I found the picture below on the Internet about a double primary winding transformer. I understand how a simple primary double secondary winding transformer works, but I don't see the point in making a transformer like the one found below (output is at the bottom of the picture, input at the top of it).

Enter image description here

Is it the same thing when you put two voltage sources in parallel so you can get twice the current at the output (so you get twice the power)?

  • 4
    \$\begingroup\$ You can wire them in series for 230V or parallel for 115V, and sell the same product in either domain. In parallel, you are correct, you lower the impedance, reducing resistance and therefore wasted power. \$\endgroup\$
    – user16324
    Jul 8, 2016 at 13:01
  • \$\begingroup\$ Wow ! Such a quick feedback from all of you ! Thanks ! If I put the same AC signal on both primary windings (something like sound for example), what am I getting at the output of the transformer ? \$\endgroup\$
    – RWIN
    Jul 8, 2016 at 13:07
  • 2
    \$\begingroup\$ About 1/5 of the amplitude, some loss of bass and treble, and the ability to drive a lower impedance load. It's quite common for PA systems to send 100V audio signals round a building, with a transformer like this built into every speaker. Search for "100V line PA" for more... \$\endgroup\$
    – user16324
    Jul 8, 2016 at 13:24

3 Answers 3


Some countries use 110VAC mains, others 220VAC. So, transformers would need to be constructed differently depending on the country from where it is used.

To avoid that, some manufacturers are selling transformers that have two primary windings. In 220V coutries, you wire the windings in series (as if the number of turns would double, thus doubling the voltage ratio), and in 110V countries, you wire them in parallel (as if the number of turns was the same, but with double wire section, thus double the current carrying capability).

With a single transformer design, they can sell everywhere.


Moreover, with some switch arrangement, you can make the transformer primaries configuration selectable, so the same end product can be sold everywhere in the world (provided that the switch is configured correctly depending on where the product is distributed).

Profit again!

  • \$\begingroup\$ You could have a four wire female stub and have different connectors connect the windings differently. Is that a bad idea? \$\endgroup\$ Jul 8, 2016 at 17:32
  • \$\begingroup\$ @JanDvorak Not a bad idea. One has to consider the cost of the switch vs. the cost of connectors, and the difference in assembling costs between the two solutions, but that is certainly very similar. \$\endgroup\$
    – dim
    Jul 8, 2016 at 18:11
  • \$\begingroup\$ The solution with a switch allows for a user mistake. User mistake that can happen will happen. And US gets the short straw by having their appliances explode rather than simply not work. \$\endgroup\$ Jul 8, 2016 at 18:18
  • \$\begingroup\$ @JanDvorak That's right. But the switch can be located internally, not being accessible to the user. Well, I think we're splitting hairs, anyway. The idea is the same. \$\endgroup\$
    – dim
    Jul 8, 2016 at 18:21
  • \$\begingroup\$ I've seen lots of products in the U.S.A., that have a voltage switch. The switch always is pre-set at the factory to 120V before they ship it to the U.S.A., and it usually is not easy to switch it by accident. The switches have been disappearing from new electronic devices---newer devices often say something like, "100-250V, 50-60Hz" at the inlet. But switches on motorized devices still are common. \$\endgroup\$ Jul 8, 2016 at 21:15

This is so that in 110 V land, you connect the primaries in parallel, and in 230 V land you connect them in series. In each case, each primary winding takes about the same current, so the same heating and same transformer action.

One problem with this setup is that if you connect the 110 V configuration to a 230 V supply, you get smoke.

Modern laptop power supplies with a 'universal' voltage input from 90 V to 260 V work in a completely different way, without using a mains input transformer like this.


The transformer you've illustrated would be easy to re-wire for 220V use.

At 110V, wiring the primaries in parallel reduces the current through each winding.

At 220V, wiring the two secondaries in series doubles the primary voltage rating. The current should be roughly halved when operating on 220V, so no the primaries will not be overloaded.


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