From: "Barry L. Ornitz" <ornitz@dpnet.net>
Subject: Re: Selenium Rectifier
Date: 02 Dec 1998
Keywords: selenium, rectifier, toxicity, safety
Newsgroups: rec.antiques.radio+phono

Mark Densel wrote in message <36659183.9596D249@agouron.com>...
>Where can I find a selenium rectifier for a 1950' kiddie record player?


I wrote a piece for the Boatanchor radio mailing list a while back on
selenium rectifiers.  When these short, they produce rather toxic products.
Thus they should always be replaced when restoring older electronics.  They
also tend to age and increase their internal resistance over time, so
replacing them will likely help the operation of the equipment too.  Just
remember that a modern silicon diode will produce a greater voltage output
than the selenium rectifier, so you may have to add some series resistance
accordingly.

                  Barry L. Ornitz     ornitz@dpnet.net


                   Selenium Rectifier Replacement
                          (c) 1997 B. L. Ornitz

Several people have recently asked about the toxicity of selenium and
its compounds since selenium stacks were common as rectifiers in many
Boatanchors.  Like others here, I too have learned the hard way just
how bad a selenium rectifier can smell when it is overheated or
shorted.  One of my first electronic projects has a small selenium
stack that I wired in backwards.  Naturally I didn't plug it into the
wall socket at my workbench - I carried it into my bedroom to test it!
I couldn't sleep in there for three days!    ;-)   Murphy Rules!

Selenium, a p-type semiconductor, is coated on steel plates in a thin
layer to produce a selenium diode.  Normally seen as a black or gray
shiny coating, the metal is a close relative to sulfur (in fact, I
have made homebrew copper sulfide rectifiers).  It was named after the
Greek word for the moon, Selene, by Berzelius in 1817 because he found
it associated with tellurium which is named for the Latin word for
earth.  If I remember my chemical history, his housekeeper accused him
of eating loads of garlic, when he had not.  This garlic odor is
characteristic of many selenium compounds.  Selenium metal is, in
itself, not terribly toxic.  Its compounds are, however, even to the
point of making some plants toxic to animals when grown in soil rich
in selenium.  [Hopefully we don't have any mega-supplement health food
nuts taking massive doses of selenium here.  Some effects of chronic
exposure include depression, lassitude, fatigue, liver and spleen
damage, yellow skin, garlic breath, giddiness and emotional
instability - and reproductive effects which nature gladly provides to
prevent the stupidity gene from being passed on.]       ;-)

Selenium dioxide is the major compound produced when a selenium
rectifier is overheated.  It can cause severe burns to the mucous
membranes and severe respiratory tract, skin, and eye irritation.  It
is also a dermal sensitizer in that it can promote allergic reactions.
Fortunately it is not consider a carcinogen.  Another fortunate thing
is the BAD smell.  When I say BAD, I mean really, really, _really_
_BAD_.  Our odor threshold for selenium dioxide is 0.0002 mg/m3.

The allowed exposure for selenium and it compounds (expressed as
selenium) is:
   0.2 mg/m3 OSHA TWA
   0.2 mg/m3 ACGIH TWA
   0.2 mg/m3 NIOSH recommended 10 hour TWA
   0.1  mg/m3 DFG MAK TWA (total dust);
   1 mg/m3 DFG MAK 30 minute peak, average value, once per shift

Note that the odor threshold is far below these.  If you smell
something really rotten, like decaying onions and garlic, coming from
your equipment, it is best to leave the area immediately, opening some
windows on the way out.  Allow the selenium dioxide vapors to
dissipate for several days before you go back.  If you have ever
smelled this odor - believe me - you will not want to go back very
soon anyway!  The odor is very distinctive to say the least.
I almost always replace selenium rectifiers as standard practice with
BA gear. If you have the schematic and know the voltages, choosing the
proper silicon diode is easy.  With no other information, a reasonable
rule of thumb is that a single stack handles about 25 volts PIV.  Thus
the typical stack for rectifying something off the power line would be
five plates.  Another "eyeball" approximation is that the area of each
plate in the stack is about a square inch per 300 milliamps.  For most
boatanchor applications where small stacks are used, common 1 amp
diodes are fine.  Remember that selenium rectifiers have a much greater
forward voltage drop with current than do silicon diodes.  Thus when you
replace a selenium diode with a silicon diode, expect a higher voltage
out of your circuit. For most selenium stacks, the DC output is about 2
volts less than the RMS input voltage multiplied by the number of plates.
This implies a considerable internal resistance.  Silicon diodes generally
have about a volt of forward drop associated with each diode, but the
internal resistance is very low.  A series resistor may be added if
necessary to drop this voltage.  For applications like the T/R relay
rectifier in a military R-390A, this series resistor is hardly necessary.
However, for large low-voltage DC supplies using selenium stacks (often
only one or two plates per leg, but all four legs of a bridge are usually
mounted in one assembly), a series resistor may be necessary when a
modern silicon bridge is substituted.  Many such supplies included
multiple taps on the power transformer, fortunately, so investigate this
before wiring in a power-wasting resistor.

From the interest of safety, I believe it is always wise to replace
selenium stacks with modern diodes. Some restorers leave the original
selenium stacks in older gear to make it look authentic.  Since modern
silicon diodes generate so little heat in comparison, they may often be
hidden in the circuitry replacing the selenium units while not being
noticed.

        73,  Barry L. Ornitz  WA4VZQ  ornitz@dpnet.net