[R-390] Selenium Rectifier Redeux

[Tim Shoppa]

"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.