| biff584 |
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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| Peter Daniel |
| What happens if you do not connect oscilloscope? |
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| biff584 |
| The circuit appears to operate normally without the oscilloscope- but I've only tried it up to about 60vAc input. |
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| grataku |
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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| JBL |
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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| tiroth |
| I second JBL's advice...though I wonder that you don't smoke things immediately if that is the problem. |
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| haldor |
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. :att'n:
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). :forbiddn: If so this is a major safety violation and is likely to kill someone. :RIP: If your power supply was grounded correctly the bridge rectifiers would be burning up even without the scope connected.
Phil |
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| biff584 |
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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| haldor |
Hi Brian,
Thanks for the info. This sounds very strange. Perhaps your transformer has a shorted winding?
Phil |
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| yeti |
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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| biff584 |
That sounds right to me- it's very old variac and I don't know anything about it. Thanks alot.
-Brian |
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| JohnG |
If it is a variac supplying a standard transformer, and you are measuring on the secondary of the transformer, you should not be having any serious problems.
I would also suspect a miswired or faulty transformer, assuming your "standard transformer" following the variac is NOT an autotransformer. Note that variacs are autotransformers and provide NO isolation.
Also, it is a bad idea from a safety standpoint to float the scope with an isolation transformer. It can put your entire scope at high voltage; knobs, probes, and everything. One touch and your DIY career (and all your other careers) could be over. The one time I have see this done in a semi-safe manner, the scopes were behind a plastic barrier and plastic sticks were used to work the controls. This was many years ago; we use amplified diff probes now. |
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| biff584 |
Just retested the power supply today- I didn't try to measure the output across the bridge- just across the cap's. Everything worked fine- no smoke.
-Brian |
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| tiroth |
| I suspect grounding one of the rails was to blame. |
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| skaara |
| quote: | Originally posted by biff584
Just retested the power supply today- I didn't try to measure the output across the bridge- just across the cap's. Everything worked fine- no smoke.
-Brian |
Isnt that the same? |
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| biff584 |
No- before I put the probe directly across the bridge, with nothing else connected to the bridge. This time I added the two filter caps, and placed the probe from one cap to the ground (center tap of transformer tied to earth ground).
-Brian |
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| skaara |
oh:)
Its working, and only that matters:D |
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| tiroth |
| quote: | Originally posted by skaara
Isnt that the same? |
Well if you literally measure "across the bridge" you'll be grounding one rail! |
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| biff584 |
| Yeah- I figured that one out the hard way! |
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| grataku |
Well if you literally measure "across the bridge" you'll be grounding one rail!
Uh? |
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| haldor |
| quote: | Originally posted by tiroth
Well if you literally measure "across the bridge" you'll be grounding one rail! |
That's corrent when using a scope, but even so unless there is something misswired or shorted in the transformer you still shouldn't be burning the bridge rectifer up.
Now if you have the center tap of the secondary grounded and try to measure from rail to rail with a scope, that would toast the bridge.
Phil |
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| SkinnyBoy |
| Do you have a fuse.. fuses are cheaper than bridges... :) |
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| halojoy |
| quote: | Originally posted by SkinnyBoy
Do you have a fuse.. fuses are cheaper than bridges... :) |
tank u very much - next ya tank mi |
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| R. McAnally |
"burning bridges" -- LOL
did anyone else see that?? |
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