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Testing an old welding rectifier

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.

Thanks,

Wes
 
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.

Thanks,

Wes

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.

Stan
 
T

Too_Many_Tools

Jan 1, 1970
0
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.

Stan

Thanks for a good informative post.

Many years ago as a kid I worked on an older battery charger whose
selenium rectifier had gone bad. I never could get it to work right
with just a diode....too high of a charging voltage.

TMT
 
D

Dr. Anton T. Squeegee

Jan 1, 1970
0
[email protected] (known said:
My uncle recently bought a 550a 3ph welding rectifier with the idea of

<snippety>

Others have already created some good advice for fixes.

I just wanted to mention that, for some bizarre reason, my gray
matter parsed the subject line as "wedding rectifier."

Go figure. ;-)
 
C

clare at snyder.on.ca

Jan 1, 1970
0
Selenium rectifiers test funny..... Thats the only thing I remember
about them.... If they smell bad they usually are.

John
And when they go bad, they REALLY smell bad. Like rotten eggs.
And about 20 volt drop instead of Silicon's .6
 
J

John

Jan 1, 1970
0
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.

Thanks,

Wes

Selenium rectifiers test funny..... Thats the only thing I remember
about them.... If they smell bad they usually are.

John
 
R

Ray

Jan 1, 1970
0
Where would I look to find 300 amp silicon bridges for a reasonable price ?
I have googled to no avail...
 
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