Selenium rectifiers

[Findm-Keepm]

Originally Posted by Kamakiri

Okay, so here's where I'm at with my selenium dilemma....

As posted in another thread, I'm replacing .320a selenium rectifiers, the rating chart is here:

http://www.videokarma.org/attachment...0&d=1380216772

These are the diodes I intend to use, 3 amp 1000 v. Since diodes are cheap, might as well step up the rating a bit, just for good measure:

http://www.mouser.com/ProductDetail/...fp8QPCBfkcQ%3d

Since this set isn't working, I'd prefer to just get a resistor value and replace it. It's been recommended to me that I get a 25 watt resistor as opposed to a 2 watt, because of how hot they run. Does that sound right?

And what value resistor would I run?

Well you know the measured current - what is the drop (in volts) across the selenium rectifier? With that, you'd have two of the three variables you need, and could plug them in ohms law for an answer.

Years ago, we made a selenium "drop box" - it was a rotary switch and 23 silicon diodes in series, with the common selecting a junction in the series string of 23 diodes. We set it for the highest forward drop, monitored the B+, and started down with the switch. When B+ was in, we stopped, multiplied the number of diodes that were in series times .7, and arrived at a voltage drop. A 1 ohm resistor in the box (in series with the diodes) provided a unity arrangement to measure current in the circuit (320mV across the resistor=320mA of current in the circuit). A simple R=E/I and we had our drop resistor value. I wish I still had the box, or the money to build another. The 23 position rotary switch we used is now some 81 dollars, so it ain't cheap to build. I've seen others that use a series rheostat from Ohmite (60 ohms, rated at 750mA max current) to do the same we did with diodes.

Measure the DC voltage drop across the selenium rectifier (Vforward) and you'll have E - the Sams gave you I (.320A), and you need "R". Simple Ohms law.

Cheers,