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# ZVS Induction Heater help? Answered

Hi, I tried to build a ZVS induction heater using this circuit http://img413.imageshack.us/img413/5136/inductionm.jpg , but I had no sucess. The work coil is 10+10 turns of 22 AWG wire around an old highlighter casing, and I am powering the circuit from a working 12 volt 7ah battery. The circuit works with my flyback transformer, but does not work on the induction heater. I am using all of the standard components in the flyback circuit, but I replaced the 5+5 turn primary with a 10+10 work coil. As far as I mesured with my thermometer the temperature of the object never increases over 1 degree (I am trying to heat copper wire or a small picture nail).
Does anyone have some suggestions?

Edit: Sorry about the delay, but I was able to heat a large paper clip using 8+8 turns of 16 AWG wire wrapped around a glass test tube. This time I used a huge 24 volt center tap 18 amp transformer to get 12 volts.

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A little more on your circuit., if you go to

http://www.circuits.dk/calculator_single_layer_aircore.htm

you will discover that your coil if wound correctly woul be about 2uH measured end to end. Assuming you copied the circuit fairly closely, the resonating capacitor is a 0.68 uF cap. this yields a ring frequency of around 136 kHz. f=1/(2 * pi * sqrt L*C) Using a 12 volt source, the peak ring voltage should be 36-38 volts (Vpk = pi * Vsupply). This will yield a Vrms measured on the cap of approximately 25-27 vac (Vpk of a sine wave / sqrt 2 = Vrms). Ring current then would be approximately 21-22 amp pk or 15-16 Arms.

I know this post is a couple of years old but if you are still interested in the circuit, the last two comments should get your project up and running and give you some approximate parameters to check to make sure it is working.

Kev

Your coil is wound incorrectly, you twisted to bite ends together and wrapped starting at the middle out to the ends. you didn't create a center tapped coil. you created two coils that are wound in opposite directions. what you need to do is unwrap one end and wind it in the other direction. you will know when you have it correct because the ends will be off opposite sides of the form, one end will be on the bottom, the other on the top. Total number of turns does not matter because just a few microhenries will make the circuit work but the number of turns have to be the same on either side of the center tap.

What you wound was two inductors that cancel out each others magnetic field. The ringing current flows through both halves of the coil. All the center tap does is allow the power supply to make up for losses. if the coils do not aid each other during the ring cycle, all you have doing work is the make up current, not the ringing current (because the isn't any). once the coil is rewound to aid the other coil, the circuit will start ringing lots of current and your heater should work much better.

Kev

If I follow what you're saying here, you had in your possession a working flyback driver circuit, based on the diagram you linked too. (I've also seen it here: http://old.4hv.org/index.php?board=18%3Baction=display%3Bthreadid=1723)

Then you took out the flyback transformer and replaced it with that 20-turns center-tapped thingy in the picture.

And then you expected the circuit to work just like it did before, and drive oodles of amperes through your "work coil" thing there, and be capable of heating up conductive objects brought near it.

Have I got this right so far?

I suspect that the driver circuit is not oscillating anymore without its flyback transformer.  That is to say that you need that transformer or one like it.  I mean you're not driving this circuit with some other oscillator.  It starts itself using some sort of feedback from the transformer.  Right?

BTW, in the process of trying to figure out what you were talking about, I found this:
http://4hv.org/e107_plugins/forum/forum_viewtopic.php?33665
There are some nice pictures at that link. Stop me if you've already seen it.

Also BTW,  making this trick work will involve driving kinda large currents,  like 10s of amperes RMS, through your work coil.  If you succeed in melting the PVC insulation off your 22 AWG wire, that probably means you're on the right track.  Try not to breath the fumes.  Burning PVC is rumored to produce dioxins.

hi
I made a indction heater out of a induction cooker that would heat three eight rod all the parts are retreaved from the cooker even the wire to form the coil
Bill

As I recall, an induction heater works on magnetic field, which in turn is proportional to the time-rate-of-change of the current source. (I could be off the mark here, sicne I'm going on memory of a few quick article scans over the years. if so, someone tell me so I can delete my reply and save a tiny bit of face)

time rate of change...

If your switching mechanism is slow, then the resultant magnetic field with be small.

As Steve suggests, get your hands on a scope.

My suspicion is that regardless of the quiescent current (essentially, the steady state current), the time derivative of that current, ie, the pulse is too slow.

You need a fast turn-on/off (sharp leading and trailing slope) in the drive circuit current, WHILE attached to the coil to generate a solid field.