Inductive spike (Tesla coil)

Thread Starter

Simranjit

Joined Apr 16, 2024
33
Hi, how can I get reduce inductive spike on drain of mosfet. The pk-pk is almost 230v lasting for 60ns. Logic circuit is running on 12v. Mosfet and primary running on 22v. I tried adding resistor in series with primary but it results in reduced performance and wasted power. My circuit is basically a slayer exciter oscillating at 87kHz with 74hc14 for sharper triggering. The mosfet is actually IRF840 (IRF540 kept on burning cause of inductive spikes). Also the heatsink of mosfet gets really hot and can't hold it for longer than 5 seconds with bare hands after a minute of use). I also tried connecting 30ETH06 as a flyback diode across primary but I barely get any corona discharge with it in circuit.

P.S. I have also implemented a phase shifter with 74hc14 but I don't really use it because output of Tesla coil is already in phase with gate of mosfet (waveforms are aligned on oscilloscope).
 

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Thread Starter

Simranjit

Joined Apr 16, 2024
33
Hi everyone I managed to solve the issue completely. I placed 5 capacitors (type CBB22) in series (1 uf each so total capacitance is 0.2 uf across drain and source of mosfet). Now there is no voltage spike and when phase shifted a little bit with the 74hc14, a potentiometer and a few capacitors to adjust for best performance the voltage swing doubled on primary. So I'm supplying 30v (drawing almost 1.6amps) and voltage swing is around 63v. I'm getting almost 11.5mm spark length. I hope this helps anybody trying to understand a single mosfet Tesla coil design. Now I'm moving to half bridge Tesla coil.

Edit: I'm attaching the the schematic. I tried to explain to best of my abilities in schematic. I would also really appreciate any feedback to improve this circuit further. Antenna length is 125mm (could be shorter). One improvement would be to use a mosfet with low RDS-on.
circuit_page-0001.jpg
 
Last edited:

MisterBill2

Joined Jan 23, 2018
19,355
I saw the similarity to the "Slayer" oscillator which is a blooper circuit in that the feedback is completely controlled by the locations of the components and the wiring. No feedback is shown in the acual circuit drawing of the slayer coil scheme.
 

Thread Starter

Simranjit

Joined Apr 16, 2024
33
I saw the similarity to the "Slayer" oscillator which is a blooper circuit in that the feedback is completely controlled by the locations of the components and the wiring. No feedback is shown in the acual circuit drawing of the slayer coil scheme.
The fee
I saw the similarity to the "Slayer" oscillator which is a blooper circuit in that the feedback is completely controlled by the locations of the components and the wiring. No feedback is shown in the acual circuit drawing of the slayer coil scheme.
Yea, I started with slayer exciter circuit and started modifying it drawing inspiration from other designs. I realize that this is not a slayer exciter anymore.
 

MisterBill2

Joined Jan 23, 2018
19,355
The remaining similarity is the use of an antenna for the feedback signal. In most actual TESLA coils there is a feedback winding so that the feedback is stable and controlled. Using a feedback winding would allow you to have more primary turns, although I am not sure what that would gain you.Probably a feedback winding would be a single turn of a larger diameter.
 
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