Unknown component...

[russ57]

This is in an elma antimag demagnetizer.
I think it is just a pair of diodes. But I'd like to be sure. It has 3 terminals and measures about 5Mohm both forward and reverse across each supposed diode.

[BillTodd]

looks like some kind of rectum frier to me .

Have you a diode test setting on your multimeter?  if so you should be able to read the forward voltage and check each diode.

Bill]

[russ57]

I do, but it is effectively open circuit.
(ie, I think it is dead). Drawing out the circuit to make sure diodes make sense.
(not some odd scr or other complexity)

[Muzzerboy]

Looks very similar to this full bridge rectum frier. But perhaps just a half bridge with 2 diodes.
https://www.ebay.co.uk/itm/124664837051

[Sea.dog]

It's a half-wave rectifier.

[russ57]

Yep, and to be precise it (was) a selenium rectifier...
I removed and opened it, to the accompanment of falling pieces.
So maybe it had failed, or maybe not, as measuring a stack of selenium junctions with a multimeter Does Not Chooch...
But from what I read of selenium if it wasn't dead, it was dying.

Replaced with 2 x 1n4007. All functional.

[russ57]

(and for interest I traced the circuit and loaded it into an online circuit simulation. It is functioning as a voltage doubler, pretty much straight off the mains. Although I could not perform the necessary topology gyrations in my head to make it look like a doubler. Charges a big cap to 700odd volts which are then dropped across a coil.

[BaronJ]

Hi Russ,

The only problem with using silicon diodes to replace a selenium rectifier is that the voltage drop will only be 0.7 volts per diode compared with maybe three or four with the selenium one, which will also vary with current draw. Depending upon application and load a few ohms series resistance might be required to get back to the original working conditions.