Even if you don’t have a Rohde Schwarz oscilloscope, you can still enjoy their recent video about using an oscilloscope to measure power supply efficiency. Of course, you don’t have to have a scope to do this. You can use a voltmeter and an ammeter, but it is very straightforward if you have a four-channel scope with a pair of current probes.
Of course, if you can measure the voltage and the current at the input, you can calculate the input power. Then again, most scopes these days can do the math for you. Then, you make the same measurement and calculation at the output. If you know the input and output power, you can calculate a percentage or many scopes can do it for you now.
