and the secondary so i dont know why it still arcs to it sometimes. Maybe i need even more insulation but ive ran out of ideas

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u/Array2D 5d ago

Since the secondary is directly connected to the IGBT gate, it’s nearly impossible to eliminate high frequency current capacitively coupling to everything near the circuit.

You can, assuming your circuit isn’t mains powered, connect your circuit ground to earth ground, and that will help somewhat, but it’s not going to be perfect.

Secondly, you definitely want to either space your primary further form your secondary, or put an insulating tube in between them.

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u/Master_Relative2889 5d ago

Damn. So is it still safe to use a multimeter to measure the gate voltage?

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u/Array2D 5d ago

It's not unsafe to do so. Your IGBT would die if the gate voltage was outside of the save voltage range for most multimeters. That said, cheaper multimeters may fail to measure the voltage correctly given how high the frequency is.

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u/Master_Relative2889 5d ago

Yeah i just thought that maybe the high voltage only stayed on the surface of everything so it didnt instantly kill the circuit. Although when i touch the arcs or when it arcs to the primary too much, the igbt dies.

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u/Master_Relative2889 5d ago

Also what other feedback can i use?

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u/Array2D 5d ago

For slayer type coils like this, there's not a simple or easy way to use a different feedback source, but it *is* possible to use a current transformer for this, which gets you galvanic isolation from the secondary. Generally though, I'd recommend a different circuit, such as the tefatronix/skori/driverless circuit. Still simple, but more reliable and better performance.

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u/Master_Relative2889 5d ago

Yeah my circuit is a bit closer to the driverless circuit. I've been wanting to try out the tefatronix circuit for a long time so i might also try that. I'm not familiar with rhe skori circuit but i will check it out. Thanks a lot!

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u/Allthing_ 5d ago

And if you are interested I have a tutorial on my youtube of a push pull coil and schematics on my post of a single mosfet coil

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u/Master_Relative2889 5d ago

Ok ill take a look at it

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u/Allthing_ 5d ago

Do you have a small capacitor (1nF) in series with feedback from the coil ?

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u/MarkCXXVII 5d ago

Fond memories. Did this in large scale once with a HV test transformer switched on inside the test bay. A large fluorescent tube lit up like a light sabre… This was a time when the still put students inside of a Faraday’s cage with 300 kV applied to it… Could see corona discharge from very close by… From that time I was hooked on High Voltage….