Electronic transformers

[ppppenguin]

If the SMPS output is AC at 20kHz+ then many types of meter will measure the current (and the voltage) inaccurately. Forget about DMMs, especially true RMS ones, and moving iron. Ordinary rectifier analogue meters (AVO8 etc) may make a half decent stab at it if the output is sinusoidal which is a bit unlikely. A VTVM should be OK-ish but with the same warning about wave shape. Current is harder than voltage here as it needs a shunt which will need to be very low resistance at these currents, which are beyond a standard AVO anyway.

I've got Crompton "Tongtest" clamp meters which work on both AC and DC and probably do something close to true RMS. No idea how they'd respond to 20kHz+. Modern electronic clamp meters? No idea.

If I needed accurate measurement I'd use a scope, either with a current probe or a 4 terminal shunt. Then there's the faff of doing RMS calcs from the waveform. Easy enough if it's sine or square, much harder if it's anything else.

PS: Rather to my surprise, you can still get new Tongtest style meters: http://www.weschler.com/_upload/sitepdfs...ampons.pdf