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Homemade HV transformer.

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Berni

Sun Nov 30 2008, 08:27PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

When i was looking at Steve's page and saw his HV power supply.I toght i might try a smaller version of it with a flyback ferrite core i had laying around.

So i sawed some small plastic pipe to size and wound some relatively thin wire on it, thrown it on the core and some litz wire for the primary. Add in some IGBTs and the testing started. First thing i figured out is that the series capacitor needs to be beefed up a bit since it got hot.It was noting special at 50V it needed like 3-5mm to start a arc and it could be drawn to about 2cm (It was nicely fat and hot tho) So i decided i replace the 50V transformer with a 1:1 transformer making it 300V input.

After the power boost the arcs became big flaming MOT like arcs.Then the dangeling output wire of the secundry comes close to the winding arcing over and melting the wire in a flash, but i got it working again by unwinding the coil until the burned part since its arced over near the end of it.

I want to add some protection to my half bridge since i don't want to blow it too esaly.Can anyone tell me how effective is a current transformer on the output and some kind of comparator circuitry that makes it shut down.

This transformer is just something i quickly sloped together to try the concept( I know its made like crap) The secondary is varnished tho.

So from what i saw til now it might be a good idea to continue trying this. Anyone has any advice?

rp181

Sun Nov 30 2008, 08:34PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

nice arcs. I your secondary 1 layer? If so, you might want to try to find a smaller secondary former, so the windings are closer to the core.

Berni

Sun Nov 30 2008, 08:52PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

Well im going to completely redo the utterly ugly secondary anyway. But im very happy with its performance, there is massive current on the output since the arcs are blinding bright and relay fat.

Yes that layer is 1 layer (since insulating layers on these things can be a pain in the you know what) I like to have clearance from the core so i don't make a ozone generator.

But first i want to build some kind of overcurent protection to keep my bridge from blowing again. I seen people use current tranformers. How well do they work?

teslacoolguy

Sun Nov 30 2008, 09:45PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
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Posts: 792

Current transformers work very well. You probably want a 1:1000 ratio on the primary side to limit the primary current or again, insulating it would be a real pain.

Myke

Sun Nov 30 2008, 10:15PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Why would you need to insulate it if it's on the pri side?

Berni

Sun Nov 30 2008, 10:30PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

Yeah of cruse i want the current transformer in the output of the half bridge.So no isolation problems. Somewhere 1:100-1000 will do fine for it i guess.

Oh any i wonder how many W dose a typical flyback core handle?

rp181

Sun Nov 30 2008, 10:35PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

yours looks like a large core, i would say at least 500W.

Berni

Sun Nov 30 2008, 11:06PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

That sounds like a fair bit of power for a flyback.I think the flyback belonged to a big B/W TV but it was dead.I also have a tiny flyback somewhere too(dead i think)

Anyway i will get to designing the peek current detection circuit tomorrow. Some opamp used as a comparator or something to cut the signal to the IGBT driver board and wait for a button to be presses to reset.Could make it even safer by shuting down on overheating too.

Is there anything else that could potentially save the bridge from blowing up? (I busted enough IGBTs)

uzzors2k

Mon Dec 01 2008, 10:46AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I've got just the thing you need.

The output leads can be shorted directly together without failure. Current transformers are quite simple, as the name implies they transform current by the turns ratio of the CT. Once the current is transformed it can be changed into voltage simply by dropping it over a resistor. No special isolation is needed for the CT itself, unless the voltage difference between the primary and driver is large. In that case just use thick wire for the primary, which you should be doing anyway.

Any estimate as to what the output voltage is when run offline? I might try making one me-self.

Proud Mary

Mon Dec 01 2008, 10:53AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I recall seeing a very similar set up on a German-language HV site - but with the secondary much tighter up to the core, and surprisingly few turns on it.

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