[stuarth]

This thread shows the importance of really understanding the behaviour of your test gear. I don’t own a DVM with claims to show true RMS, but I would expect a True RMS meter to read the same, give or take a diode drop, either side of a full wave rectifier, assuming no smoothing cap. After all, they have the same energy or heating effect.

I can see applications where you would need to know the true RMS value of a DC and AC signal superimposed (heater chains with half wave rectification, LED lighting, etc). When would you need the RMS value of just the AC bit of the rectified voltage, or of the 2v ripple on a 200V supply?

A crude measure of the true RMS voltage or current (if your meter insists on AC coupling) could be made with the old trick of an appropriate light bulb and a light meter. For example, put a small bulb in series with your heater chain, note its light level, then connect the same bulb to a DC supply and note the current needed for the same brightness. The light meter can be any photosensitive device and a multimeter, you’re always measuring at only one light level, so linearity and calibration of light levels are not important.

Stuart