My uncle recently bought a 550a 3ph welding rectifier with the idea of converting his welder to dc. When he tried checking out the bridge it conducted both ways.
So drilled out all the rivets that connected the sections and tried testing it one section at a time. Using my DMM on diode check, I drop .3 volts one way and 1.2v the other way.
The sections remind me of a selenium rectifier if it helps. Could some one educate me on what I'm seeing and how to properly test this? It sure doesn't test like a normal silicon diode.
Been a loooong time since I've had to deal with selenium rectifiers, but conduction characteristics aren't anything like a silicon or germanium diode. They've got a large forward voltage drop and the reverse characteristic isn't anything like completely "off". In a lot of battery charger circuits, they were used as a primitive voltage regulator in addition to DC-producing duties. That's the reason large ones look like giant heatsinks, they're very lossy. Really big ones had fan cooling. It's unlikely your solid-state meter puts out anything like enough voltage to test a stack properly. I always used a scope and hooked the test victim up to AC(read Variac), if they put out anything like half-wave DC, they passed. Mostly, when I encountered an iffy one in an old TV set, I replaced it with a 10 cent silicon diode and a 5 cent resistor. That was back in the days when it paid to fix TV sets.
Usually, failure amounted to a small fireball and letting copious amounts of stinky smoke out, somewhere between a garlicky belch and a sulfur candle.
Big amperage silicon diodes are/were readily available on the surplus market, your relative would have been far better off buying some of those. I've seen 300 amp bridges for $15-20 in the past, google should be able to locate some.
Nothing more useful to add, but perhaps interesting...
Another thing about those old selenium rectifiers - you could take the separate plates out, remove any existing coating, and they worked as poor efficiency solar cells. When I was about age 12 I scavanged counless TV's from the junk bin of the local repair shop to cannibalize for parts, and this is what happened to some of them...
Forget the bridge. Buy discrete diodes and "roll your own". On my old E.V. application I used 600 amp diodes I bought surplus for $5 each, including heat sinks.
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