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Variac Mains Variable Transformer

It is rare that a variac is needed, but when it is there is nothing else that will do the job. If you think you might need to use this then you shouldn't be using it! Take the time to ask someone knowledgeable about what you want to do, and discuss whether a variac is or isn't the right tool for your job. The members email list is your friend!

The dial indicates the coupling percentage, not the output voltage. Hence if you want a 115Vac output from a 230Vac input, set it to 50%. Yes, you can get an output higher than the input.


  • do not implicitly trust the dial's value. The dial is not firmly attached to the shaft, and can relatively easily be rotated beyond the endpoints. It can also be rotated (with thought and care) so that it correctly indicates the coupling percentage.
  • the meters are poor and “sticky”
  • if maintenance is to be done, note that the box is hinged; accessing the interior is not obvious
  • do not exceed the maximum current/power

For any variac:

  • they are auto-transformers, not an isolating transformers. They cannot reduce the chance of electrocution
  • a UUT with an SMPS should never be driven from a variac - and that's most modern equipment!
  • in particular, don't try to “recondition” potentially dodgy PSU capacitors in a switched mode power supply (SMPS) by slowly raising the voltage from zero

The problem with using it with SMPSs is that SMPSs are designed to deliver the rated power output. Clearly the SMPS's input power must be the same as its output power (give or take the efficiency!). Hence if the input voltage is abnormally low, then the input current will be abnormally high - and that can destroy the SMPS's internal components.

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