There is a little switch on the back of older computer power supplies to select between 110 and 220 volt AC input. I imagine this switch enables/disables a 220 to 110 volt transformer. Or perhaps a different channel within the same transformer unit is enabled/disabled

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    \$\begingroup\$ Please unaccept my answer and accept the other - I intend to delete it as it's not correct \$\endgroup\$
    – nanofarad
    Jun 16, 2022 at 1:03
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    \$\begingroup\$ ↑ This is the reason, why it is advised to wait at least 24 hours before accepting answer: to give chance for discussion, corrections and other, more correct or more complete, anwers to be posted. \$\endgroup\$ Jun 16, 2022 at 12:34
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    \$\begingroup\$ All of this also only applies to old power supplies (that still have the manual switch). These days, good quality power supplies use "Active Power factor correction", which is basically a big step-up / boost converter which will take any input voltage and create a stable, high voltage (~400+ volts) DC supply rail. \$\endgroup\$
    – Manawyrm
    Jun 17, 2022 at 8:48

1 Answer 1


It is a very simple trick:


simulate this circuit – Schematic created using CircuitLab

If SW1 is open, the 4 diodes work as a standard bridge rectifier. If V1 is 220 VAC you can expect a rectified voltage around 310 VDC between DC+ and DC-. Calculate SQRT(2) * Uac.

Assume now, that V1 is 110 VAC only. If SW1 is closed, D2 and D4 never conduct. D1 is a half wave rectifier charging DC+ up to 155 VDC relative to U_CENTER. D3 also is a half wave rectifier charging DC- up to -155 VDC relative to U_CENTER. This again creates 310 VDC between DC+ and DC-.

If you connect 220V with closed SW1 you have 620V between DC+ and DC- for a very short time until the capacitors explode or a fuse melts.

This often happened in Europe with devices imported from 110V land.

The power factor of such a circuit is no longer welcome.

  • \$\begingroup\$ "This often happened in Europe with devices imported from 110V land." Been there... \$\endgroup\$
    – JFL
    Jun 16, 2022 at 9:46
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    \$\begingroup\$ What you mean "the power factor of such a circuit"? \$\endgroup\$
    – Joshua
    Jun 18, 2022 at 4:21
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    \$\begingroup\$ Cool..I always thought it was something like in nanofarad's answer :) @Joshua diode bridge rectifiers draw all their current on the peaks of the mains voltage sine wave. Therefore current is not synchronous with voltage distorting the mains supply. This is called "a low power factor" and forbidden for commercial PSUs larger than 75W in Europe. \$\endgroup\$
    – tobalt
    Jun 18, 2022 at 9:11
  • \$\begingroup\$ @Joshua en.wikipedia.org/wiki/Power_factor \$\endgroup\$ Jun 19, 2022 at 1:09
  • \$\begingroup\$ Traditionally SMPSUs fed the rectifier straight into the primary capacitors. This meant they ended up with poor power factor. Adding an inductor helped a bit but the power factor was still not great. \$\endgroup\$ Oct 4, 2023 at 20:19

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