Faulty Toridal Transformer

[brian_mk]

The transformer is 15VA with two primary windings, 120V each.
There are two secondary windings, 12V each. (Red & Black, Yellow & Orange).

The two secondaries are supposed to be electrically isolated but there appears to be an internal short.
In my intended application, the two secondaries must be kept separate.

If I measure the DC resistance of the secondaries (transformer disconnected), I get the following:-

Red - Black: 0.6 Ohms
Yellow - Orange: 0.6 Ohms
Red - Orange: 1.8 Ohms
Black - Orange: 2 Ohms
Black - Yellow: 2 Ohms
Red - Yellow : 1.6 Ohms

This doesn't make much sense to me.
Can anyone come up with a diagram showing the winding resitances and the short that agrees with the measured results?

[Andy Watson]

How accurate is your meter?
Have you allowed for the resistance of the test leads?

[edpalmer42]

Does it matter?  The transformer is defective.  If it's not potted, you might be able to repair it.  Otherwise, it has to be replaced.

[brian_mk]

Just out of interest, I decided to unwind the faulty toroid to see if I could locate the short.

Both the primaries and secondaries are 'bifilar' wound.
The two primary windings (many hundreds of turns of thin wire pairs) form the outer layers, so I had to cut those away first.

The secondary windings comprise about two layers of thicker wire.
As I gradually unwound the secondary windings, the resistance of the 'short' gradually increased to several tens of Ohms.
The short only went away completely after I unwound the secondaries to the point where the upper layer began to overlap the first.

Using a continuity tester I managed to find the spot where the enamel coating had failed.
I could see no obvious signs of heating so I guess the failure was down to the wire enamel coating or mechanical abbrasion during the winding process.

I think I could have made use of the faulty transformer in a single 12V supply provided the other secondary remained unused.
For the sake of £20 for a replacement, probably not worth the risk.

Is there anything useful I could do with a now empty toridal core?

[Xena E]

You could use it to make an LF current transformer if you wanted one?

X.

[edpalmer42]

It's too bad the primaries were on the outside.  I thought that, for safety reasons, the primaries were on the inside so that the users were better protected from the high voltages.  In fact, I remember modifying a rather large toroidal transformer by adding a center tap to the secondary.  It was right on the outside - no unwinding required.

[Gyro]

Yes, that's really unlucky. I've never come across a mains toroidal with the primaries on the outside. I've modified or rewound several with extra secondaries or modified voltage. The primaries are normally safely out of the way under decent layers of insulation with double insulated leads emerging through the secondary.

I suppose there's no particular reason for 'normal' layup. It does provide mechanical protection for the primary and probably allows manufacturers to stock standard part wound transformers, for completion according to demand, too. The secondary windings are usually of thicker wire too, so benefit from a larger corner radius.

It sounds as if it was maybe a poor quality transformer - especially if the insulation failed due to mechanical stress.