There has been discussion of "True RMS" meters, so I thought it would be handy to have a signal to test them.

RMS capability is most needed when waveforms are peaky; this means that the crest factor (ratio of peak to RMS) is high. Many meters whose specs I've looked at allow a maximum crest factor of 3 for rated accuracy.

Peaky waveforms often occur in power supplies. The current drawn by a capacitor input filter following a transformer/rectifier is usually rather peaky, but rarely exceeding a crest factor of 3. This fact suggests that a rectifier waveform might make a good test signal.

This circuit generates some waveforms that can be used to verify meter performance:

It's not necessary that the transformer be a 24 VAC output unit. You might use a 12 VAC, or a 36 VAC unit. The 1000 uF capacitor voltage rating should be appropriate for the transformer secondary voltage. A 63 volt rating would be useable for most cases. The resistors should be selected to be "matched" in the following sense: R1 and R2 should be as nearly identical as possible--within .1% and R3 should be 10 times the value of R1 and R3; R1 and R2 don't have to be exactly 1k but they should match. So, for example, R1 and R2 might be 1023 ohms; then R3 should be 10230 ohms. Hopefully, you'll have a selection of nominally 1k and 10k resistors and you can then use the ohmmeter function of your DMM to pick out a matched trio of resistors.

Here is a scope capture showing the waveforms across R1 and R2. The voltage across R1 is orange and across R2 is blue.

The current through R1 is the same as the current through R2, except that the R2 current is full wave rectified. The current through R3 has the same DC value as the current through R2. Because we're looking at currents (via the voltage across resistors), we don't have to deal with the diode voltage drops.

Average responding meters are calibrated to read the RMS value of an undistorted sine wave. This means that they measure the mean (average) absolute (full wave rectified) value (with any DC component removed before rectification) of an applied waveform and multiply that value by 1.1107 because this number is the ratio of the RMS to mean absolute value of a pure sine wave.

The voltage across R3 is proportional to the mean value of the rectified current; mean value because the capacitor does averaging. If we measure the DC voltage across R3 and multiply that value by 1.1107, then divide by 10 (because R1 is 1k and R3 is 10k), that is the value an average responding meter will give for the voltage across R1. So, in effect, we have a very peaky waveform whose value as read by an average responding meter is known.

The waveform across R1 has exactly the same "True RMS Ac+DC" value as the waveform across R2. To check whether or not a "True RMS AC+DC" meter is performing the "AC+DC" function correctly, just measure the voltage across R1 and R2; they will be the same if the meter is correctly measuring "True RMS AC+DC".

To determine if a meter is measuring RMS at all, first measure the voltage across R3 with the meter in DC mode. Then measure the voltage across R1 with the meter in AC mode. If the reading is about 1/10 of the DC reading across R3 times 1.1107 (within the accuracy specification of the meter), then the meter is an average responding meter. If the reading is about 40% to 50% higher than 1/10 the DC reading across R3, then the meter is a "True RMS" meter. And, of course, to double check the "True RMS AC+DC" capability, compare the readings across R1 and R2; they should be the same (if the "+DC" option is turned on).

I wired up this circuit, picking out resistors for R1 and R2 with values 1000.6 and 1000.3 ohms and for R3, 10001 ohms. My transformer had a 36 VAC secondary and I fed the transformer with a variac. I adjusted the variac so that the DC voltage across R3 was 10.064 volts. Then the voltage across R1 measured with a Fluke 187 in "True RMS AC only" mode was 1.485 volts; an average responding meter read 1.15 volts. With the meter in "True RMS AC+DC" mode, it reads 1.485 volts across both R1 and R2.