[Boatanchors] RE: selenium rectifier... replacement strategies
Dave Roscoe
[email protected]
Tue, 11 Jun 2002 10:50:31 -0400
I enjoyed reading Dr. Barry's scholarly, informative , and interesting note
on Selenium rectifiers. I have a vivid memory of a fellow student in my US
Navy Electronics Technician School in 1953 , exceeding the ratings of one.
He is right. You NEVER EVER forget the odor. Unbelievably gross. I still
gag when I think about it.
However, I take umbrage at him accusing the ARRL , by referring to the
constant 700 mv forward drop of a Silicon diode , as propagating a "myth".
It is , in fact, a FACT !
If you use the term "relatively" before the word "constant".
The slope of the E/I curve is ALMOST vertical throughout its USEABLE range
of currents, with a forward drop of 0.7 volt ( + or - )
"Close enough for government work" , as the saying goes.
Dave Roscoe W1DWZ
----- Original Message -----
From: "Barry L. Ornitz" <[email protected]>
To: <[email protected]>
Sent: Monday, June 10, 2002 5:38 PM
Subject: [Boatanchors] RE: selenium rectifier... replacement strategies
> Well, it is pretty obvious that virtually no one on this
> list knows any chemistry!
>
> The result of a burned out or overheated selenium rectifier
> is selenium dioxide, not selenium sulfide or hydrogen
> selenide. But people are not kidding when they talk about
> the toxicity.
>
> Selenium rectifiers do age, with their internal resistance
> increasing with age. They generally do not lose their
> reverse voltage rating, however, until their forward
> resistance makes them all but worthless. From a safety
> standpoint, they should always be replaced. If you want an
> old radio to look authentic, leave the selenium stack in
> place, but hide the new silicon diodes beneath the
> chassis. Modern silicon diodes take extremely little
> volume compared to the old selenium stacks for the same
> current rating.
>
> A power selenium rectifier, rated at the same operating
> current, will generally have a higher forward voltage drop
> than a silicon diode. Since an individual selenium
> rectifier is only rated to around 25 volts or so, to get
> higher voltages you have to place a number in series. So
> each individual selenium rectifier will have a greater
> voltage drop than a single silicon diode, and when you
> stack several in series, you can get a sizeable voltage
> increase if you replace the stack with silicon diode(s).
>
> I won't name names here, but someone said the voltage drop
> of s silicon diode is constant at 0.7 volt. This is quite
> incorrect, and I wish the ARRL would quit propagating this
> myth in some of their publications and articles. A real
> silicon diode behaves almost identically to an ideal diode
> in series with a resistance over an exceptionally large
> range of currents (typically over 8 to 10 orders of
> magnitude). At low currents, the V-I characteristic of the
> diode dominate. The voltage drop across the diode is a
> logarithmic function of the current through the diode. But
> at higher currents, the voltage drop is dominated by the
> diode's internal resistance and the voltage drop becomes
> linearly proportional to diode current.
>
> A selenium rectifier actually behaves in much the same
> way. Based on my research, there are some interesting
> differences between selenium diodes and most other
> semiconductor diodes (there appears to be what might be
> described as a double junction), but the same basic diode
> equations can be applied with some changes in parameters.
>
> I don't want to go into all the gory details because I have
> not finished my research yet, but it appears that it should
> be possible to predict selenium rectifier operation based
> solely on the active area and the number of plates.
>
> I will say, however, that at low currents, an individual
> selenium rectifier (not a stack which is individual units
> in series) can have a lower forward voltage drop than a
> typical silicon diode. But at currents at which they are
> typically operated at in power supplies, their voltage drop
> is far in excess of a similarly rated silicon diode. This
> is in agreement with the fact that many early televisions
> used selenium video detector diodes over silicon diodes
> because they were more sensitive. But this is also in
> agreement with the fact that selenium power rectifiers are
> far less efficient than silicon diodes. They are just
> operating on different portions of the actual selenium
> characteristic curve.
>
> I wish I could post some of the curves here. I have
> comparisons between copper oxide and selenium rectifiers of
> the same area, a 1N93 germanium diode, and 1N4148 and
> 1N4007 silicon diodes. I have curves shown in both linear
> (which is better for showing the diode internal resistance)
> and logarithmic (better for showing low current operation)
> scales. I will be happy to email copies of these graphs in
> Adobe Acrobat PDF format to anyone who is interested, but I
> will warn you they are large files.
>
> Most of my characteristic data was taken from manufacturers
> specifications and other published data. I have recently
> discovered a local source for a number of never-used
> selenium stacks, and I have collected a number of selenium
> units at hamfests too. These will all go on my Tektronix
> semiconductor curve tracer before I complete my work. I
> have a Sarkes-Tarzian engineering handbook on selenium
> units, but I would appreciate any other information the
> members of this list might have, especially data from
> Federal or International Rectifier. If Ed is willing to
> loan a few units for non-destructive testing, I would like
> to talk to him offline. I also need to make a trip to the
> University of Tennessee's library to copy about a dozen
> articles from some engineering journals of the 1940's on
> selenium units. [I sure wish there was an engineering
> school closer to Kingsport than Knoxville!]
>
> As I noted earlier, I eventually hope to be able to
> provide some simple equations that describe selenium diode
> behavior. These could be used to calculate the required
> series resistance to be added to a silicon diode when
> replacing a selenium unit. Several readers of this group
> have detailed the experimental method of getting the
> correct resistor value which is a heck of a lot simpler
> than what I am doing. But those that know me (like Ed
> Tanton), know that I like doing this sort of research for
> my own fun.
>
> I am including with this post (already far too long!) an
> article I wrote several years ago on selenium and selenium
> rectifiers (even longer!). It has been updated a little
> recently. The voltage drop estimations are crude in this
> post, but they should help you get started. If you do want
> to see the preliminary graphs, drop me private email. If
> you have technical data on commercial selenium rectifiers
> of years ago, I would sure appreciate a copy.
>
> So before the article, let me summarize...
>
> Selenium rectifiers do produce poisonous gases when
> overheated.
>
> Selenium rectifiers do age with increasing forward
> resistance with age.
>
> In power supply applications, when replacing a selenium
> rectifier or stack with a properly sized silicon diode, you
> will obtain a higher output voltage, often high enough to
> overstress downstream circuits. A properly sized series
> resistor can reduce this output voltage to the desired
> value.
>
> 73, Dr. Barry L. Ornitz WA4VZQ [email protected]
>
> Now on to the article...
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Selenium Rectifier Replacement
> (c) 1997, 2002 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 had 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, only modertely 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 considered 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 in
> 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 plate in a stack handles about 25 volts RMS.
> 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 [email protected] (c) 1997, 2002
>
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