Does my meter measure true RMS, AC+DC?

A question that comes up in the forum, especially from people building a transformer/rectifier power supply, is: "What is my meter measuring?"

True RMS DMMs are becoming more common, even at moderate prices, but it may not be clear just what their capabilities are.

Those of us who have been in the business for years usually have a collection of instruments, but the beginner may only have a DMM. So, I thought I would find a way for a beginner with just a DMM to determine what his meter can measure.

This method won't distinguish an average responding, RMS indicating meter from a true RMS responding meter.

The idea is to measure an AC voltage and then compare that reading with the same voltage that has been full wave rectified.

See the attachment. Use a transformer with a convenient output voltage, say at least 12 volts, but higher is better.

Using circuit 1, measure the output voltage. This voltage is just the transformer voltage, with two diode drops in series to compensate for the two diode drops the bridge rectifier will insert. Call this voltage the reference voltage.

Now use circuit 2, and measure the output of the bridge rectifier. This voltage has a DC component and a meter which measures true RMS AC+DC will give a different reading than one that only measures the AC component.

As an example, I used a transformer with a nominal 24 VAC output and measured circuit 1 with a Fluke 189 meter. This is the reference voltage and I got a value of 25.86 VAC.

I then measured circuit 2 with 3 different settings of the meter and got the following values:

DC 23.01
AC only 11.88
AC+DC 25.88

Your meter may not have separate AC and AC+DC modes; if so, they you will have only one reading for RMS AC.

Divide the readings you got from circuit 2 by the reference voltage (the reading from circuit 1); normalize the readings, in other words. Then, if your meter measures a normalized value about .4352, it is not including the DC component in the RMS measurement. In other words, it's "true RMS AC only", not "true RMS AC+DC".

The values I got are shown below, along with their theoretical values if the measured voltage were a distortion free sine wave. Actually, the grid voltage is somewhat flat-topped these days, which causes a deviation from the theoretical value.

The AC+DC value measured from circuit 2 should be identical to the value from circuit 1.

This method allows a person to tell whether their meter can measure a true RMS voltage which includes the DC component.

Nice method, Electrician. This would be a good check to see if the DMM really does perform as indicated in the marketing info/manual.

CAUTION: Electrician's recommendation of using a transformer is mandatory. Under no circumstances do this type of testing with direct line voltages -- it's dangerous and you can easily be killed. I would recommend that you not use a voltage over 24 VAC. You can get 24 VAC transformers for doorbells and sprinkler systems power. You can also use a wall wart that specifically says it converts AC line voltage to a lower AC voltage (i.e., it's just a transformer).

If one has access to a function generator, here's another way that doesn't require building anything. Set the function generator to a 50 or 60 Hz sine wave (depending on where you live) and connect it to the DMM. Set the generator's output to produce a 1 volt reading on the DMM. Then switch the function generator to a square wave output. If the square wave's peak-to-peak amplitude is the same as the peak-to-peak amplitude of the sine wave, then you should read about 1.4 volts on your DMM if it reads RMS. You may want to use a scope to check that the generator is doing what you expect -- and many folks won't have access to a generator or a scope. However, if you're a student, you may be able to convince someone in the physics or EE department to let you use their equipment for a quick check.

To check whether your meter can measure AC + DC, just connect a battery to the DMM on the AC scale. It should read the battery voltage if the meter does indeed read AC + DC. If the function generator has the capability of supplying DC offsets, then you can use that instead, both for a straight DC voltage and an AC voltage with a DC component.

As I said in the first post, I was trying to find a way that would be available to someone without any other equipment than the DMM. I assumed that since questions about what the meter measures often arise in the process of building a capacitor input rectifier filter, that the person doing the building would have some rectifiers and a transformer! Said transformer probably not being more than 48 volts max--probably more like 12, 24 or 36 volts.

Now, the next problem is to determine whether a given meter is even measuring true RMS AC only. I haven't been able to think of a good method that doesn't require using waveforms with high crest factor, and that would mean either having access to a signal generator, or building something to generate narrow bipolar pulses. If anybody has any good ideas about this, let's hear them.

One could use one of the many 555 circuits to provide a pulse train to the meter. An op amp or transistor could be used to make it bipolar.

But a fundamental problem is that the person needs to know the shape of the wave so he can calculate the RMS value, then compare that value with what the DMM reads. This requires a scope. I don't see a way around this unless someone comes up with an easy-to-build circuit with one or a few commonly-available components that produces the same shape of waveform for a variety of sine wave amplitude inputs.

Wait -- there is such a thing -- just use the charging and discharging of an RC network. This could be driven with a sine wave or a square wave and is simple enough that the person could be pretty confident of what the RMS value should be.

someonesdad said:

One could use one of the many 555 circuits to provide a pulse train to the meter. An op amp or transistor could be used to make it bipolar.

