Tesla coil problem (resonant transformer)

[soare_cristian16]

Tesla coil (audio jack controlled) problem (resonant transformer)

Hello !
For some time I've been trying to make a Tesla coil. Since I didn't have a neon sign transformer, I decided to go on the solid state way.

I've made this schematic: **broken link removed**

The AC signal on the primary circuit is generated by the audio jack from my PC (I've written a program which can give me a sinusoidal / square wave at any frequency). I'm using an old pc speaker amplifier to get the signal powerful enough to operate the mosfet and it's working well (I've tested it with an led). This was my attempt in replacing the old capacitor + spark gap solution with an already tuned AC signal. I hope it isn't a bad idea.

Specifications:
-secondary coil: 2000 turns 0.1 mm, base diametre: 5 cm, L~ 44.8 mH
-C1 is a set of 5 ceramic capacitors in series (each 3kV, 10 nF). The set should work as a 15 kV 2 nF rated capactor. I use this set to bring the coil's resonant frequency at about 17 khz (with a 44100 hz soundcard it gets harder to go above 20 khz)
-primary coil: 3 turns 2 mm
-source: ATX 12 V, 18A (the coil should give me about 8 kV)
-mosfet/power transistor: IRFP260NPBF N channel

After building the coil, I tested it ....and nothing happened... (except the heating of the mosfet)

Could you please help me with some advice ?

If it eventually works, I will be able to modify the signal to generate musical tones. I've started this project to help musical Tesla coil enthusiasts to get the job done without having to know any PIC programming or other alternatives.

Thank you a lot in advance for helping.

 

[WimRFP]

Hello,

Your capacitor is hanging into the air, so you put 2nF in series with some pF. Therefore the resonant frequency of you secondary will be far higher.

Your mosfet with primary is not the best solution. Your mosfet has to dissipate the inductive kick from the primary coil. It would be better if you would make a half bridge inverter scheme. Then the inductive kick is supplied back to the power supply (via the body diode of the high side switching mosfet). There are various half bridge driver chips available that will save you from designing the driver circuitry.

If you have this functioning, you should make your frequency variable and go DOWN in frequency very slowly (from above the resonant frequency) untill you see a rise in supply current. If the supply current stops increasing, don't go further down in frequency as this will result in increased mosfet dissipation. By staying at or slightly above the self-resonant frequency, your power stage operates in a zero voltage switching mode (ZVS mode), reducing switching loss significantly.

You have to play with the coupling between prim. and sec. to find the sweet spot. There is lots of tesla coilers stuff on the web that may be helpful.

 

[soare_cristian16]

Thank you so much for your reply !

If I remove the toroid / sphere, what can I replace it with ?

I'll start looking for the half bridge inverter. Still, if I place a diode before the primary coil will it give this mosfet solution a chance to work ?

Also, are 12 V and 18 A enough to power the primary (and to obtain at least 1-2 cm spark on the secondary) ? I know that usual Tesla coils are powered by thousands of volts, but I saw some solid states powered by just 12 V.

Again, thank you very much for your reply.