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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I advise a normal ZVS driver with a rewound mot power supply over any exotic high-voltage versions. This way you don't need big ferrite isolation transformer and circuit has been sucessfully used as induction heater. If you are keen not destroying your mosfets, you can even add an over-current protection circuit to save your mosfet's lives. hint: you can pass both of your source leads through a CT core in opposite directions for it to feel AC.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Inducktion wrote ...
James wrote ...
Marko wrote ...
The matching transformer allows you to convert some voltage to current, so that the transistors don't have to carry the full current passing through the work coil. In effect it's like a gearbox allowing a motor to produce enormous torque by reducing the speed. The current in the work coil can be tens to hundreds of Amps without needing exotic mosfets. It also dramatically reduces the effect placing different objects within the work coil has on the operating frequency and lets you use a work coil without a center tap. Almost any large ferrite will work to some extent, like I said, you can take one from a dead flyback or take apart the transformer from an ATX power supply.
This is one of reasons why a matching transformer is used with series induction heaters, but not very true for ones with parallel circuits. In these the transformer is either 1:1 or a small stepdown (2:1 or so) and it's primary purpose is isolation.
Theorethically you could run a ZVS driver from mains at 1200V staight into work coil, but I'd definitely use a tansformer simply for isolation reasons.
In any case if you want to get into serious induction heating, apart form the big ferrite cores (look at my soviet ferrite thread and search the store for gigantic soviet toroids), you'll also need to spend several hundred bucks on a proper conduction cooled capacitor as well.
The other problem with ZVS is, that when ran from high voltages, it frequently tends to blow up. That's the main reason you don't see anybody using it. You might have fun heating some bolts red hot with it powered from a rewound mot or so.
Marko
I'll post this in this thread
He determined why the ZVS driver had a tendency to explode and came up with a refined design that solves this. The resonant capacitor he uses is 3-4 bucks in small quantities and the circuit is capable of running continuously at ~1kW and pushing up on 2kW for short periods. He's currently developing a much larger heater that can do >8kW but for most people the basic design is likely more than enough.
His design can be built for under $100 if you buy everything retail, and much much cheaper if you do some scrounging and can get by with somewhat lower power. Take the basic ZVS circuit, add some real mosfet gate driver ICs and a schmitt trigger flip-flop and the reliability is greatly improved at higher powers. The stock "Mazzilli" circuit is good for ~60V input though if you use 3W resistors. I found I can easily heat screws and nails and such red hot in a few seconds using parts salvaged from old CRT monitors. As I said already, I *strongly* recommend starting with the proven "Mazzilli" circuit since it's dirt cheap, ridiculously simple, and proven to work. When you reach the limits and want more, tweak it and experiment, or build John's design as it's proven robust and also quite simple.
Most importantly, have fun and don't burn yourself.
I already have the IGBT's, the capacitors, and the diodes needed. The only issue with that circuit, are the IC's... I don't have money to buy them, unfortunately. Being 17 without a job and a love for electronics really sucks. :/
I got free samples of all the ICs involved. On Semiconductor charges $11 S&H but will send you up to 5 of each part you request for no additional charge. Texas Instruments, Maxim IC, Fairchild, and Vishay have all sent me parts completely free of charge, not even shipping. You can say you're working on a design of some gadget, but I've also had good luck just contacting them, saying I'm working on a personal project and asking nicely if they can send me a few samples to try. You can legitimately say you're a student and those companies will probably send you stuff as long as you don't get greedy and ask for too much too often.
Oh and the big ferrite isolation transformer, monitor and TV flyback transformers all have cores good for at least a couple hundred Watts and I have used them successfully with the Mazzilli driver. There's no need to buy fancy cores, although Fluxeon does sell the cores for a reasonable price. He quoted me $12.50 each for those used in the heater, and that's with winding tubes and mounting hardware.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Just wondering do the NAND gates have to be the EXACT same ones? I got some 74HC132N Quad NAND schmitt triggers from Fairchild. And I'm also going to use a very similar gate driver, inverting just as it is in the schematic. Surface mount though, so i'll have fun soldering that stuff.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Inducktion wrote ...
Just wondering do the NAND gates have to be the EXACT same ones? I got some 74HC132N Quad NAND schmitt triggers from Fairchild. And I'm also going to use a very similar gate driver, inverting just as it is in the schematic. Surface mount though, so i'll have fun soldering that stuff.
Any NAND should work. It is just to make sure the switching devices do not turn on at the same time and destroy themselves.
I have already made a PCB of a beefed up ZVS like this, i thought i was the first one...
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Arcstarter wrote ...
Inducktion wrote ...
Just wondering do the NAND gates have to be the EXACT same ones? I got some 74HC132N Quad NAND schmitt triggers from Fairchild. And I'm also going to use a very similar gate driver, inverting just as it is in the schematic. Surface mount though, so i'll have fun soldering that stuff.
Any NAND should work. It is just to make sure the switching devices do not turn on at the same time and destroy themselves.
I have already made a PCB of a beefed up ZVS like this, i thought i was the first one...
Snazzaroo. 8D
I'll be working on this as soon as I get the stuff I need, I.E. the gate drivers and the actual perf board...
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Microwatt wrote ...
Why can't you find the resonant frequency of the flyback. get a crystal that resonates with the same and use that with a mosfet?
Because crystals oscillate at sine waves. Sine waves are TERRIBLE for driving mosfets. They like to be on or off, not inbetween. You can, but it generates alot of heat and is inefficient.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No, you can buy a crystal oscillator for a few bucks with a square wave output. People use the 4MHz ones to drive Class-E SSTCs.
The real reasons why nobody uses crystals are:
The resonant frequencies of flybacks are low, <100kHz, and you don't get a particularly good choice of crystals down there. 32.768kHz is the only off-the-shelf value I can think of.
The frequency doesn't need to be that stable. A good RC oscillator, like the one in a TL494 or 555, is fine.
The easiest way to find the resonance is to connect up the flyback to your power oscillator and tune for maximum smoke. To do this the oscillator has to be adjustable, which RC ones are, and crystals aren't.
The RC is cheaper.
So it wins on three counts.
Getting back to the original topic, RTFArchives, you will find plans for Kim Ladha's self-oscillating induction heater, a simple circuit successfully copied by many people here. I think the thread title was "OMG Induction Heater" or the like. I prefer a circuit using the 4046.
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The best/easiest oscillator...
