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is my power supply set up suitable for zvs use?

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Davebmx

Thu Feb 16 2012, 06:27PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

Hi guys im trying to build a zvs driver to eventually use as part of induction heater setup. However i dont know wether my current variac/rectifier is sufficient. I am using a 1a 240v variac follower by an 800v 15a full bridge rectifier. Will i need a transformer after this before the zvs driver? or am i totally off base?

Forty

Thu Feb 16 2012, 07:30PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

If the variac can only provide 1amp then it won't be very useful. You'll need a fair sized step down transformer either after the variac or just straight from the wall. A microwave oven transformer with the secondary windings removed and new windings added is a common way (just search "rewound MOT") of making such a step down transformer. After the transformer you'll then put the bridge rectifier to turn the AC into DC. Then you'll want a large electrolytic capacitor from + to - to smooth out the DC signal. That's about as simple a DC supply as you can get. Can also add things such as fuses, a bleeder resistor for the capacitor, an "on" indicator, power switch, cooling fan, etc.

You couldn't put a transformer after the bridge rectifier because transformers need AC to operate and the bridge rectifier would have just rectified the AC to DC.

Davebmx

Thu Feb 16 2012, 07:38PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

Many thanks! I see where i've gone wrong haha unfortunately im pretty new to all this high power stuff! Its a little scary working from the mains but i will do my best to work safely. This has raised a question though for me. Say for the driver circuits in those small bench top induction hobs how do the achieve the initial stepdown trans in such a small low profile space?

regards

Davebmx

Tue Feb 28 2012, 10:28AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

Just an update Forty. I have found a Mot and am in the process of rewinding it to get a useable voltage. I should have mentioned that this is to produce only a very low power induction heater capable of only warming a small amount of metal to around 125 centigrade! i don't need kw's of power on this one so do you think the 1a variac can be used with the transformer to control the final output of the heater?

Forty

Tue Feb 28 2012, 04:58PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I'm pretty sure the bench top ones use an SMPS for their power supply. If you wanted something like that then you could use a computer power supply. If you wanted to try a low power zvs you'd have to put a big power resistor in series with the inductor to limit your current. The way the zvs circuit is usually made it will draw all the current it can pretty much. If your variac is only rated for an amp then you'll want to make sure you don't draw more than that from it for extended periods (or at all if it has a built in fuse.)

So you've got 240 watts to work with. If you rewind your mot for 12v then you'll have up to 20 Amps (probably more like 18), if you rewind it for 24v then 10 Amps (~9A) etc.

probably the easiest way of going about doing it is just build your zvs heater, rewind your mot, and put a 240v 1A slow blow fuse in series with the mot's primary side. If it works then you're golden, and if the fuse blows then we'll come up with an alternative.
If you need a fuse (or 5) then littlefuse.com will sample you some.

Davebmx

Tue Feb 28 2012, 05:30PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

many thanks for your reply forty! i am rewinding the mot as i type this, no mean feat ha! to produce 12v. I have already constructed a very crude driver circuit which i will post pictures of very shortly once i have finished the rewinding process. so so far it goes wall outlet>1a slowblow fused plug>mot (rewound for 12v) >rectifier bridge>smoothing capacitor>zvsdriver>large resistor?>tank circuit with workcoil. sound somewhere about right?

many thanks

Forty

Tue Feb 28 2012, 05:49PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

You won't really need the resistor if you've got the fuse in place. either one is just there to protect your variac.
If you only need to heat up to 125C why not just use a hot plate?

Davebmx

Tue Feb 28 2012, 07:43PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

I need the core temp to be around that also i want to keep the surrounding temp down as much as possible. and also heating speed needs to be pretty quick. I picked up some 1amp slow fuses and im going to pop one into my variacs fuse holder. i also got a 240v switch so i turn the variac off quickly should stuff go wrong!

Davebmx

Tue Feb 28 2012, 10:50PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
Location:
Posts: 40

Ok so i hooked up my very, very crude attempt at a zvs circuit to see what would happen. I have the 1a slow fuse in the variac fuse holder. I thought i had put just enough turns to get 12v from the mot at 240v input but it turns out i'm closer to 20v after the rectifier and smoothing capacitor. Will this be an issue? I'm using a 275v 5uf mkp cap for smoothing does this sound ok? the 800v 15A rectifier doesnt get warm at all. My tank circuit is connected directly to the zvs circuit and uses 13 0.15uf caps in series and the work coil is 5 turns of brake pipe. Does this sound relatively close or am i way off? The heater doesnt seem to work as there is no voltage measured in the tank circuit at all even when the variac is turned up full. should i be using a transformer from the driver circuit to the tank circuit instead of wiring directly? here are some pics of my godawful attempt! please excuse the crudity i am a serious beginner

Forty

Wed Feb 29 2012, 01:58AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

In all three pics it looks like the drain and source of the lower mosfet are rather close or touching, so make sure they aren't. Your filter cap should have much higher capacitance as it is there to provide energy. Since you're using 20vdc, a 25v rated electrolytic capacitor would be enough, and finding one or a few with several thousand microfarads shouldn't be hard.
from what i've seen of other people's induction heater designs, and my own experience with the mazilli zvs, I believe you'll want to put those 13 0.15uf capacitors in parallel for your tank cap.

Fixing those two capacitors might not solve the problem though. your connections look to be correct, but they're a little long which might introduce some stray inductance and capacitance. If you could get rid of some of that wire then the mosfets might be happier. It looks like you've got them insulated on the heatsink so the drains aren't shorted, that's good.
I can't think of any other reasons why it's not working, so try fixing those things and see if it helps.

-much larger, 25vdc electrolytic filter capacitor
-parallel tank capacitors to lower resonant frequency
-check that legs of fets aren't touching each other
-shorten wire lengths

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