On Fri, 23 Oct 2009 12:48:20 +0100, "N_Cook"
wrote:

Why would a SMPS high speed Unitrode rectifier stud diode 1N5812 , 20 amp,
fail sitting around in a normal human occupation heated environment for 10
years unused?

I'm not an expert on packaging, but it's possible that package leakage
allowed moisture to enter the device. Moisture then rots the aluminum
(or copper?) wire bonds and possibly the surface layers. If you're
curious, a helium leakage test will show if there are any leaks.

Currently reads a stable 6.2 ohms.

Currently *what* is reading 6.2 ohms? Forward resistance? What does
a good 1N5812 read? What's the reverse resistance? Got a curve
tracer?
http://en.wikipedia.org/wiki/Semiconductor_curve_tracer

Replaced with a new one but I intend
passing ever increasing current through the old one to see what happens.

I don't have any info on what's inside the diode, but if it's a bunch
of diodes in parallel, and some of them have opened thanks to internal
corrosion, my guess(tm) is that you'll see something resembling a
good, but much less than 20A rated rectifier.

Any other ideas concerning cause or analysis ?

Yeah, try non-destructive testing before you destroy it. Otherwise,
break it open and look inside before you melt the guts into a blob of
silicon.

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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
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