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Old 11th December 2002, 01:51 AM   #1
biff584 is offline biff584  United States
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Question burning bridges

I've been having a problem burning bridge rectifiers- I'm trying to test the power supply for a P3A amplifier before I mount it to my chassis. I'm testing it by plugging it into a variac to slowly increase the voltage. I wanted to see the waveform produced on the output of the bridge, so I connected a scope across it (the circuit was just the variac, the transformer I plan to use, and the bridge)- every time I've done this I see the pulsating DC waveform nicely until I reach about 30vAc input, then the bridge smokes. One of my prof's thinks the oscilloscope may, in effect, be acting as a connection and shorting the output. Does this sound right?

-Brian
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Old 11th December 2002, 01:58 AM   #2
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What happens if you do not connect oscilloscope?
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Old 11th December 2002, 02:00 AM   #3
biff584 is offline biff584  United States
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The circuit appears to operate normally without the oscilloscope- but I've only tried it up to about 60vAc input.
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Old 11th December 2002, 02:24 AM   #4
grataku is offline grataku  United States
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Biff,
what prof may that be? Political sciences?
I have never heard of a scope doing that, but then again what do I know? I do forget to switch my multimeter back to voltage reading from AMP reading all the time. That is essentially a short.
The scope I deal with can be set at either 1 Mohm or 50 ohm even if it set to DC coupling I find it hard to believe it would act as a short.
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Old 11th December 2002, 02:32 AM   #5
JBL is offline JBL  Canada
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Make sure that you put the ground pin a the good place.

Also I think that some scope have a switch to make their input floating.
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Old 11th December 2002, 02:58 AM   #6
tiroth is offline tiroth  United States
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I second JBL's advice...though I wonder that you don't smoke things immediately if that is the problem.
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Old 11th December 2002, 04:20 AM   #7
haldor is offline haldor  United States
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Please describe your power supply. Is it transformer isolated or are you using an off-the-line switcher?

If you are using an off-the-line switcher supply then the "ground" terminal of the power supply is probably somewhere between 60 to 100 V potential relative to earth ground. When you connect the ground clip of your scope probe you are shorting the power supply ground to earth ground through the scopes line cord and the regulator goes "poof".

If you are using a traditional transformer then something in your wiring is seriously amiss.

I suggest powering up the amp without the scope and measure between the power supplies "ground" and earth ground. You probably won't be happy with what you measure.

Incidentally, I think your power supply is not connecting chassis ground of your amp to earth ground (outlet). If so this is a major safety violation and is likely to kill someone. If your power supply was grounded correctly the bridge rectifiers would be burning up even without the scope connected.

Phil
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Old 11th December 2002, 04:40 AM   #8
biff584 is offline biff584  United States
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The power supply I was testing was a variac supplying a standard transformer connected directly to a bridge- nothing else (b/c I just wanted to see the output from the bridge)- not in a chassis, just on the bench. I will try it again tommorrow double checking the earth ground before I begin. Thanks for the help.

-Brian
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Old 11th December 2002, 04:56 AM   #9
haldor is offline haldor  United States
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Hi Brian,

Thanks for the info. This sounds very strange. Perhaps your transformer has a shorted winding?

Phil
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Old 11th December 2002, 07:41 AM   #10
yeti is offline yeti  Germany
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Default bridge

If i got you right, your variac isn't mains isolated.
Usually the ground connection of a scope is earthed.
So you'll get a short when you try to connect it to the bridge.
Try to use an isolation transformer for the mains of your scope, then your potential will be earth-free.
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