At 05:29 AM 2/3/2000 +0000, you wrote:
>There is step-start in place and functional. There is glitch protection.
>One more time, the failure DID NOT occur on start-up. It DID NOT occur
>during a big-bang or other disaster. The whole thing was loafing along at
>about 1/3 design limits when the damn diodes just rolled over and died.
>Never before have I experienced diode failure at anything but turn-on or
>big-bang. I have almost convinced myself that it was just a bad batch of
>cheap diodes, but yet at the level it was running at, anything should have
>survived.
>
>So the question remains, if 3 amp diodes blew with only 700 ma. load, what
>does it take to keep it running at 2 amps?
>
Under the circumstances Larry, it appears that one or more of these diodes
were destined for a short life anyhow. Their death likely resulted in a
domino type death of the others. They obviously didn't fail due to undue
stress at the time of expiration. These things are mass produced and
there's no garantee that they'll live-up to their specs - as you've seen.
If they've spent some of their life in the supply delivering 2 amps DC, its
likely that the peak current through the strings was considerable higher 2
amps and this may have weakened them. As Rich has mentioned, glass
passivated rectifier diodes are more robust, and more expensive, and
probably aren't required for normal amateur service. But if I were in your
shoes, I'd probably take a look at them. There is also a more robust diode
that will take 6 amps. I believe its a 6A10 or a 6D10 (or something like
that) and the package type is R6. Surge current rating is 400 amps and I
think the max operating temp is lower than the 5408's. Either Mouser or
Digikey carry them (can't remember which). They're alot more money than
the cheap 5408's, but I think you're past the point of looking for the
cheapest solution. Regardless of how heavy the current rating is, Rich's
recommendation for stringing lots of them to get a total PIV of 2 - 3 times
the operating voltage is good advice.
Phil
>73,
>
>Larry - W7IUV
>
>
>
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>
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