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Sorry for poor English. I have a transformer from an old TV and try to generate HV discharge. I've added a capacitor (C1 on the scheme) to get resonant frequency around 17kHz. Used coil 5-6 for feedback for the transistor T1 (IRF3808) and protected the gate with 5k potentiometer RV1. Sometimes the circuit works, sometimes not. I have tried to switch pins 5 and 6 of the transformer. Moved the tap of RV1 both directions. Checked that the transistor works. Everything looks fine, but the circuit does not work most of the time.
What is wrong with the circuit?

scheme

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  • \$\begingroup\$ Please show oscillograms of gate drive waveform. \$\endgroup\$ – winny Oct 28 '18 at 10:39
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Q1 probably needs to have a little bit of DC bias in order for it to start oscillating so, I suggest that pin 6 of the transformer (as shown in your diagram) is raised by a small DC voltage in order to take a few tens of milli amps through Q1's drain. Try using a potential divider - maybe 10 kohm and 680 ohm. The 10k would connect to 30 volts and pin 6 and the 680 ohm from pin 6 to ground.

Bear in mind that these are approximate values and not calculated. I would also urge you to use a simulator to try and get it working with the "better" values. You might also consider reducing the amount of AC fed back to the gate because, when it is optimized, given the same turns are used on both primary coils, there will be too much positive feedback.

See also this question and answer regarding a tuned collector oscillator and fixing it to produce a cleaner sine wave - it's very similar to your set-up so it will help you understand the importance of restricting the positive feedback levels in order to produce a decent sine wave.

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  • \$\begingroup\$ Thank you. Bias current indeed improved stability of starting the oscillation. \$\endgroup\$ – user54041 Nov 5 '18 at 11:07

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