Dave M wrote:


Actually, they ARE heat sinks. Selenium rectifiers were much less efficient
than silicon, or even germanuim rectifiers, with a forward voltage drop of
about 1V per cell (plate).

** But silicon diodes like 1N4004s or 1N4007s you see everywhere have a similar voltage drop - about 1V at 1 amp.

https://www.onsemi.com/pub/Collateral/1N4001-D.PDF


That means that if your rectifier has 16 cells,
then the rectifier, as a whole, will have AT LEAST 16 volts of forward
voltage drop. Multiply that by the current through the rectifier, and you
have the number of watts that has to be dissipated, thus, the need for the
heat sink plates.


** But the *REAL* reason is the low reverse voltage capability - 25V per diode rather than 400V or even 1000 volts.

FYI: Silicon diodes are remarkable devices - a finger nail size, 4 diode bridge is adequate for a 1kW DC supply using only the PCB foil as a heatsink.

https://au.element14.com/vishay/w10g...1497580?st=1.5 amp bridge

Think there is another reason too in that Selenium diodes cannot be allowed to run as hot as Silicon, or their life span is drastically shortened.


.... Phil