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I'm an electrical engineering student and I've been super excited to put a zvs induction heater together.

I finally got all my parts, turned it on, and smoked a MOSFET almost instantly. Happened twice actually. It's just one that gets hot the other seems to do nothing. My work coil is about 2+2. I originally tested it to be about .2mH but I took a turn out. My choke started at something like 70mH and I took 2 turns off of that. The caps are .1uF 1000V and the MOSFETs are irf540s. I've tried troubleshooting on a number of sites but can't really figure out what I'm doing wrong. Any suggestions going forward? circuitpaint drawingcan take a closer one if needed

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  • \$\begingroup\$ The hand drawn schematic you posted shows that you shorted the 460's to the 10K's to LM781. Hopefully that's just a poor drawing? \$\endgroup\$
    – horta
    Mar 25, 2014 at 18:45
  • \$\begingroup\$ ya i did mess up the drawing the center line is supposed to be the ground and the 460s come from the lm781 \$\endgroup\$
    – barrett
    Mar 25, 2014 at 22:06
  • \$\begingroup\$ My gut tells me your circuit is not oscillating reliably. I don't think you should feel much magnetic field in your work piece at working frequencies. If only one MOSFET is getting hot then it may not be starting. If you PC-PSU fan is turning on it means that a lot of current is being drawn not conclusive if you have oscillation though. Make sure all your high current wires are short and thick and your feedback connections are reasonably symmetrical. \$\endgroup\$
    – KalleMP
    Aug 24, 2017 at 19:56

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I think I see your problem... You state that you had a 70 mH choke originally but then removed two turns to get down to the picture showing 40mH choke. I don't really think from your pictures and from your description of two turns changing the inductance that you have an inductance that large. If it's that enormous coil with electrical tape around it acting as your choke, then you have something in the uH area of inductance I would expect. How are you measuring your inductance? With an inductance multiple orders of magnitude too small and very likely a resistance equally small since there's hardly any turns on your choke, you end up with huge amounts of inrush current with no resistance or inductance to slow down your current. Right away you'll heat the mosfets up and blow them out.

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  • \$\begingroup\$ The choke I didn't actually measure that's just what it said when I ordered it. It was from Ebay so it could be wrong. The work coil I measured with a real expensive RLC meter at school. I suppose I could try to actually measure it though I'll let you know what I find out. \$\endgroup\$
    – barrett
    Mar 25, 2014 at 19:13
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    \$\begingroup\$ So i calculated it you were right I was at about 4uf not mf. So I have some magnet wire and 2 of those cores I could stack them and use a ton of windings. According to the Al value I'd need about 140 turns to get 2mH. I was thinking about trying it with .5mH from 70 turns. Otherwise I guess I can just order a new core and wait would prefer not to blow any more transistors haha \$\endgroup\$
    – barrett
    Mar 25, 2014 at 22:08
  • \$\begingroup\$ Glad my shot in the dark yielded some useful results. If you have an inductance meter to check your inductance, and the time to do it, I would just hand wrap them. Otherwise, going the shopping route is easier to hit your specs. \$\endgroup\$
    – horta
    Mar 26, 2014 at 14:26
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    \$\begingroup\$ Ya I wrapped as many times as I could around the 3 toroids I had and it finally worked. Still needed to be bigger so I ordered some cores with higher permeability. Plus I fried my last mosfet so I have to wait for more of those haha. \$\endgroup\$
    – barrett
    Mar 27, 2014 at 21:59
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    \$\begingroup\$ @KalleMP This is 3 years old. You also put this on my answer so the asker will never see your comment. Mb move it to the question if you think that'll help future people? \$\endgroup\$
    – horta
    Aug 20, 2017 at 3:17

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