How do you test a Selenium Rectifier?

A DMM won't read correctly. No enough voltage compliance on the current source to read ohms correctly. Just replace it with a 1N4007 diode and about 100 ohms series resistor.

--
"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com

Selenium stacks have high Vf & high reverse leakage. DMMs don't apply enough voltage. Probably the simplest way to test is to power it up. They fail by Vf increasing, causing overheating & lots of toxic selenium fumes, so best not tested indoors.

Replacement is probably a wise option. But add a resistor & fuse, seleniums act as current limits. There are lots of old scopes still running with selenium EHT rectifiers in them.

NT

True, but it *might* give some rough indication, like if the thing has somehow shorted out. I have a few of those lying around somewhere; I'll measure 'em with a DVM just out of curiosity and see if I get readings like those reported by tabbs.

Not kosher if you want to keep the internals original! Many collectors are extremely fussy about that.

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I replaced some parts on my 1934 set, but you'd never know by looking inside. The new parts are inside the old, which are repacked exactly as original. With selenium you could possible hide the diode somewhere not noticeable, keeping the selenium in place.

NT

To maintain the authentic smell?

--
Adrian C

The same way as with any other diode. However the selenium drop about a volt for each plate that is on it. Each plate is only good for about

25 volts , so you may have 5 to 10 volts to overcome. Therefore most meters will not have enough voltage to over come that drop.

Get a voltage source of around 10 volts or more (maybe even a 9 volt battery will be enough) use your meter on the voltage scale and put hte battery , diode , voltmeter in series. Then turn the voltage or diode around and measure again. There should be a couple of volts dropped one way and many more (almost all) the other.

Yes, I'm aware of this practice, but for some super-fussy types, it's still not good enough.

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+1

And watch out for confetti. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

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** Most DMMs have separate ohms and diode tests ranges, the ohms ranges are for " in circuit" resistor testing and will not make a good Silicon or Selenium diode conduct.

** Really ? Only the wimpiest rectifier has 100 ohms series resistance, like a Germanium detector diode.

The OP's Selenium rectifier powers an AM / FM radio,( minus less the tube filaments ) so ought to be good to half an amp - so maybe try 22 ohms @ 5watt.

.... Phil

nside. The new parts are inside the old, which are repacked exactly as orig inal. With selenium you could possible hide the diode somewhere not noticea ble, keeping the selenium in place.

obviously there won't be any smell from a selenium rectifier that isn't see ing power.

NT

I like my kit to work. If some folk want theirs dead, I guess that's their call. There was no chance of it working without repair. FWIW having old kit that doesn't work is missing much of the experience.

With the selenium rectifier, you could still use it if it still works, but its toxicity is a problem that needs addressing somehow. That gets difficult, though not impossible.

NT

Seleniums have a shelf life. By their very nature the go bad over time. Thus any selenium out there is bad as they have not [at least in the US} been made in a shelf life time. CP

And, IIRC, a horrible smell of cabbage!

Mike.

Cabbage that's already been digested...

wow I always thought those were just heat sinks

I seriously would not have guessed that I would learn something new about selenium rectifiers today. :-)

mark

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). 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.

I remember selenuim rectifiers being used in car battery chargers, capable of charging at 50 amps. The plates, as I remember, were about 8" square, and had to have a fan inside to cool the rectifier since there was so much heat to be removed.

Cheers, Dave M

snipped-for-privacy@yahoo.com wrote:

I remember selenuim rectifiers being used in car battery chargers, capable of charging at 50 amps.

Did not most battery chargers use copper-oxide rectifiers? As I understand it, they are both more rugged and more weather resistant than selenium. On the other hand, they are very similar in appearance.

Peter Wieck Melrose Park, PA

Yes, the actual plates are heat sinks, but I was loose with my words and just using that as a refference as to how many sections there are. That is each plate coresponds to a diode type junction in series. So if you count 5 plates it is similar to having 5 dioides in series. Each one good for about 25 volts of inverse voltage and dropping about a volt. That would make around 5 volts forward drop and good for about 125 piv.

Using the term plate was to me just a quick way of estimating how much voltage drop and PIV the stack could handle.