An electrician friend of mine asked me if I could recommend a regulator to step down 125vac down to 118vac with a load of 2.5 amps. He would like 3 amps to be safe.

Normally I deal with low voltage DC and would use a buck-boost regulator to get a very consistent output voltage despite some fluctuation in the input and it has a trimpot for setup/calibration to prevent people from accidentally bumping it or messing with it easily. I don't know if there is a good equivalent for AC loads. Will something like this work, or is there something else that may work better? The load is non-inductive, it is a sensor, so the current draw should be fairly stable. I know we could achieve a roughly similar result with a potentiometer as a voltage divider or a resistor, but we're looking for something more stable and efficient that will adapt to some fluctuation.

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    \$\begingroup\$ 125V down to 118V is a strange requirement. Is it not within tolerance already? For AC you would typically use a transformer. \$\endgroup\$
    – user253751
    Sep 1, 2020 at 17:32
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    \$\begingroup\$ Sounds to me like a job for an Autotransformer or Variac. You could probably scrounge an autotransformer with multiple taps from an old UPS which has AVR functionality. \$\endgroup\$
    – brhans
    Sep 1, 2020 at 17:33
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    \$\begingroup\$ The linked device does not change the voltage, it just turns it off some of the time so that the average voltage is lower. \$\endgroup\$
    – user253751
    Sep 1, 2020 at 17:33
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    \$\begingroup\$ What kind of sensor needs 300W? \$\endgroup\$
    – Finbarr
    Sep 1, 2020 at 17:45
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    \$\begingroup\$ I know of 3 posibilities to convert AC to AC: 1 - transformer, 2 - current converter and 3 - motor–generator (an M–G set). I doubt you find a (1) with the correct ratio, (2) are not a weekend project, but maybe you can find a device which can do 125 to 118V - maybe even a used one at ebay. (3) easy to build, but I guess it would be hard to find motors with the correct ratio when combined. I know (3) was used at a great scale in Rolling mills for steel production... \$\endgroup\$
    – schnedan
    Sep 1, 2020 at 17:51

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. A step-down transformer arrangement.

In the arrangement of Figure 1 the secondary voltage can be subtracted or added to the mains voltage depending on which end of the secondary is connected to the live wire. Paying attention to the dot convention shown will result in a voltage reduction. 7 V is not a standard voltage but a 120V:6V transformer should be close enough. Power rating will be 7 V × 2.5 A = 20 VA minimum. I'd go for 30 VA or more.

The whole exercise seems suspect.

  • As commented 300 W is a heater, not a sensor.
  • 118 V is within the normal variation of 120 V mains supply.
  • With the transformer arrangement offered here voltage will vary in proportion to mains voltage.

Constant voltage transformers exist. They (the ones I'm familiar with) use a transformer core with a small air gap and a tertiary winding with a capacitor. The combined effect is to give a fairly constant RMS output voltage although the waveform may be quite distorted.

enter image description here

Figure 2. A constant voltage transformer. Image source: Circuits Today.

I don't know if they come in < 1 kVA versions.

  • \$\begingroup\$ Brilliant cheat! \$\endgroup\$
    – winny
    Sep 1, 2020 at 19:45

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