Smitty Two wrote:

In article ,
"Arfa Daily" wrote:

"Smitty Two" wrote in message
news
In article ,
Smitty Two wrote:

In article ,
"Arfa Daily" wrote:

"Smitty Two" wrote in message
news Please tell me whether this is an inverter or a bridge
rectifier, if you
recognize it out of circuit context.

http://members.cox.net/prestwich/51.jpg

Selenium stack bridge, by the looks of it. Equipment it's in ?

Arfa

Ancient European slide projector with rotary carousel. Trying to
help a friend get the thing going again, via email since he lives
in another city.

Problem with it is that this motor:

http://members.cox.net/prestwich/52.jpg

turns very slowly.



All right then, here's what happened with this. (Recall that the
motor is 240VAC and has a secondary winding to power some simple DC
stuff through the bridge) My friend disconnected the AC supply to
the rectifier, and presto, the motor returned to normal speed.

Thinking something downstream could be drawing too much current, he
reconnected the rectifier supply wires and then disconnected the DC
side of the rectifier. Motor slowed down. So he replaced the
rectifier, and all is well.

But I'm still puzzled, and since I was never on-site I didn't do
any of the tests myself. If the rectifier had some fault that was
drawing too much current and it pulled the voltage down, how could
he have measured 240 on the slow-turning motor, and 17VAC / 12VDC
on the rectifier?

As it was a selenium stack, I would suspect that one arm was leaky.
Enough to make it draw excess current, but not enough to represent a
'serious' failure that would load up the supply really hard - such as
happens when one arm of a silicon bridge fails short circuit. With
the other three arms functioning normally, the result may well have
been a DC output sufficient for the rest of the circuitry to work.

Out of interest, did your friend replace with a silicon bridge, and
re-measure the AC in / DC out voltages ? Looking again, assuming that
a resevoir cap follows the rectifier, with 17v AC in, you would
expect to see around 24v DC at the output, rather than the 12v that
was apparently measured with the defective bridge in place.

Arfa

Arfa, are you sure about that? I thought you were supposed to divide
by the square root of 2, not multiply by it. Oddly, my friend says
there is no smoothing cap in the circuit. I did not ask him whether he
checked voltages again, but I will.

Arfa is spot on ! 24v as near as makes no difference.

--
Best Regards:
Baron.