Identifying buck-boost transformer windings?

Imagine you are asked to install a used buck-boost transformer. Imagine you could normally do this in a few minutes. Except if the leads were cut short such that identifying characters on the leads' insulation were missing.

Identifying 2 leads belonging to any one winding is straightforward ohm meter work. Maybe use of a ESR meter might help separate the X windings from the H windings?. But identifying which specific winding is which and which end is

How would you go about identifying the windings? Maybe use a Variac to input voltage to each of the windings then measure the output of the others? What outputs should I expect at, for example, the H3/H4 winding with a voltage on H1/H2 winding? How to identify backward connection of a winding?

Are the two H windings identical? The two X windings?

Suggestions welcome.

This is a 208 -> 230 (ie, 12 & 24 v buck-boost voltage) single-phase autotransformer in N. America.


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Yup, put a small voltage on one winding, and see if you get a much larger voltage on another. If so, you can tell which is the H and which is the low voltage one. Once you have identified the high and low voltage windings, you can put them in series to determine which way the phasing goes. If you get, say 50 V out of each of two windings, and then put them in series and get zero, you have them phased backwards. Reverse one winding, and you should get 100 V.


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Jon Elson

Hey dave.. Imagine only sending this to groups that would be interested. Woodworking , probably not


Leon, Frequently wood- and metal-working groups are interested in mains wiring for their equipment. They're not completely disinterested in my experience...



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