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Simple Induction Heater using ZVS

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radhoo

Tue Mar 01 2011, 11:21PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

As per the title says, an induction heater with an without an isolation transformer:

Schematics:

More details here:

Inducktion

Wed Mar 02 2011, 01:20AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Yeah....That brings about a question I've had, is it absolutely necessary to have the isolation transformer? Because from what I've noticed, isolation transformers really bog down power and frequency... Would a 170 V DC ZVS induction heater need the isolation transformer? What does it do exactly? Thanks.

radhoo

Wed Mar 02 2011, 10:14AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Inducktion wrote ...

is it absolutely necessary to have the isolation transformer?

No. The video shows 2 different tests, one with a ferrite transformer and one without it.

Inducktion wrote ...

.. Would a 170 V DC ZVS induction heater need the isolation transformer? What does it do exactly? Thanks.

Be careful with bigger voltages. For my setup I've used small IRF540 mosfets, and surprisingly this heater worked , were another one based on IRFP460 failed (the transistors were getting hotter than the target).
The isolation transformer limits the power getting to the work coil. As per the video, you can see the difference: current consumption going from 3A (with transformer) to 5A (without transformer). This ferrite core transformer also sets the frequency on 74KHz.

Ceriand

Sat May 07 2011, 12:00AM

Registered Member #3865 Joined: Thu May 05 2011, 06:24AM
Location:
Posts: 1

How well does this run on 12V? Can you use a lower value inductor for the 2000uH one, and how much current is flowing though it?

Marko

Sat May 07 2011, 01:10AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

One thing I would point out (and I think I did before) is that using a ferrite transformer to isolate an induction heater the way that is pictured is an extremely poor practice. Putting a ferrite transformer in between the work coil and the tank cap forces it to pass all reactive power of the tank circuit, which can be tens of kVA even for a small few hundred watt heater. It's expected that a typical flyback core or whatever you put there is not going to be able to handle this and the power output of the heater will suck.

On the other hand, if you put the capacitor after the transformer, then the transformer only needs to handle the real power going into the circuit, which hundreds of times less.

I'm not sure why newbies seem to have hard time getting that but it's a very natural and practical thing to do. You have a copper tube work coil with a cap bank connected to it directly with good connection that can stand hundreds of amps, and you can water cool it if you want... and to feed power to that assembly you only need to bring two thin wires carrying just few amps to it. Those can come from secondary of a transformer or directly from your royer circuit, and it can even be located remotely and connected to the tank by a coax.

Trust me, when you wire a transformer this way, with 1:1 transformer you will not notice any difference compared to not using the transformer, and apart from that, you can change the power output simply by adjusting the turns ratio of your transformer! A 1:2 transformer will feed double voltage to the tank, quadrupling the power while your circuit still works from the same supply voltage! You can make an induction heater that runs from 12V and still pushes kilowatts out this way. You only need to take care that your transformer doesn't have too much leakage inductance and parasitic capacitance, making it's resonant frequency lower than of your tank circuit which would be a bad thing. For smaller, HF circuits a small core will do.

For power levels of few hundred watts transformers from PC power supplies (in various series/parallel configurations) can be of great use.

The main reason such transformers are used in industrial induction heaters is isolation of the work coil from mains. If you're already using a rewound MOT or whatever transformer to provide low voltage for your royer circuit, you already have such isolation and a ferrite transformer may not be needed at all if you can get enough power without step-up.

I think it's by far simplest recipe for a newbie induction heater: A rewound mot, and a royer based on some IRFP260's, 250's, 150's or similar driving a tank circuit directly. Has been on this forum, proven to work at 1kW or so and melting aluminum stuff. Quite cheap also (if you get the mot for free), just make sure to use a nice bank of several tens of good polypropylene caps in parallel for your tank cap. Ebay WIMA MKP caps and like will probably work quite well.

One final thing to notice, if you're using a transformerless approach, connecting the DC link choke to the middle of the work coil might be awkward and you'll hardly get a symmetric voltage waveform that way. It's much better idea to put a separate choke to each of mosfet's drains in that case.

Marko

Steve Conner

Sat May 07 2011, 07:25AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

+1, thanks for the informative post Marko. :)

It's still better than the old style of hobbyist induction heater that didn't even use a resonant tank circuit. The work coil was connected straight to the output of an H-bridge which had to supply all of the reactive power.

One reason to put the matching transformer in the work circuit as Radhoo did, is if the capacitor you have is the wrong size for the job. For instance, you can use one of those high voltage, low value ceramic tube caps, by making a tank coil with lots of turns and series feeding it straight from an inverter. Then you can step the output down to drive a single turn work coil. The transformer is usually air cored.

The tube-driven induction heaters of the 40s and 50s all worked this way, except the tank circuit was shunt fed by a great big vacuum tube. Those things just threw so much RF power at the problem, some of it was bound to stick.

Marko

Sat May 07 2011, 03:42PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi steve,

For tube induction heaters, it's true that they had an output transformer wired this way, but it was always air cored since ferrite of required size was hardly an option at that time, and they didn't have low voltage conduction cooled PP caps they could connect in parallel to the work coil like today. So they had no choice but to use HV doorknob caps and match them down to a small work coil over an air cored transformer.

Such a transformer is still a very tricky part and seems to waste hundreds of watts, according people who built tube IH's here.

Marko

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