[R-390] Selenium Rectifier Redeux

[Michael Murphy]

Selenium Rectifiers being in the "dry disk" family of rectifiers do seem to
be prone to failure due to contamination by water. They must have been
cheaper and more reliable that the device that they replaced, namely the
vaccum tube rectifier! Radios and TV's from the 1950's through the 1960's
seem to be full of them.

Mike Murphy WU2D

----- Original Message ----- 
From: "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
To: <r-390 at mailman.qth.net>; <N4BUQ at aol.com>
Sent: Thursday, November 24, 2005 2:54 AM
Subject: Re: [R-390] Selenium Rectifier Redeux


> "Barry" <N4BUQ at aol.com> wrote:
>
> > Another question about the selenium rectifiers.  Is it possible that the
> > failure rate of the this component is due to the relatively low voltage
> > applied to it?  I don't know what these are rated, but I'm thinking they
are
> > probably good for 100VAC or more.  With only  28VAC applied, perhaps
this
> > doesn't stress these very much.
> >
> > Also, with the low voltage in this case, if the rectifier should fail,
is it
> > likely to go out in a blaze of glory?
> >
> > Just wondering...
> >
> > Barry - N4BUQ
> >
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>
> My impression with Selenium rectifiers in consumer equipment is that
> the top causes of selenium-stink-failure were:
>
> 1.  Audio output stage drawing too much current (usually due to leaky
>     coupling caps)
>
> and
>
> 2.  Failed-shorted electrolytic filter caps on the outputs of the selenium
>     rectifiers.
>
> In other words, yeah selenium rectifiers burned up, but it was due
> to the load being higher than their rated current.  The burnt-up
> rectifier isn't the cause of the failure just a symptom. Although
> they were so stinky that they were often blamed as the cause.
>
> The load in the case of a R-390A are some very simple very well defined
> relay coils.  I'm not going to say that they never ever fail shorted
> and take out the rectifier, but that isn't going to be a real common
> failure mode.
>
> Certainly the low PIV doesn't hurt selenium rectifier life either!
>
> Tim.
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