How to measure power supply efficiency ?

If efficiency is significant to you, you will need to evaluate it carefully. For a regular DC power supply operated from the AC line, you will need to measure the input AC power and the DC power being output by the supply as in the following plot:
power supply efficiency

The best instrument for measuring the AC power used by the DC power supply is an oscilloscope. You’ll need to measure the AC voltage and AC electric current going into the power supply. The best way is probably to use a non-inductive current shunt for the current measurement and two independent differential amplifiers to measure the AC input voltage to the power supply and the AC voltage across the shunt. The power wave form can be gotten by multiplying the current and voltage waveform using the oscilloscope. With appropriate bandwidth in the scope and amplifiers, this will be an accurate measurement, show you the power factor, and tell you about any power line harmonics associated with the DC power supply’s operation. If your scope can’t do the multiplication, you can still measure the RMS values of voltage and current, measure the power factor, and multiply the three together.

For measuring the power consumed by the load, you can use the voltage and current meters of the DC power supply as long as you know they’re precise. For confirmation, you can use a DC load instead, set to the same load features.

The calculated power supply efficiency in percent will then be 100%*P(out)/P(in)
where Pin is the measured AC power in and Pout is the measured DC power out, both in the same power units.

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