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Old 19th August 2004, 05:32 AM   #1
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Exclamation Unequal Power Supply--Please Read

Tonight I roughed out a power supply for the Holton AV800 amp I am building and when I checked the rails of the supply the negative rail was about 96v and the positive rail was about 92v.

What the heck?

Both windings have identical output.
I am using some large capacitors and I only used two per rail instead of the planned 5 per rail, could the capacitor tolerance account for the discrepency?

Thanks for your comments.
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Old 19th August 2004, 06:50 AM   #2
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What do you mean if you say: "both windings have identical output"?

Jan Didden
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Old 19th August 2004, 07:14 AM   #3
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I am assuming he means that when read with a multimeter/scope not in the circuit the secondary outputs (positive and negative) with respect to ground are opposite but equal in magnitude and when he has the rectifying and filter caps hooked up, the rails are uneven. I don't know the answer to why, just thought it would help to clear things up though.

Its possible one rail contains a bad cap that has a modest (READ more than miniscule that all caps have) resistance. Try swapping caps between rails and see if the problem reverses itself.
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Old 19th August 2004, 07:40 AM   #4
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Well, you assume a lot. I'd like to hear it from the horse's mouth before starting on some speculative course. After all, if you don't know the indications, how can you solve the problem if you don't know what it is?

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Old 19th August 2004, 08:01 AM   #5
ekaerin is offline ekaerin  Sweden
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Frazzled,
I would try to put a light load on the supply. Maybe a some 100mA on
each polarity to GND.
Then measure again.
With that voltage bleeder resistors are called upon for discharge in
case of service after a failure. Try put in 14.7 kOhm / 1 W on each rail.
Maybe that is already enough to EQ your voltage.

/ Mattias
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Old 19th August 2004, 12:19 PM   #6
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dswiston You are correct. When I measure the AC voltage from each winding without rectification or filtering (2 secondary windings for +/- supply) I get 68.2V from each.

I have 5K 8watt bleed resistors on each rail. The resistors were matched and are exactly 5K (that took some time to do let me tell ya)

I am going to try switching the caps around to see if that has any effect, it was just too late last night to start fooling around and i was tired and didn't want to make any stupid mistakes.

Thanks for the help.

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Old 19th August 2004, 12:26 PM   #7
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Default Re: Unequal Power Supply--Please Read

Quote:
Originally posted by Frazzled
Tonight I roughed out a power supply for the Holton AV800 amp I am building and when I checked the rails of the supply the negative rail was about 96v and the positive rail was about 92v.
The amp may not have totally equal current consumption from each rail. (I don't have the schematics in my head right now). In other words: Nothing is wrong
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Old 19th August 2004, 12:35 PM   #8
MikeB is offline MikeB  Germany
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It seems the positive rail has some strange defect. 68.2volt from
the windings should result in some 96.4v.
Is something heating up ? What is the voltagerating for your caps ?
Maybe the rectifier is defect, or does not withstand 200volts ?
I do not believe that a cap is defect, it is very likely it would have blown
already. (With these voltages: detonated !)

Mike
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Old 19th August 2004, 12:59 PM   #9
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Sorry forgot to specify that the supply is just sitting on the bench not hooked up to the amp.

As I said I am going to switch the capacitors with 4 different ones this morning to see if there is any effect.

Quote:
It seems the positive rail has some strange defect. 68.2volt from
the windings should result in some 96.4v.
Is something heating up ? What is the voltagerating for your caps ?
Maybe the rectifier is defect, or does not withstand 200volts ?
I do not believe that a cap is defect, it is very likely it would have blown
already. (With these voltages: detonated !)

Mike
Nothing is heating up and measuring current on the primary side gives a reading of near ZERO. I have tried different bridge rectifiers from different manufacturers and they are rated at 600V @35 amps.

I'll update shortly.

Thanks for the suggestions!
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Old 19th August 2004, 01:05 PM   #10
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This is indeed fishy. Unloaded, the caps should charge to the same (peak) value. If there was a difference loaded, I could imagine that there was an error in the bridges, and that the lower side would only have half-bridge rectification.
A difference in cap leakage could also cause it, there will be a relative large difference between zero load and a little load. Let's see what your cap switching gives.
Can you post the schematic, how you have interconnected the secondaries, bridge, caps?

Jan Didden
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