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Registered Member #2018
Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74
Hello Simone
Welcome to the forum.
The second link is from my page. You should remove the bracket at the end of the link for let it working correctly
Your problem can have several different causes. For me it seems your circuit works okay up to the gate of the powerfet, but it looks like your final stage is getting out of the class-e mode when powering it with low voltages. This happens with my coil as well. My coil doesn't work correctly with too low voltage. It needs about 120V. Below that voltage it starts to sizzle angry and the powerfet is getting hot. Have you got an ajustable power supply for your coil?
You may also try to change the coupling between primary and secondary a bit (lower or rise the primary a bit). Sometimes it needs a bit trial and error before working properly.
Registered Member #61232
Joined: Sat Oct 22 2016, 06:27PM
Location:
Posts: 5
Well it's nice to be welcomed directly by you!
I have a transformer with many output so I tried with 100v.but the waveform at the gate is still a mess... Tried with almost any cap value(and with the variable cap too) , I tried varying the turns of the primary but nothing seems to work. Maybe my FET is damaged and I'm losing time but I don't know what else to try
Registered Member #2018
Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74
Simone, what kind of oscillator have you got in your setup? Is it a quartz with fixed frequency or is it a 4046 PLL or something similar with variable frequency? If you've got a quartz, the secondary coil is critical. The resonance frequency of the secondary coil has to be a bit higher than what the actual quartz frequency is, because the flame will lower the frequency a bit and then pulls the coil in tune. If you've got a variable oscillator, it is much easier to find the optimal tuning point.
Registered Member #61232
Joined: Sat Oct 22 2016, 06:27PM
Location:
Posts: 5
Crystal osc + some small transistor then CD4093.. I measured the coil and it resonates at something like 100kHz below (4.21 MHz) the freq of the crystal.
Registered Member #2018
Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74
That may be a problem. I would remove a few windings until the resonant frequency is a bit higher than the crystal oscillator frequency. Together with the arc, it will pull the resonant frequency to the osc.-frequency.
Registered Member #61232
Joined: Sat Oct 22 2016, 06:27PM
Location:
Posts: 5
I've removed some wire so now the resonant frequency is 100 kHz above the crystal freq. I've also replaced the FET (with a new one) and the waveform at the gate it's changed very much... as soon as i can i'll post some shot. so i think that maybe the previous FET was effectively damaged and the gate driver wasn't properly tuned.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
You ideally want about +/- 15 volts on the gate for good clean Class E switching. If your gate voltage is only just making it to +10V with no B+ applied to the final power stage, the amplifier might go unstable from insufficient drive when you apply B+ to the MOSFET drain. Definitely better to drive it a bit harder.
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