Can a DMM be trusted?

[ppppenguin]

When you know or suspect an awkward waveform and/or AC superimposed on DC you need to understand what you want to measure and the limitations of your testgear. On these difficult waveforms AVO type meters will give the wrong result. DMMs, whether true RMS or otherwise, may or may not give the right result.

Taking the opposite case to a diode dropper, let's measure the unsmoothed ripple on a DC supply. You would normally want the DC blocked, whether you are looking for a mean or true RMS value. Better to use a 'scope and see the frequency and peak-to-peak voltage. Which is the voltage that's of most value.