Given a transformer with a dual primary winding (generally used to step down a series 220v or paralel 110v), can I instead hook up one winding to a 110v input and use the second as an isolated 110v output?
What kind of power restrictions or undesired behavior might this cause?
EDIT: This transformer, for example: http://mouser.com/ds/2/410/media-1067445.pdf
EDIT2: Added 24v to the image secondary coil for clarity
You can if you only need functional isolation. If safety isolation is required you need a transformer designed for that.
Since you are only powering half the primary you can only put in half of the primary current so you only get half the VA rating of the transformer, so compared to a real isolating transformer you end up with twice as much iron and an unused secondary.
Safety: These units are designed with 4000VAC isolation between the primary and secondary, and also,between each winding and the core.
If there had been a comma after "winding" it would be a different story, as it is there's probably only about 250V isolation between the two secondaries, they may well be wound bifiliar (intermixed)
If you are planning on using the isolated primary to do something like supplying Nixie tubes, and the secondary supplies the logic, and safety depends on mains isolation of both, I would not suggest using this kind of transformer. It will work, but it will not have adequate isolation for safety.
There are dual split-bobbin construction transformers and they would be much better, but I suggest checking with the manufacturer to see if they Hipot test between primaries and if they would recommend this service.
The below illustration is from a Hammond 229A12 6VA part:
As you can see, each coil is independently isolated by the bobbin structure.
Why not connect the two "110" windings in series? (Do observe polarity - connect the "dotted" lead of one to the "not dotted" of the other.)
Then apply 110 across the two of them, and you get 110 out, isolated, across the "220" secondary. Or turn it around: Put 110 into the "220" winding and you'll get 110 out across the pair of series windings.
110V in, 110V out, and no loss in rated power.
There is no rule that says that a "220" winding absolutely must be operated with 220 across it.
