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-   -   trafo outputting higher than rated Vdc (http://www.diyaudio.com/forums/solid-state/44581-trafo-outputting-higher-than-rated-vdc.html)

timsch75 1st November 2004 12:50 PM

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

richie00boy 1st November 2004 02:01 PM

I wouldn't worry. Probably just mains at lowest limit on one day, highest the next.

UrSv 1st November 2004 02:07 PM

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).

timsch75 1st November 2004 05:59 PM

Both times I measured there were no connections after the bridge

blmn 1st November 2004 06:05 PM

Put out the snnubers and check de DC output again.

I think it has not to be with the snnubers, but...

K-amps 1st November 2004 06:23 PM

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.

timsch75 1st November 2004 07:25 PM

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.

cunningham 1st November 2004 07:54 PM

Rated voltage is at rated current. If the transformer is not loaded, you will see slightly higher voltage.

timsch75 1st November 2004 08:02 PM

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?

Tim__x 1st November 2004 08:11 PM

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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