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Suppose I buy a cheap 12 V, 100 W toroidal transformer. They used to be very common in the halogen light era, so they're easy to find and very cheap.

Is it possible to remove the secondary winding carefully and leave only the primary, sand a little varnish off certain turns so I can tap into them, and solder several outputs from this autotransformer?

In an extreme version of this, if the windings are perfectly flat, I suppose it'd be possible to sand one side enough for a brush to run across it and convert it into a variac.

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If the primary is a single layer, I suppose so. Otherwise, only whatever is outermost accessible. I would not recommend sanding a broad area, as a short circuit is likely.

Be careful that the wire may lay in multiple layers along the inside, but fewer around the outside, i.e. the adjacent turns are not from the same layer but a different layer entirely, so a short circuit between them might be particularly disastrous.

Better to cut one turn only, peel it out, strip the ends and patch them back together, adding a wire lead -- the joints can be protected with heatshrink or other sleeving, and the adjacent turns insulated with tape or glue in case they got nicked in the process.

It's also possible to use the secondary itself as a "buck" winding, in combination with the primary. A smaller transformer can be used, and must be the correct ratio of course. Perhaps if these 12V transformers are very cheap/common, multiple could be used, primaries in parallel, secondaries in series, for various combinations of voltages (including wired as autoformer).

Needless to say, you lose valuable isolation when wired as autoformer. Use only where appropriate, and take adequate precautions.

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  • \$\begingroup\$ thanks. the question was more on the sense of "will the reactance of the transformer be affected by removing the secondary coil and cause it to act weird?". I don't think so since IIRC the inductance of one winding is not affected by the other. \$\endgroup\$
    – hjf
    Dec 30, 2022 at 18:24
  • \$\begingroup\$ Right, the mere presence of a secondary has no effect on general transformer action. \$\endgroup\$ Dec 30, 2022 at 18:51

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