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Inducktion Heater!

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Inducktion

Mon Feb 14 2011, 12:38AM

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

Hey all, I'm sure nearly 90% of you guys out there have read my other thread about my sine wave oscillator circuit and all of my discoveries along the way. Well, very recently I tried something a little different, and took out the transformer I had used, and left it air cored.
I also changed out the transistors to something a bit more efficient in the ways of heat and power loss, and decided to use some MOSFETS I had laying around. Originally, the circuit hadn't worked very well, much to my displeasure, and resulted in very little mosfet heating, or really much of anything happening at all. When i took out the transformer, and turned it on, the coil started to smoke! Obviously there was ALOT of amperage flowing, which made no sense to me as before the current draw was only around 100 milliamps. I quickly turned it off, and felt the mosfets for heat. Ouch. I didn't have them heatsinked and I ended up listening to the death of 10000 skin cells in my fingertip sizzle away. So, I then proceeded to hook up my oscilloscope to the tank cap, and add heatsinks to the fets, and turned it on. A sine wave, and a big sine wave at that too, appeared on the scope.

Mind you, the scope was set at 10 volts a cm! The sine wave has a bit of harmonics in it, probably fixable with a cleaner design since all I have it on is a breadboard at the moment.
Looking at it, and thinking quietly to myself, the quiet hum of the oscilloscope as it took measurements in my ears, I quickly grabbed a screwdriver, jammed it into the work coil, and plugged it in. In less than 5 seconds, the screwdriver was glowing dimly red hot. 10 seconds, bright. Whew. Hot. I unplugged the batteries, and they were already dead, unable to supply any more current or voltage to my power hungry circuit. I sighed, and proceeded to come up here and ask Grenadier to kindly make a schematic of my circuit.

Really easy, and really simple. The tank cap needs to be high current rated, and the work coil definitely needs to be thicker wire than what i have now. The mosfets are any type, although personally I used FDA28N50F fet's from fairchild. From what I could tell from my little experimentation, they don't even get all that warm, but still do need heatsinks. However, I'd recommend against TO-3P packages since they have the extra metal flange on the top, just adds more unneeded inductance, and thus unwanted oscillations.

The way the circuit apparently works, is the sine wave goes up one peak, then turns on the opposite mosfet, allowing it to conduct current, thus adding more juice to the oscillations, keeping them going. This repeats, thousands upon thousands of times a second, and allows for some serious current draw from what I can see and ...*cough* feel.

Wow. All this in two days, and it just started out with me and a simple idea.

And please, don't be afraid to squash my excitement and tell me this is already made. I don't mind, after all, nearly everythings already been discovered in the way of oscillators. Correct? ^^

And I WILL post pictures of the screwdriver red hot in a day or so, my batteries need to charge up. I only have 1.6 AMP hour ones, heh.

TILL THEN HV'ERS.

EDIT;

Okay, apparently, what I built here was a really simple Royer oscillator circuit, without the choke on it. If from what I understand about chokes, they limit current, and therefore prevent stuff from blowing up. My circuit didn't blow up more than likely due to my poopy batteries being unable to supply the necessary overkill current. But, it works, and is ridiculously simple to make. I'm going to continue to fool with it and see how well it performs with a decent power supply. I'll probably end up adding a choke to prevent it from dying, or using better fets, either or, depends on what happens. :)

Conundrum

Mon Feb 14 2011, 09:30AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

this is amazingly simple!

i guess it works in much the same way as the conventional Royer circuit used on backlight inverters.
Might try this with some cheap MOSFETs (IRF720 IIRC) I have here.

BTW another way to limit current is to use something like an LM317T (constant current) with lots of bypassing.
This would also allow some control over the current draw so it doesen't blow up with full batteries but still heats the load fine.

-A
"Bother!" said Pooh, as he induction heated a HDD...

IamSmooth

Mon Feb 14 2011, 03:39PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

See his basic royer circuit on this page

Dr. Dark Current

Mon Feb 14 2011, 03:54PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The gates will not last long

Inducktion

Mon Feb 14 2011, 09:25PM

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

Okay! I tested it out using a non current limited source, up to 32 volts, and it ran just fine, drawing a max of about 16 amps heating a screwdriver! I would still be somewhat wary about this circuit to be honest, don't go plugging it into a wall outlet just yet, if you have built it. If for some reason you want to, I'd seriously recommend using a variac to up the voltage until either 1. Something starts to smoke, and dies, which isn't recommended but there's not really any other safe way of doing it, or 2. You can't raise the voltage any higher.

Sure did heat a nice sized screwdriver though, reached curie temp in about 20-30 seconds of constant heating. The tank cap definitely needs to be high quality, MKP, FKP, something good, and the mosfets need decent heatsinks as well, or a fan. I'd recommend fets that can handle at LEAST 20 amps, and that's cutting it a bit close, 30-40 amps is probably a bit more into the safe range.

Feel free to make the circuit now guys :)

ScotchTapeLord

Tue Feb 15 2011, 01:37AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

You cannot go over 30V without anything protecting the gates! They are only rated for +/-30V and God knows what they're getting from that tank.

If you want to keep it simple but alive, please put a few ohms on each of the gates and have back-to-back zeners from each FET's gate to source... and don't go too far above 30V.

Inducktion

Tue Feb 15 2011, 02:50AM

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

Alright, hold on... I'm going to modify my circuit and schematic appropriately.
Thanks for the advice! ^^

Wolfram

Tue Feb 15 2011, 07:20AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Isn't this just basically the classic Royer ZVS oscillator, without the DC bus inductor and the gate protection components?

Dr. Dark Current

Tue Feb 15 2011, 10:28AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Yes it is.

Inducktion

Tue Feb 15 2011, 08:46PM

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

Ah, never mind, it doesn't like zeners or resistors on the gates, weird.
Ah, well, it works well for voltages under 25-30, depending on what fets you use. :P

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