[AMPS] Power Supply Diodes

[Larry Molitor]

At 10:52 AM 2/7/00 +0000, Peter Chadwick wrote:

>Are we reaching some agreement that my suspicion about peak currents being
>the problem was correct? I've always tended to assume (like, it seems many
>others) that a 3 amp diode is good for 3 amps. Now I've started actually
>thinking about it, I am beginning to wonder in just how many places I'm
>pushing diodes on their peak currents........
>
>There's a lot to be said for choke input!
>
>73
>
>Peter G3RZP

Amen, Peter!

Since I have never before been able to make a HV, cap filter, supply work 
to my satisfaction, I am going to make a real effort on this one. Since I 
asked the original question, there have been many informative opinions 
offered, both on the list and private email. Enough so that I actually went 
back, dug out the reference books and checked the design. It appears that 
the only things wrong were use of off-brand diodes and failure to over-react.

As Steve said, "When in doubt, over-react!". Well, I had no doubts at all. 
I knew this would work in a choke input supply. A string of 3 amp diodes 
with the PIV they had would have run 6 amps forever in a good choke supply. 
I guess this should give me a hint why my "C" filter supplies are always a 
disaster.

The majority of responses favored insufficient peak current margin. Second 
place were those who thought the problem was a "transient". Third place 
goes to insufficient PIV margin. The one vote for "alien death ray" was 
lost in the noise.

Here's what I plan to do: Replace the bad diodes and add one in each leg 
thereby increasing the PIV margin somewhat. While I am running this setup 
expecting to blow up the whole thing again because the problem really was 
"peak current", I'll be looking for some 12 amps diodes that I can afford. 
If I can't any, then I will use 6 amp diodes.

I may look at a half hearted attempt at paralleling two strings of 6 amp 
diodes. Yes, I know, you can't get equal current sharing. Hopefully with a 
bit of series resistance in each parallel branch, I can equalize it just 
enough to give a bit more margin.

This probably won't get done, because I don't have enough space to put all 
those parts in and still provide adequate air circulation.

I'll keep the list posted on any future developments.

73,

Larry - W7IUV


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