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Old 1st November 2004, 12:50 PM   #1
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Default trafo outputting higher than rated Vdc

I'm working on the PS of my gainclone and have a couple of CHIA YU LU (sp?) trafos that I'm running through MUR860 diode bridges. When I hooked it up and powered it, I read ~+-22Vdc. A few days later after putting snubber caps (.022mfd, 200V) across each diode, it now reads ~+-26Vdc. I'm no EE, so could someone please clue me in on what might have happened? Can caps raise the output voltage? If so, it's certainly news to me. BTW, I smelled and heard nothing indicating any problems and the voltages were steady across all four output voltages.

thanks
tim
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Old 1st November 2004, 02:01 PM   #2
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I wouldn't worry. Probably just mains at lowest limit on one day, highest the next.
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Old 1st November 2004, 02:07 PM   #3
UrSv is offline UrSv  Sweden
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Did you have any capacitors after the bridge or were you measuring with no caps?

A difference of almost 20 % sounds a bit high for normal mains variation (I'm used to a few percent maximum).
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Old 1st November 2004, 05:59 PM   #4
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Both times I measured there were no connections after the bridge
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Old 1st November 2004, 06:05 PM   #5
blmn is offline blmn  Brazil
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Put out the snnubers and check de DC output again.

I think it has not to be with the snnubers, but...
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Old 1st November 2004, 06:23 PM   #6
K-amps is offline K-amps  United States
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Is the amplifier loading the PSU or is this a stand alone PSU? A heavy bias can lower supply rails. However if the PSU is stand alone, I'd rip out each component and see whats wrong, a 22 to 26 volt jump is not good.
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Old 1st November 2004, 07:25 PM   #7
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The amplifier chips are not hooked up to the PSU yet. I just checked the voltages coming out of the bridge with it hooked up to nothing.

The thought of pulling off those 16 caps off to test is a bit of a drag. It was a minor PITA soldering those p-t-p across the mur860's.
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Old 1st November 2004, 07:54 PM   #8
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Rated voltage is at rated current. If the transformer is not loaded, you will see slightly higher voltage.
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Old 1st November 2004, 08:02 PM   #9
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I made the same measurement before without snubber caps and without load and got +-22Vdc. I have another set of trafos that measure +-18Vdc at the bridge without load, which is its rated voltage. So two out of three measurements saw no significant increase over rating without load. What would be different here?
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Old 1st November 2004, 08:11 PM   #10
Tim__x is offline Tim__x  Canada
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If there is no load snubbers act as tiny little supply caps. Put a very light load on the psu, 47-100k and then measure it.
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