So far I thought that when an unregulated linear power supply is rated as, say, 12V 1A, it means that when there is no load, the output voltage is 12V and the maximum safe current is 1A. Now I'm in doubt: maybe the rated voltage is the voltage when there is a certain amount of output current.

A cheap power supply I bought recently caused this doubt: the open-circuit voltage is about 20% higher than the rated voltage.

  • \$\begingroup\$ Only 20% high off load? That's good! \$\endgroup\$
    – Neil_UK
    Commented Sep 8, 2021 at 14:15

1 Answer 1


The unloaded voltage will definitely be higher, as that is simply the transformer peak voltage output that is rectified into a capacitor.

And the transformer has a rated AC output voltage at a rated load current, and the unloaded transformer output will be higher than the rated voltage.

Also the mains voltage has some voltage tolerance, so the output will depend also from actual mains voltage coming to your socket.

So the output will be 12V under load conditions, but it might be impossible to know what the expected output current is designed to be to get that voltage.

  • 1
    \$\begingroup\$ Also, as load current increases, the ripple amplitude goes up, pulling the average output voltage farther below the peak voltage. The net effect is that the power supply has a significant source impedance that is a combination of effects from the transformer, rectifier, and output filter. The nominal output voltage is usually seen at the nominal load current. \$\endgroup\$
    – Dave Tweed
    Commented Sep 8, 2021 at 10:42
  • 1
    \$\begingroup\$ ... Assuming the input voltage (and frequency) is nominal as well, of course. \$\endgroup\$
    – Dave Tweed
    Commented Sep 8, 2021 at 10:47

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