"Smitty Two" wrote in message
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"Arfa Daily" wrote:
"Smitty Two" wrote in message
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In article ,
Smitty Two wrote:
In article ,
"Arfa Daily" wrote:
"Smitty Two" wrote in message
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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.
Hi Smitty
Peak DC out is root 2 (1.414) *times* the RMS AC in, so with a resevoir
cap of sufficient size, and not too much loading and source impedance,
that's about the level of DC that you would measure on a normal multimeter.
You are thinking of calculating the AC RMS figure from the DC where you
*divide* the DC level by root 2 (or multiply by 0.707).
The fact that there is no resevoir cap, throws a bit of a spanner in the
works when it comes to measuring the 'DC' out from the bridge because,
whilst by strict definition it *is* DC, what you will actually have is a
train of unipolar pulses, which the meter will make some attempt to
integrate into a DC level that it can display. Obviously, this will be a
misleading figure, and may even differ between an analogue, and a digital
meter. The meter reading of "12" might well have been 'correct' in this
case - if you see what I mean !
I wonder what additional circuitry this supply is powering ? It can't be
anything 'electronic', as such a raggedy-arsed rail would play havoc ...
Arfa