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is my power supply set up suitable for zvs use?

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Davebmx

Tue Mar 27 2012, 02:03PM

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

ok heres an update. built another heater the other night. turned it on straight from the wall socket instead of using the variac. it appeared to buzz for a few seconds and then one of the fets burst into flames! i measured the voltage from the smoothing capacitors and it is exactly 20.9 volts with a full mains input. Now if my heater circuit was built properly then i have a sneaking suspicion that the problem is caused by the initial load causing the voltage to drop way below the 15v for the mosfets causing the previous lock on and burn out problem. I had a reasonable size bolt in the work coil. Does this sound like it might be the reason? I'm definately rewinding the mot anyway just to be sure so that it will deliver maybe 40-45v so i have a load tolerance

Forty

Tue Mar 27 2012, 04:38PM

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

Enjoy another text wall!:

If the buzz was coming from the circuit then you've got a problem with AC coming through. If it makes any noise it will be a high pitched whine. I just did a quick calculation and it turns out you probably need some more filtering. With 6600uf, at 60hz (you might have 50hz, I don't know) and with a full bridge rectifer, you'll have over 13 volts of ripple at a 10A draw. double that for 20A, or half for 5A.

That ripple basically means that if you're drawing 10A, your output signal is still AC with a peak to min voltage difference of 13v. That's quite a bit. To get it down to a 1v ripple you'd need about 83,000uf, which is also quite a bit (of course with only a 25v requirement, capacitors with >1000uf should be very easy to find in salvage)

I've recently read about some simple filter designs (here:

that might work for you, but you'll need a pretty large inductor. I've seen large inductors used in lighting ballasts that should work, or you could also use a second large transformer with one of the windings shorted out. Might you have anything like that?

If you just want to stick with capacitors i'd first recommend salvaging them. If you need to stick together 20-30 salvaged caps then so be it. It doesn't take very long at all and it's free. You could also buy several capacitors and parallel them (

) or buy one big capacitor (

)

Rewiring the MOT for 40-45v will require different capacitors obviously, and also higher gate resistance for your mosfets.

Hmm just thought of something that might solve both of the problems. I doubt the mosfets care that they're switching a ripply signal (the series inductors smooth it a bit as well) but their gates certainly do.
When people want to up the input voltage for their ZVS circuits they tend to isolate the transformer power from the gate section. They then power the gate section with a nice constant 12v supply, and send the higher power through the inductor(s) for the switching side. The 12v supply could come from any DC source (battery, computer power supply, or a 12v voltage regulator chip fed by your 20-25v main supply) that provides a steady voltage. This should stop your fets from exploding quite so often because they'll at least be switching properly.

So I guess to summarize what to do for that last part:
1. find a 12vdc source. The regulator chip i mentioned will be in a 3pin TO-220 package and be marked with a 7812. There might be letters before, after, or in the middle, but that should be fine. Might even find one with 4 legs but that'll work too.

2. disconnect the high side of your power resistors that go to the gates of the mosfets.

3. connect the +12v supply to the disconnected ends of the resistors, and connect the ground so that it's shared with the other ground.

4. Find some more capacitance for your filter if you can.

Davebmx

Tue Mar 27 2012, 04:48PM

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

ok looking for some more caps. im assuming if i use a 12v power source i wont need as much smoothing? I have a 12v lead acid battery that should do the job right? also thanks for the excellent reply most helpful!

Davebmx

Tue Mar 27 2012, 04:55PM

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

also do you think i need to rewind the MOT? if i aim for 40v around what capacitance will i need for the smoothing bank?

Davebmx

Tue Mar 27 2012, 05:00PM

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

would this do the job?

Forty

Tue Mar 27 2012, 09:34PM

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

Well if its a 12v DC source then there's nothing to smooth right :P A battery will work fine, and so will that regulator you linked to. I get most of mine from salvaging circuit boards, since 12v is pretty common.
Any computer power supply will also have a 12v regulated line (they make decent bench top supplies if you happen to have one laying around

)

I wouldn't rewind the MOT. The MOT can output a lot of current, so upping the voltage would increase the power into your circuit. For the goals you have in mind for the heater, you're not going to need that much power most likely. Let's just stick to getting the heater running for now, and then you can increase the power later if need be.

The equation I used for the ripple voltage is v= I/(2fC)
where I is the current, f is the frequency, and C is the capacitance.

Oh and not that it bothers me, but the forum rules say not to double (or triple) post. You can use the edit button to add on to your last comment if you need to. It's only the two of us in this thread really but it might save you from getting yelled at elsewhere.

Davebmx

Tue Mar 27 2012, 09:41PM

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

Thanks for the heads up i will keep it to one post at a time! ok cool i wil solder together another heater with the 12v reg in it and then i will post it on here for you to have a quick look at before i power it up. should have it built by the weekend if not the start of next week!

Harry

Wed Mar 28 2012, 03:51PM

Registered Member #4081 Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139

On the 12v idea, I built this circuit, used about 4- 5uF and it drew about 20 - 30 amps. So a battery or computer PSU (as forty said) would be ideal.

Davebmx

Wed Mar 28 2012, 04:14PM

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

Thanks harry! That means i dont need to buy more caps!

Ok i have been soldering away and have stuck together 8 10000uf caps and my 3 2200uf caps in parallel to make 86600uf! i have built the circuit onto a pcb i made so that the traces are much bigger and there is very little chance of a short happening because i have cleaned out the spaces in between. I have also stuck the fets on a large heatsink and they are also insulated from it aswell. I have tried to keep it as simple as possible so that i can get it working. could you please have a look at the pics and see if there is anything i have gotten wrong or anything i should be aware of? I have dispensed with the variac so i will be running from the mains. What fuse should i put in the plug? Also is it safe to discharge this cap bank with a screwdriver? On firing it up i will put a bolt in the work coil so that it is under load.

Davebmx

Thu Mar 29 2012, 10:54PM

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

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