Odd problem with simple PS

Hi I wonder if you could provide some insight?

I have build a simple power supply (couldn't get much simpler...) - a soft start circuit, transformer, dual rectifiers and two caps.

The really odd bit is the point between the two caps doesn't sit at 0v relative to chassis earth. It sits at -25v. If you connect the two together then it blows the fuse. I've checked all my wiring and the ps outputs the correct split rail, I just don't understand why there is the -25v potential!

Any help / guidance would be very much appreciated...its for my new UCD180 amps
 
johnnyx said:
How is it referenced to earth? Did you connect one secondary wire to earth? The secondaries should be floating until you connect a point to earth.
If the positive rail is earthed, and then you connect the centre point to earth, it will blow a fuse. Is it 0-18v, 0-18v?


Definitely the transformer, I swapped another one in and that was fine.

My wiring scheme was to make the connection to earth only at one place - between the ps caps on the power supply. In the transformer that is working correctly there is 0v potential between the ps caps centre point and earth. With the duff transformer there is a -25.5v potential, hence the fuse blows if I connect the two points as you would expect. What kind of transformer fault mode is this? I checked the duff transformer and one secondary reads 2v AC from earth, the other 25V AC or thereabouts (neither side of the secondary grounded). The good transformer has the same measurement of 25V on both secondaries.???!!!