# Would analog multimeters measure correct dimmer's output?

Because I decided to regulate 230VAC with a dimmer (or SCR chopper), I want to measure the outcome (which is very big noise!) with a voltage and current meter.

I know I am supposed to take true RMS instrument, but would analog work too?

I don't find it logic for NOT to work, because the voltmeters are actually just a non-spinning motors, which could be interpreted as: "I use dimmer to regulate speed of motor, which is in this case a voltmeter". And the same could be said for current meter.

edit:

How about, if I add a Graetz bridge and measure DC component there? I don't actually need to show actual AC, if I can do it with DC. (In case I even do get something near a proper voltage value.)

• Modern voltmeters contain no motors. True RMS voltmeter might work. Capturing the waveform on a scope would be a good idea if you are able. I don't really understand your question, but I believe you are confused about something. The only meters that contain motors that I know of, are the meters used by the utility company to measure power delivered to one's home. – mkeith Jun 19 '16 at 20:27
• @mkeith Analog multimeters usually display their readings using a galvanometer - a device that uses electromagnetism to drive a needle gauge. Indeed not unlike a motor, even if it's not an actual motor :) – marcelm Jun 19 '16 at 21:02
• @marcelm, yeah, that is true. Then again, I haven't seen one of those for 30 years or so. I am surprised they are still around. – mkeith Jun 19 '16 at 21:36
• @marcelm, nowadays, when an analog needle type display is desired, it is often a stepper motor that is used to move the needle. For example in automotive gauges. Kind of interesting, I guess. ;-) – mkeith Jun 19 '16 at 21:43
• A dimmer circuit may "reduce" but without a feedback control loop, it does not "regulate". To take a measurement, you have to consider what your goal is - most practical loads do not have a response that is linear in relationships to either RMS voltage or duty cycle over the wide range of a typical dimmer/TRIAC controller, so your measurement may not be meaningful. Typically for example one adjusts a dimmer to the subjectively desired light output, without taking a measurement. – Chris Stratton Jun 19 '16 at 22:01

Figure 1. Waveform from SCR phase-angle controlled voltage.

Moving coil and moving iron analog multimeters have their scales calibrated to give the true RMS reading on full-wave sinusoidal waveforms. They will not read correctly on non-sinusoidal waveforms such as those shown in Figure 1.

Higher quality digital meters will have a True RMS converter on the AC modes. This will measure and display the correct reading.

Power in a circuit is given by $\frac {V^2}{R}$. The significant aspect of this is that the parts of the waveform where the voltage are high have much greater effect due to the square term. True RMS readings tell you the equivalent DC voltage that would give you the same power.

There are many tutorials on the web that will give you a primer on true RMS and why we use it.

• Ah well, I'm losing my hope here on my knowledge. On the other hand, so..just a thing here: I've got analog meter, which I connected to a dimmer, and it worked! I also measured it with my Voltcraft VC-155 and showed simmiliar. Link to the analog meter: ebay.com/itm/… If I add this one, will not work properly, I assume? ebay.com/itm/… – Jakey Jun 19 '16 at 21:13
• The moving coil meter is not true RMS. The digital meter specification is Measurement type: Averaging so it's not true RMS either. – Transistor Jun 19 '16 at 21:18
• Okay, I understand that. But that is actually..the efficial voltage, that comes out? The actual voltage, that is connected to a lightbulb (for example) later? – Jakey Jun 19 '16 at 21:23
• @mkeith - filaments bulbs are resistive in nature, but their resistance is heavily temperature dependent, so neither their power consumption nor to an even greater degree their light output has a linear relationship to RMS voltage. So its not clear that even having an instrument which measured the true RMS voltage of the dimmer waveform would be of much practical use. – Chris Stratton Jun 19 '16 at 22:07
• @Jakey: Why do you care what the voltage is in this situation? If it's just to be able to get back to the same setpoint in future then any meter - linear or not - will suffice. If you need to calculate actual power you may need a true RMS measurement. – Transistor Jun 19 '16 at 22:16