But a fundamental problem is that the person needs to know the shape of the wave so he can calculate the RMS value, then compare that value with what the DMM reads. This requires a scope. I don't see a way around this unless someone comes up with an easy-to-build circuit with one or a few commonly-available components that produces the same shape of waveform for a variety of sine wave amplitude inputs.

Wait -- there is such a thing -- just use the charging and discharging of an RC network. This could be driven with a sine wave or a square wave and is simple enough that the person could be pretty confident of what the RMS value should be.

Click to expand...

The charging and discharging of an RC network will still give just a sine wave, won't it? Driving a CR network with a square wave will give spikes (it will differentiate the square wave) and would provide a nice signal, but for that we need a square wave. I'm still trying to give the person without much beyond a DMM and the transformer/rectifier circuit a way to determine if they have a true RMS meter.

I think I have a way. See the attachments. They show a simple bridge rectifier with a couple of extra current sense resistors. I've shown .1 ohm resistors, but they could be any low value that won't overheat with the applied load current. But whatever value is used, they should be fairly well matched. Pick a couple of resistors out of several that are as well matched as possible.

The circuit in the first image rectifies the 24 VAC voltage from the transformer and applies a 2 amp load. The 6 ohm load shown is just for illustration; my actual load resistor was adjusted to give close to a 2 amp DC load current.

The second image shows the current in the CS1 sense resistor (purple) and in the CS2 sense resistor (green).

I placed a .5 μF capacitor in series with each meter when making readings to make sure that no DC will be applied to the meter in case it has some sensitivity to DC.

I used a Fluke 189 true RMS meter and an older Triplett analog meter. The Triplett is the style of meter known as average responding RMS indicating. This meter measures the rectified AC waveform and then multiplies its value by PI/(2*SQRT(2)) = 1.1107; this assumes that the waveform being measured is a pure sine wave.

When the 2 amp load is applied, the voltage across the two sense resistors is as shown in the second image.

When making the measurements, wait a number of seconds for the transient due to the .5 μF cap to settle out.

The measurements across the two sense resistors is:

The Triplett is looking at a doubly rectified voltage when it measures the voltage across CS2, and it measures nearly the same as it measures across CS1.

The waveforms across CS1 and CS2 are typical for a bridge rectifier of this sort.

So, if your meter (with a .5 μF cap in series to avoid any DC sensitivity) measures nearly the same across the two sense resistors, your meter probably isn't true RMS. Notice two readings with the Fluke 189 are substantially different.

Without the series .5 μF capacitor, and with the Fluke in AC+DC mode, the voltages across the two sense resistors read identically.

For making measurements of currents in rectifier circuits it's good to use a true RMS meter; then you get a true measure of heating in components.

electronics1 said:

Where I can find the electronics sketches and explanations about how the multi meter measure AC voltage and DC voltage​

Click to expand...

electronics1 said:

Where you (The Electrician) find the electronics sketches that you put in this chinning ?​

Click to expand...

I made them with a circuit simulator.

The Electrician said:

The charging and discharging of an RC network will still give just a sine wave, won't it? Driving a CR network with a square wave will give spikes (it will differentiate the square wave) and would provide a nice signal, but for that we need a square wave. I'm still trying to give the person without much beyond a DMM and the transformer/rectifier circuit a way to determine if they have a true RMS meter.

Click to expand...

Yes, I figured it would be pretty straightforward for the user to charge the capacitor through the resistor with a square wave from e.g. a 555 circuit. That would give a waveform with the familiar exponential charging and discharging waveforms (i.e., make it integrate, not differentiate). But I also see your point about wanting to keep the user's construction work to a minimum.

someonesdad said:

Yes, I figured it would be pretty straightforward for the user to charge the capacitor through the resistor with a square wave from e.g. a 555 circuit. That would give a waveform with the familiar exponential charging and discharging waveforms (i.e., make it integrate, not differentiate). But I also see your point about wanting to keep the user's construction work to a minimum.

Click to expand...

I meant to say "The charging and discharging of an RC network with a sine wave will still give just a sine wave, won't it?".

So, we would need a square wave.

I was just making some measurement on a rectifier/capacitor setup, and I remembered that I have some moving iron ammeters, which are RMS responding and also respond to DC. These can be bought inexpensively on eBay. Here is an example:

http://cgi.ebay.com/Yokogawa-1-5-am...emQQptZLH_DefaultDomain_0?hash=item414d37f655

The fact that the scale is compressed at the bottom end is a dead giveaway that this is a moving iron meter, responsive to RMS AC+DC. It's a lot cheaper than a DMM, and if one has one (or more ) of these, you can leave it in circuit to measure some particular current while you use your DMM for measuring something else. It can even measure the ripple current in the filter capacitor!