[Boatanchors] selenium rectifier

[Reed Park]

Mike & Judy Wells wrote:

> I am wondering if there is any problems or concerns about replacing a
> selenium rectifier with a silicon diode? Is there a higher voltage drop
> across the selenium rectifier? I would think so, but is it significant
> enough to worry about?
>
> Mike W0FD

Hi Mike.

Yes, there is a higher voltage drop across the selenium rectifier
than a silicon diode. The silicon diode drops about 0.7 V across
it no matter what the current drawing through it. You can do lots of
damage replacing a silicon diode for a selenium, depending on the
circuit. As an example, I just repaired a battery operated portable
tube type radio that also plugged into the wall to use the mains power.
All the tubes were 1 volt types, like 1R4, 1T5 etc and all the filaments
were wired in series. The selenium device rectified the 115 volts AC
for the B+ "AND" supplied the DC for the filaments. (these tubes have
no cathodes, so need DC to operate) Of course there is a suitable
dropping resistor in series with the rectified 115 V. If I had just cut
out the defective selenium device and wired in the silicon diode, the
resulting higher voltage would have burned out some of the filaments
so an additional resistor had to be added to drop the voltage to an
acceptable level.

By the way, since the filter capacitors had to be changed anyway (they
were dried out and had leaked all over the place) I made sure the new
ones would handle the slight increase in DC voltage resulting from
the lower voltage drop of the silicon diode.

Many years ago, when I serviced VHF 2 way radios that used selenium
rectifiers, the boss had a great idea. Replace the selenium with silicon
as the part was smaller and cheaper. We did. A few days later, all those
radios started coming back into the shop as the filter caps were failing
because the voltage was to high for them. Live and learn.

Regards
Reed

--

ARROW Research  (Avro CF-105)
Reed Park
252 Gauvin Road
Dieppe, New Brunswick
Canada    E1A1M1

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