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Old 26th January 2009, 08:34 PM   #1
pjanda1 is offline pjanda1  United States
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Default Bad Schottky??

I just tried to fire up the PS for my "El Cheapo." It doesn't do squat. I am using EliD's Cockeyed Bridge, and the Schottky on the trafo CT seems to have infinite resistance in either direction. That should not be the case, right?

Here's the deal. I was completely absent minded and just soldered the diode in with no regard for cathode orientation. (The complete lack of markings did nothing to remind me either). So, I could have killed it by installing it backwards. I tried to discharge the static from myself every time I touched it, but its wintertime in Minnesota. My basement might be the driest, most static prone place on earth.

So, before I order another one of those buggers (1200V Schottky's are a pain; I wish I needed more stuff from digikey), is it for sure dead? My DMM should be able to pass some current through it, no?

pj
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Old 26th January 2009, 09:03 PM   #2
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Default Re: Bad Schottky??

Quote:
Originally posted by pjanda1
My DMM should be able to pass some current through it, no?
This all depends on the particular meter you have. Some will not produce conduction in a diode. Although for a schottky I would imagine they should. Some have dual ohms positions. One for continuity testing with an audible tone which will make the diode conduct. Don't be afraid to use that position. Also, try measuring another diode if there is one in the circuit somewhere.

However, in case you are not sure of your meter, try connecting the diode in series with a standard light bulb and see if it lights up. This will confirm if it is open or not. An open diode may, or may not, show signs of cracking if it was abused.
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Old 26th January 2009, 09:35 PM   #3
pjanda1 is offline pjanda1  United States
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My DMM is a cheapie from Sears. I tried the audible tone setting to. If I can't measure the Schottky, how do I know which side is the cathode? (so I could connect it to a lightbulb) If it ain't dead yet, I'd hate to do it in now.

I was putting a few volts through the PS with a Variac. Where the diode was connected to the PT CT, I had a couple dozen volts. On the other, 0. Doesn't that mean it was probably either backwards, open or backwards and now open? Is the logical next step to turn it around and see what happens?

Or is there some standard polarity for a diode in a TO220-2 package? I googled and googled to no avail. This one is a 1200V 10A Cree Schottky.

pj
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Old 26th January 2009, 09:48 PM   #4
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Probably silicon carbide. I don't know the typical forward voltage.
I doubt you could kill it without blowing something else up first.

The tab itself might be a connection? Maybe one leg isn't used?
You going from leg to leg, or leg to tab?

It might even be two diodes in one package.
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Old 26th January 2009, 09:56 PM   #5
pjanda1 is offline pjanda1  United States
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I was trying leg to leg. One leg (the left one as it faces you, and the leg I had connected to the power transformer) is a dead short to the tab.

pj
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Old 26th January 2009, 09:59 PM   #6
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Quote:
Originally posted by pjanda1
My DMM is a cheapie from Sears. I tried the audible tone setting to. If I can't measure the Schottky, how do I know which side is the cathode? (so I could connect it to a lightbulb) If it ain't dead yet, I'd hate to do it in now.

I was putting a few volts through the PS with a Variac. Where the diode was connected to the PT CT, I had a couple dozen volts. On the other, 0. Doesn't that mean it was probably either backwards, open or backwards and now open? Is the logical next step to turn it around and see what happens?

Or is there some standard polarity for a diode in a TO220-2 package? I googled and googled to no avail. This one is a 1200V 10A Cree Schottky.

pj

Assuming that I found your part (DigiKey P/N C2D10120A-ND) Here is the datasheet showing pinout and orientation.

According to the datasheet, the left leg is supposed to be shorted to the tab, which brings up another question....Are you using insulating spacers if you are heatsinking to chassis?

http://www.cree.com/products/pdf/C2D10120.pdf
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Old 26th January 2009, 10:04 PM   #7
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Is this your diode? C2D10120
http://www.cree.com/products/pdf/C2D05120.pdf

Cause there's another C2D10120D. Note trailing D!
Its two 5A diodes in one package, not the same...
http://www.cree.com/products/pdf/C2D10120D.pdf
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Old 26th January 2009, 10:14 PM   #8
pjanda1 is offline pjanda1  United States
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Now why didn't I think of that? It isn't the D version, but the regular C2D10120. I do have it backwards. I'll turn it around and see what happens.

I've got it flappin' in the breeze: not screwed to the chassis, no heatsink. I did some reading about heat dissipation for a TO220 case. Given the low voltage drop and fairly low current, I was thinking it could fend for itself inside the chassis. I may have some ventilation issues in general, but I can always make more holes. I figured I'd gauge that with it up and running.

Thanks a bunch guys. Unexpected problems often get me so flustered I miss the obvious answer.

pj
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Old 26th January 2009, 10:46 PM   #9
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Can someone educate me ( a newbie) on this diode? The specs look impressive, as does the price..

Do these offer anything for HV rectification that FREDs, etc don't?
Are they noisy? Does zero recovery mean "not soft" recovery?

What applications would require these?
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Old 26th January 2009, 10:46 PM   #10
pjanda1 is offline pjanda1  United States
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And it works! The title of the thread should have been, "Bad Builder??"

pj
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