I have hand wound a transformer which has a 220v primary winding, two 25v secondaries and a 15v secondary:
I have centre tapped the 15v secondaries (to later rectify to +-25v) and when measuring the voltage between A-B or B-C, I get 25v as I would expect. Similarly between D-E I get 15v.
However, If I measure the voltage between B-D I get 5v, and B-E gives 15V.
A-D is also at 24v and A-E at 34v. I would have expected these to all be ~0v.
Why am I getting these voltages? I would have assumed that the 15v winding would be isolated from the 25v centre-tapped winding? I have checked there are no shorts between any the secondaries. I should also point out that this is under no load.
As I said I hand wound this transformer (well the secondaries), so is this something to do with the direction of the windings? Or unbalanced windings? I would like the 15v winding to be isolated from the 25v ones. Is this not possible with a shared core?