Cheap DMM's and AC ripple measurements

[wingerr]

Why is it that the cheaper DMM's don't use any filtering for DC on the AC measurement modes? With pretty much every one of the non-Fluke branded meters I tried, trying to measure the AC of a DC signal would give off the wall results. Only with my Flukes would it read zero on a battery. All the others would read something like double the 12VDC.
The intent was to check the charging output from a motorcycle R/R, to see if it might have had a bad diode by reading a high AC ripple. A blocking cap would proably help, but seems like that should be built in to the meters-

And if a meter has a true RMS measurement mode, what would it read if connected to a DC signal? Should it read the straight DC?

 

[dknguyen]

True RMS is only something that applies to measuring AC signals I believe. It does not affect DC measurements (if it does, then it gets the RMS of the DC waveform rather the instantaneous value).

But anyways, you answered your own question when you said "cheaper". Cost. Large capacitors are needed to filter out low frequencies but high quality capacitors are needed to pass high frequencies. Since frequency characteristics and size tend not to be found together in a capacitor material, you end up needing expensive capacitors.

 

[wingerr]

That's the weird thing with cheaper meters, using AC mode to measure a signal with any DC component results in a reading of "20VAC" for a 12V DC signal, for example. Try measuring a battery with the meter in ACV mode, and see if it reads 0VAC as you might expect, or if it shows some non-zero value-

 

[Nigel Goodwin]

That's the weird thing with cheaper meters, using AC mode to measure a signal with any DC component results in a reading of "20VAC" for a 12V DC signal, for example. Try measuring a battery with the meter in ACV mode, and see if it reads 0VAC as you might expect, or if it shows some non-zero value-

Basically you're trying to use a meter for something it's not designed for, the AC ranges on multimeters are intended for measuring 50/60Hz mains - nothing else, so there's no need for blocking capacitors.

So it's nothing 'weird', it's working exactly as intended, it's just that you are trying to use it in a weird way

 

[wingerr]

the AC ranges on multimeters are intended for measuring 50/60Hz mains - nothing else

So it's nothing 'weird',

Then maybe it's weird for them to offer a millivolt AC mains range; for a lilliputian hair dryer maybe.

Cheap has its uses though-
I still like the three dollar HF meters for sprinkling around in every room because they work fine for most of my uses; just have to watch for the Lo Bat indication, which says "wacky reading alert", instead of the Flukes that let me run it as low as the display is still visible.

 

[Hero999]

I like the mV AC range as it can be handy for measuring cable voltage drops and current, if used with an appropriate series resistor.