# Identifying the winding that has multiple taps?

## Main Question or Discussion Point

hi guys
Do you know a way of identifying the winding that has multiple taps?
Do you know some material on internet that explain this better?

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vk6kro

Suppose you had a transformer with multiple primary taps so that it could be used on different mains input voltages.
It might have a single 15 volt secondary winding. This will normally be wound with thicker wire than the primary.

The transformer will work backwards. That is, if you feed 15 volts AC into the thicker winding from some other transformer or power source, then the primary will develop voltages that are similar to the intended input voltages. They may not be exactly right, but they could be close enough to let you estimate what the transformer was for.

If you only had 6 volts AC and the thick winding was not labelled, you could still try it and get an idea of the type of primary winding on the transformer.

For example, you might get 55 V, 60 V and 65 V. If you lived in America, you would suspect that this transformer was intended to give about 12 volts out and these were primary taps for 110 volts +/- 10 volts.

If it was a discarded transformer and didn't smell like burnt paint, you could just try it with 110 volts on the primary. Nothing to lose and you might find a useful transformer.

Be careful with high voltages.

Another note - transformer designers usually wind the primary first. I think it's because it eases the burden of laying down multiple secondary's. In any case, it seems to have become a tradition.

The primary winding / windings will always have a hefty bit of insulation between it / them and the secondaries. Many transformers were wound with two primaries - each for 115 volt. When used in the USA, they are wired in parallel. In Europe, they are wired in series for 230V.

PS - A 100W lamp in wired series might keep you from a disaster and allow you to measure the relative voltage of the windings

Careful measurement of the dc resistance between the common and the individual taps will determine the sequence of the taps and relative number of turns between taps.

Bob S

Bob S,
I want to know I way of identify a winding that has multiple taps.
If this way is visual or if I can do an essay to find it.

Bob S,
I want to know I way of identify a winding that has multiple taps.
If this way is visual or if I can do an essay to find it.
Find all the taps on a transformer coil that have a finite (low) common resistance. Very carefully measure the resistance (using milliohm scale if necessary) between all taps and the common tap, and deduce the sequence of taps on the winding (more resistance means more turns) based on the relative resistances.

Bob S