Bipolar PSU strange output voltages?

[composition4]

Hi, I've just tested a +/- 12V power supply and I'm getting some strange voltages...

The part of the circuit I'm testing is pre- voltage regulators, just raw rectified and filtered DC.  So I'm expecting maybe around +15V on the top rightmost terminal (with respect to the 0V centre terminal) and -15V on the bottom terminal.  Instead I'm getting about 24V DC on the top terminal and -10V on the bottom...

I tested it at the output of the transformer and it seems right (+/- 12V AC) so it's either my layout or bad bridge rectifier or filter caps.  The square pads are where the positive cap leads go, just for reference.  So before I replace these, can anyone see anything that I'm overseeing?  Pretty basic stuff, just can't understand what I've done wrong and I'm guessing I'm overlooking something (it IS pretty late here!)



Oh, the yellow trace between the transformer and the rectifier is a jumper, by the way
Thanks!
Jonathan

[petemoore]

  Output voltage may drop when load is attached.
  Rated for 12v @ X-high current, a PS may measure 18v with no load, near 12v when current load is heavy.
  Regulators could be used to provide a fixed voltage, [set up right] which is not dependant on the current draw.

[composition4]

Thanks for reply Pete... yep I do have regulators after this section of the circuit, they're just in a different enclosure... haven't tested with them yet.

Yeah I thought maybe it's that the transformer might need a load, but then when I tested the output of the transformer and it was right (+/- 12VAC), I figured that lack of loading isn't the issue.  And the transformer is a little 175mA pcb mount, so definitely not a high current transformer.

Hmm any more suggestions?

[merlinb]

Theres nothing wrong with the layout, so I would suspect something is wired up wrong (backwards cap?) or something is faulty.

[Gurner]

I've studied your board, and believe your problem likely relates to having no components on it.

[composition4]

I replaced the caps and the bridge rectifier... same result!  I really am stumped, it's such a simple little PCB...

I'm guessing the problem is at the bridge rectifier, because the caps wouldn't make the voltages strange like that.  And I've definitely got the pinout of the bridge rect right, as I've used the same ones in another (single sided) PSU circuit yesterday and they work as expected.

I'm trying to figure out what could be making the voltages the particular values of +24 / -10, but can't even work out what would be doing that

And gurner: "hah hah"

[composition4]

Okay, solved.  Replaced the original faulty electro with another faulty electro - by chance I replaced the same one with the next faulty one.  Pretty slim odds of that happening I imagine, but it shows it WAS the caps after all.  After a THIRD replacement all is good now, getting 29-0-29V DC now, unloaded of course - I expect this will go down once loaded with the voltage regulators and circuit load.

Thanks guys
Jonathan