Odd problem with simple PS

[Zodiac]

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

[Zodiac]

//some fault finding later// it appears the transformer is the culprit. Very odd. I've never seen this before, but one of the secondaries is referenced to earth as you would expect, the other is floating at around 15v. Why is this the case? It was an el-cheapo transformer, have I got a case for a refund?

[johnnyx]

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?

[Zodiac]

Originally posted by johnnyx
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.???!!!

[AndrewT]

Hi,
have you got 0-18 & 0-18? i.e. 4 secondary wires.
or have you got 18-0-18? i.e. 3 secondary wires.

[johnnyx]

Continuity checks will help identify the fault. I had a new transformer that had a short between the mains primary and the inter-winding screen. It blew a fuse whenever it was switched on. Most disconcerting.

[Zodiac]

Originally posted by AndrewT
Hi,
have you got 0-18 & 0-18? i.e. 4 secondary wires.
or have you got 18-0-18? i.e. 3 secondary wires.

Hi its two separate secondaries i.e. 0-18, 0-18

[Zodiac]

Originally posted by johnnyx
Continuity checks will help identify the fault. I had a new transformer that had a short between the mains primary and the inter-winding screen. It blew a fuse whenever it was switched on. Most disconcerting.

What should I be looking for, doesn't appear to be any shorts that shouldn't be there?

[AndrewT]

Hi,
you have said there are no shorts.
Check again to be sure each winding is separate from the primary, from each other and from the core.
Check the Vac from each winding again, don't connect these to any part of the downstream circuit.