Re: Selenium rectifiers in Tek equipment


Brad Thompson
 

Tom Lee wrote on 2/9/2021 2:50 AM:

Exactly. I never followed the lore because, as a teenager, I wasn't aware of the lore. I foolishly replaced blown seleniums with whatever rectifiers I had on hand from Poly Paks ("U test 'em and save"). <snip>
Hello, Tom and the group--

Polypak's other motto: "We bot [sic] thousands, no time to test."   In practice, one could learn a lot
from a homebrew curve tracer add-on for an oscilloscope and a bag of diodes-- intermittents and weird
breakdown characteristics showed up on screen. Also, sometimes a PolyPak would turn out to contain
pretty good surplus parts.

Back to selenium rectifiers: years ago, an industrial-controls company manufactured its own isolated interface
modules that used triacs as AC  switches for 120 and 240 VAC. To protect the triacs against voltage
spikes, the designer selected transient suppressors containing stacks of fingernail-size selenium plates
encapsulated in black epoxy and resembling candy gum balls (licorice flavor).

One lot of 240-volt interface modules exhibited a problem: the suppressors were bursting into flames, emitting
selenium fumes and stinking up the test area in the process.

The selenium "gum ball" manufacturer sent an engineer to investigate the problem, which turned out to contain several
smaller problems revealed in a meeting.

--The interface-module manufacturer didn't perform incoming inspection on the "gum balls".
--The "gum balls" were totally unmarked (their uneven surfaces precluded marking with ink stamps).
--The "gum ball" encapsulation process applied poorly-controlled amounts of epoxy.
--Overencapsulated  120-volt suppressors were mixed with 240-volt parts.
--The manufacturing group selected larger "gum balls" for installation in the 240-volt interfaces.

Someone at the meeting asked the "gum ball" engineer whether fumes from
the smouldering selenium parts was hazardous. "Of course not!" he replied,
"I've been working with selenium for years." ...And proceeded to cough a lung-shattering
liquid cough that completely spoiled his credibility.

Fortunately, GE had announced its line of metal-oxide varistors (MOVs) which were
better characterized and marked with part numbers.

73--

Brad  AA1IP

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