What are the ratings of vintage Selenium Rectifier B20/16-2,4?

Question

Sort-of a follow-up to How to ground a vintage device that was powered without a ground? in that the device is the same.

I finally took the leap and opened up the old charger (depicted in the linked question). The plan is to replace the Selenium rectifier/s by Silicon. The Se rectifiers have two marks as follows

• Vertical '15360'
• Horizontal 'B20/16-2,4'

Here is what I hope to find out

• What is/are the ratings of this/these Selenium Rectifiers?
• Should I use 1N4xxx, or 1N58xx in lieu of the Selenium?

Answer 1

You're unlikely to find specs on the selenium rectifier. However, you can probably get a pretty good idea from context. You must know something about the device this is in.

Chances are, a 1N4004 can be used as a replacement, although you might have to put a resistor in series. The old selenium rectifiers had significant resistance, and sometimes the circuit relied on that, like to limit inrush.

These rectifiers were usually limited to the power supply, so taking a measurement of the power supply voltage or otherwise inferring it would be useful.

Answer 2

That looks like a bridge or half bridge rectifier (bridge I would guess).

I agree with Olin's comment about maybe needing a series resistor.

The magnetics look like they could be designed to have a very high leakage inductance so it may be kind of a constant current charger. I have a hunch there is something interesting going on there, it may also be saturable to limit the terminal voltage of the charge.

Typically Se rectifiers have a forward drop of about 1V per junction, so if there is only one junction or two per diode, then 2 or 3 (respectively) Si diodes in series for each Se diode would be reasonably close.

The nameplate indicates up to a couple amperes, so maybe some 1N5404 3A diodes would work for you. Quite possibly the 2.4 on the Se rectifier stack refers to the current rating, and perhaps 16V is the voltage rating.

Needless to say, if you fiddle with the diodes and don't get it right, you could create charging parameters that damage (or fail to properly charge) a battery. Murphy's law (and Vf voltages) would make the former more likely.

If it has that characteristic horrible (and apparently toxic) rotten garlic/onions smell that Se rectifiers give off when they are dying you can be pretty sure it needs to be replaced.