[Drake] Re: Selenium Rectifiers

[Barry L. Ornitz]

Henry Hollwedel, K2UYS, offered a dozen selenium rectifiers to 
the list...

My advice is that these pieces of the past are best left 
there.  Selenium rectifiers do age and increase their internal 
resistance with time.  They also tend to lose their voltage 
rating.  If you are restoring old equipment containing 
selenium rectifiers, it is best to leave the rectifier in 
place for looks, but to wire silicon diodes into the circuit 
below the chassis to do the real rectification.

You can estimate the ratings of selenium rectifiers from the 
number of plates in series and from the size of the plates.  
The voltage rating is approximately 25 volts RMS per plate.  
Thus a stack for rectifying line voltage would have 5 plates.  
The current rating is based on the surface area of each plate 
with one square inch providing about 300 mA of current.
Ordinary 1N4007 silicon diodes will replace virtually all 
small selenium stacks used in radio equipment (except for 
those very large plate units for battery chargers).  Because 
of the much higher internal resistance of selenium rectifiers 
than silicon diodes, expect a higher voltage if a selenium 
stack is replaced by a silicon diode.  You can compensate for 
this by adding resistance in series if the extra voltage is 
too high.  For example many AC/DC radios used a selenium stack 
to provide B+ and 150 volt electrolytic capacitors were used 
when the 120 volts line was rectified.  If you use a silicon 
diode, the voltage will be closer to 170 volts unless a series 
resistor is used.  Also remember many selenium stacks are 
internally wired as bridge rectifiers.

Selenium is extremely toxic and if a rectifier shorts you will 
fill the room with noxious, poisonous fumes.  Fortunately the 
smell is so bad few people stay around to get poisoned.

I have a file with more details that I will post after this 
message.  Please read it if you are seriously planning to use 
such rectifiers.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net