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Landoctor 8th September 2007 06:49 PM

Power Supply problem
 
I am having a problem with a standard dual full bridge rectified power supply(+32,-32). I am getting the ~32V (24VAC input) out of each filtered output from the power supply, but the potential between the two "grounds" of the power supply output has ~25V or so of ac across them? I don't understand how this is happening. Does anyone have any ideas on where to start looking? I have changed all of the diodes and checked all solder connections.
Thanks,
Donovan

KSTR 8th September 2007 07:41 PM

I understand that you have two *isolated* 24Vac secondaries, running into completely seperate full wave bridges and filter caps, right?

You should check the isolation between the secondary windings with an ohmmeter (power off, of course), should read MegOhms. If isolation is given, then you can combine one of the (+) and one of the (-) to get your common ground. The voltage you measured supposedly comes from capacitive coupling between the secondaries which are often bifilar wound. You also can check the cross current instead of the voltage, should definitely not be above a few mA. Preferred style of measuring would be the voltage across a, say, 100 ohms 10 watts resistor. If it runs hot you're in trouble...

Usual Disclaimer:
Please use proper fuses and the "lightbulb trick", and most preferably a ground fault interruptor when checking xformer issues live.

- Klaus

Landoctor 8th September 2007 07:46 PM

Sorry, I forgot to mention that this is using a center tapped secondary. ~48VCT transformer going into the bridges.
I have tried two different transformers that are both using the same style secondaries with the same results.

KSTR 8th September 2007 07:50 PM

Then you can't use separate bridges. Your common GND point is the center tap. Just like in here:
http://www.paia.com/KRUKits/K114/k114.pdf

- Klaus


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