trafo outputting higher than rated Vdc

[timsch75]

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]

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

 

[UrSv]

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]

Both times I measured there were no connections after the bridge

 

[blmn]

Put out the snnubers and check de DC output again.

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

 

[K-amps]

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]

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]

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

 

[timsch75]

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]

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.