I am trying to build induction heater with the help of the following schematics(taken from this page):
enter image description here

So I am using 220:10Vrms(15Vpeak) transformer with fullbridge rectifier and a capacitor to get 15V DC. If I disconnect everything from this simple "PSU", I do get 15V DC.

But, when I actually connect the circuit and measure my voltage rails I get this strange behavior on my oscilloscope, as you can see it shows 5V with some ripples.
enter image description here

The circuit draws 0.27 Amps on the primary and 1.84 on secondary of a transformer, which I measure with the voltage clamp.
enter image description here enter image description here

My coil is 1.2 uH (measured with LCR meter). I use 0.47 uF 250V Polypropylene capacitor. This should result in approximately 220 kHz resonant frequency.
enter image description here

I also use the choke of 2.2 mH on center tap.
So, questions:
1) The circuit doesn't work for me, i.e. if I probe the coil there is nothing on it, zero. If I put metal object inside(screwdriver) nothing changes. Also the overall current consumption is not affected(looking at clamp meter) if metal part is introduced.
I already tried to:
Re-wire everything from scratch, making sure everything is connected properly.
I checked the mosfets with simple testing circuit - they both work fine.
I tried to measure MOSFET output on the Drains without coil or cap in the circuit - it also shows zero on both mosfets.
2) Why if I measure voltage on my 15V rail it shows 5V with ripples? It shows perfect 15V DC if nothing is connected to it. Is this some kind of feedback? I have a choke to eliminate high frequency feedback, but overall it doesn't make sense, since the circuit does nothing to cause feedback as I think. The capacitor after full-bridge rectifier is 1000uF, seems enough too.

  • \$\begingroup\$ What size ac transformer did you use - how many VA is it rated at? Ripple on the supply is normal when the output load is moderate to large. 15V dropping to 5V tells me you probably only used a transformer rated at ~15VA. \$\endgroup\$
    – Andy aka
    Commented Nov 7, 2015 at 16:47
  • \$\begingroup\$ @Andyaka, I don't know the amperage of this transformer, but I suppose it should be around 30W or something, just picked it up because it was 15V, some Chinese one. Anyway - it does not get hot, even if I turn it on for minute or two. Isn't it suppose to be warm, if it is 15VA and I use to for higher current? \$\endgroup\$ Commented Nov 7, 2015 at 16:51
  • \$\begingroup\$ In the link it shows the current being about 3 amps therefore the VA of the transformer should be at least 15 x 3. \$\endgroup\$
    – Andy aka
    Commented Nov 7, 2015 at 16:58
  • \$\begingroup\$ @Andyaka even if it is, 15 x 3 is when metal object is introduced and causes current to rise. Isn't it? In stand-by mode it should be lower and I suppose I should see something on the oscilloscope. And I suppose trans should get hot if it is overloaded. I'll try to find suitable trans in my lab now to try to replace it anyway.... Will write back if it is the case. \$\endgroup\$ Commented Nov 7, 2015 at 17:04
  • 1
    \$\begingroup\$ Put a power resistor of maybe 10 watts in series with the choke to current limit things then gradually lower the resistance when you are more confident. Running it at 25V doesn't mean your mosfets are safe because of back emfs from L1. Also you'll need 15V zeners across the gates to stop them zapping. Read the data sheet on the MOSFET - chances are the max gate voltage ratings of the device are 20V and 25V will/could/possibly damage them. \$\endgroup\$
    – Andy aka
    Commented Nov 7, 2015 at 17:14

2 Answers 2


I have been afflicted with this circuit several times before .Its on the internet so it must be good?! .If L2 is saturated the loop gain of the osc is very low and it can stall .If input current is not limited then T1 and /or T2 will blow.Make sure that L2 can hack the prospective currents .Use a linear variable Psu for runup to avoid blowing fets .C1 must be high Quality because the circulating high frequency currents could be many times the Dc supply current .Use lots of lower value film caps in paralell for C1.If you use a seperate transformer with a ferrite core with a seperate gate winding for your feedback you will be better off .It is better to start your osc in a controlled class A situation .The ZVS royer is commonly used with BJTs but to make it reliable with mosfets requires a little more nouse .If you can get it to go on your lab supply then you can do a buck convertor with current limit and soft start for a reliable product.Under steady state conditions the peak drain volts is PI times the supply volts But with a hard start you can get twice this so check your ratings .Remember that the mosfets must have gain to start .If the biasing turns one hard on and one off you will stall.

  • \$\begingroup\$ idk, seen many people doing this circuit and having it go with ease, but yeah, this looks little suspicious. Do you have any example of well-designed, yet not complex, induction heater? Maybe even with BJT(actually I have a lot of power BJT, would be good to use them). By the way I am not aiming for melting steel or something, I basically want to be able to heat steel up to 120-150 C, not really red-glowing stuff. I will try your advice tomorrow, can't do that at the moment. Thank you. \$\endgroup\$ Commented Nov 8, 2015 at 0:53
  • \$\begingroup\$ If you have lots of BJT then reduce your frequency and do your ferite transformer between the collectors and to the bases .Give the bases about 3 volt .The difference in power consumption under no load Vs with the object to be heated is a guide to how efficient your "furnace" will be ".I used TO220 fets with no heatsink to heat a hollow steel needle using the gap field of a small planar core .This worked fine but wasnt needed so didnt go to production . \$\endgroup\$
    – Autistic
    Commented Nov 8, 2015 at 1:05

15VA is not enough to get this circuit to oscillate. you need at least 12 vdc on the gates to get it started. If your voltage is dropping to 5vdc on the rail as soon as you connect it means that you are not delivering enough current to your circuit.

first you need to get a larger transformer over 200VA. Also 1000uF cap needs to change to 4700uF. R1,2 should be 470 ohm 5W wire wound to ensure that the difference in the resistance is large enough by virtue of the tolerance to create an offset between the two sides of the circuit. This will provide the ability to oscillate. Your diodes should be UF4007 and not 1N4007.

It would also help if you placed a 12v zener diode in paralell between source and gate with the diode pointing towards the gate on both fet's. this will ensure that you do not exceed the max gate voltage which is around 15V. Also your tank capacitor should


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