I have a dual primary (0-120V,0-120V) transformer with dual secondaries (0-6,0-6). For a quick test of an amplifier board, I need a 12-0-12 supply.

What would happen if I wired the 2 primaries in parallel (or used just 1) and connect them to the local 240V supply? Logic says the number of primary windings would halve and I would get the 0-12, 0-12 output that I need.

Or would it just smoke?

  • 1
    \$\begingroup\$ Logic says you will probably saturate your core and trip the breaker or worse. It’s all about volt-seconds. \$\endgroup\$
    – John D
    Commented Mar 7, 2019 at 15:51
  • 2
    \$\begingroup\$ Why not use the inputs in series for 240 VAC, the outputs in series for 12 VAC and 2 half wave rectifiers for +/- 12 V \$\endgroup\$
    – Colin
    Commented Mar 7, 2019 at 16:13
  • \$\begingroup\$ ..and that is why (as a digital hardware guy) I have never really understood switch mode power supplies.... \$\endgroup\$
    – par42
    Commented Mar 7, 2019 at 17:18
  • \$\begingroup\$ run a voltage doubler on the outputs then stack them to get a bipolar supply. \$\endgroup\$
    – dandavis
    Commented Mar 7, 2019 at 18:31

1 Answer 1


If the one of the primary coils is rated for 120V and it is connected to 240V, smoke will most likely ensue.

The problem is transformers have a saturation point, this is by design to limit the amount of power that can be transferred for safety reasons. If there is a short on the secondary side, then the power is limited and the full power of mains is limited which reduces fires and wielding that may occur in a product.

The other ratings that shouldn't be exceeded is the coil voltage rating and coil current rating on the primary side.

So check the ratings on the transformer and don't exceed them. Don't put more voltage on the primary coils than rated. Don't put more current through the coils than rated, and don't put more power (the VA, or volt-amp rating) on the transformer.


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