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Danielle

Fri Aug 03 2007, 03:01PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
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Posts: 267

I go to a great electronic store where I bought 2 SCRs that are 3" dia and 1.5" tall, hockey puck type. I know they are very high current but I got them for $5 each and they didnt tell me anything about them, and the PN dosnt exist. I was wondering for that price if it would be worth it as I can get 6 more is there a why to finde the brake down voltage without a curve tracer? I would be willing to sacrifice one to send different HV pulses through it and see when it dies. I would like a better why though.

the PN is something like 240 1010 I need to check it again later and I will also post a pic.

Thanks for the help,

Daniel.

Dr. Dark Current

Fri Aug 03 2007, 03:44PM

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

You cannot "overvolt" a SCR because when you apply too high voltage, it simply turns on. Just use a high impedance power supply, slowly turn up the voltage, and the highest voltage you get just before it turns on is the breakdown voltage.

Danielle

Fri Aug 03 2007, 03:53PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
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Posts: 267

thanks I will try this I didnt know thats how it would work but would I destroy it?

Barry

Fri Aug 03 2007, 04:43PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

You won't destroy it unless too much current flows. So just put in a series resistor for current limiting, to make it a "high impedance power supply".

Pick a resistor value that limits current to, say, anything under an amp and it will be fine. Be sure the resistor's power rating is sufficient (hint: power = voltage * current).

Cheers, Barry
PS - Rocketry76, please keep working on that punctuation to fix your run-on sentences.

GreySoul

Fri Aug 03 2007, 08:35PM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

Also, those hockey puck SCRs need to be clamped under significant pressure to function properly.

Problem is, without the spec sheet that came with the SCR you probably won't be able to find out the pressure required for the SCR to function properly.

I can get rather large SCRs of that type for $5-10 all day long from ebay or surplus houses, since they're essentially worthless without the proper heat sink and clamping pressure.

The best high current SCRs I can get are the large stud mount for $3 each, and some of those are good to 300+ amps.

-Doug

...

Fri Aug 03 2007, 08:49PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The clamp isn't really all that insanely critical. Really, just make it as tight as you can, and you should be fine

Ideally you want to have about 3tons of force, but as long as you are in that general range it should give you a nice long life.

In general, the 3" diameter "H" package is usually good for about 1500a continous, or or 25,000a peak.

Here is a datasheet for a similar SCR

Danielle

Fri Aug 03 2007, 09:09PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
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Posts: 267

wow I didnt realize what I had as current ratings go it was in a bin called "really high current SCRs" can I use my voltage boost converter for my coil gun but put a HV low capacitance capacitor across it? I built it to withstand 1600V but have only tried 1200V.

Steve Conner

Sat Aug 04 2007, 09:50AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Heh, "About 3 tons". I wouldn't worry too much about getting the exact clamping force. Even if you get it wrong and the SCR dies prematurely, you've probably still had more fun for $5 than if you never tried using it at all.

I've used a MOT with a 22k power resistor in series with the output, a variac, and a scope with a 100x high voltage probe, for doing this kind of thing. I was testing the breakdown voltage of IGBT bricks, but I can't see why it wouldn't work with SCRs too. You slowly turn up the variac until you see the device start to break down and clip the tips off the sine wave. Then note the voltage and turn it back down quick. :P

Danielle

Sat Aug 04 2007, 05:16PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

thanks that sounds like a great idea. I will try it as soon as I can get access to a scope probably on next Monday. For now can I check it a different way, or should I just wait?

...

Sat Aug 04 2007, 11:13PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Should work to just put a voltmeter across the resistor, and see when it starts to drop a ton of voltage. Of course you would want to keep the resistor at the gnd side of the secondary so it isn't floating at hv.

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