Why You Shouldn’t Quite Forget The Moving Coil Multimeter

Did any of you have an AWS multimeter? Was it the best? Radio-electronics magazine, August 1981.
Did any of you have an AWS multimeter? Was it the best? Radio-Electronics magazine, August 1981.

If you were to ask a random Hackaday reader what their most fundamental piece of electronic test equipment was, it’s likely that they would respond with “multimeter”. If you asked them to produce it, out would come a familiar item, a handheld brick with a 7-segment LCD at the top, a chunky rotary selector switch, and a pair of test probes. They can be had with varying quality and features for anything from a few dollars to a few hundred dollars, though they will nearly all share the same basic set of capabilities. Voltage in both AC and DC, DC current, resistance from ohms to mega ohms, and maybe a continuity tester. More expensive models have more features, may be autoranging, and will certainly have better electrical safety than the cheaper ones, but by and large they are a pretty standard item.

If Hackaday had been around forty years ago and you’d asked the same question, you’d have had a completely different set of multimeters pulled out for your inspection. Probably still a handheld brick with the big selector switch, but instead of that LCD you’d have seen a large moving-coil meter with a selection of scales for the different ranges. It would have done substantially the same job as the digital equivalent from today, but in those intervening decades it’s a piece of equipment that’s largely gone. So today I’m going to investigate moving coil multimeters, why you see them a lot less these days than you used to, and why you should still consider having one in your armoury.

Evidently the Triplett Model 60 had hidden depths. Radio-Electronics magazine, August 1981.
Evidently the Triplett Model 60 had hidden depths. Radio-Electronics magazine, August 1981.

There was a period through the 1980s into the 1990s, when the transition between moving coil and digital was in full swing. Everyone wanted a digital meter because they were cool. They had both precision and accuracy, instead of saying “That’s about 5 volts”, you now knew it was exactly 4.98 volts. It didn’t matter that the difference between the two readings had no effect whatsoever on the circuit in question, you had a cool multimeter, and you could measure down to 0.01 of a volt. Engineers walked Tall in their aviator sunglasses to their Ford Sierra/Merkur, and drove off into the sunset knowing that they had Tamed The Last Fraction Of A Volt.

So we can make fun of 1980s fashions, both sartorially and in test equipment, but what’s the real benefit of a digital meter? It lies in the internal resistance. When you hook a moving-coil meter into a circuit, you are doing work, drawing energy from the circuit being measured to move that pointer. Imagine the meter itself in a hypothetical inexpensive moving-coil multimeter, that has a full scale deflection of about half a milliamp.  To take a reading at full scale the meter must then draw 0.5 mA from the circuit being measured. So to give a full scale deflection of 10 V for example the meter must have a resistor in series with it of value 10 KΩ, and measuring a voltage thus involves placing what is in effect a 10 KΩ resistor into the circuit in question. Most decent moving coil multimeters had meters with much lower full scale deflections that required greater series resistances, but the principle was the same. A moving coil multimeter loads the circuit it was testing, altering its characteristics.

High-impedance voltage measurement, 1950s style.
High-impedance voltage measurement, 1950s style.

The problem of meter resistance was one for which there were solutions in the age before digital meters. Many decades ago you could buy a valve (tube) voltmeter, in which the meter was driven through an amplifier with a high impedance input. Even when these were replaced in the solid-state era with FET amplifiers they remained expensive devices, and the average bench would not have had one. When mass-market digital meters arrived in the 1970s they incorporated FET input stages, so immediately there was the possibility of a multimeter with an impedance in the mega ohms rather than the kilo ohms. These meters could be hooked up to a circuit from which they would draw negligible amounts of current, and whose characteristics they wouldn’t affect in the slightest. People bought digital multimeters, and never looked at their moving coil devices again.

Moving Back to the Coil

My Avo 8 isn't as clean as this one. Megger Ltd. [CC BY 3.0]
My Avo 8 isn’t as clean as this one. Megger Ltd. [CC BY 3.0]

So why should you have a moving-coil meter on your bench, if digital meters are so good? Aside from looking a bit retro and never having to worry about the batteries running out, that is. The answer lies in being able to measure voltages that change. If you have ever worked with radio or with analogue circuits that require adjustment you will know something of this, tuning for a peak or a trough is extremely difficult when you have no view of the trend. A typical use for a moving coil meter here is to set up an RF amplifier by tuning a tank coil, there will always be a peak in the current drawn by the amplifier as the coil reaches resonance.

The moving coil meter of choice here for the last 30 years or so has been a venerable Avo 8. This is the classic mid-twentieth-century multimeter, a large and heavy Bakelite unit with an extremely high quality meter lurking within it. The full scale deflection of the meter itself is a relatively tiny 50 µA, so its impedance when measuring voltage is better than the cheap meter example quoted above, but in 2017 it’s a bit of a museum piece. Thus this article will now turn into a mini-review of a more accessible moving coil meter, as  I’ll take a readily available model from 2017 and put it through its paces.

5.8V on the MF47 when it should be 6V.
5.8 V on the MF47 when it should be 6 V.

The TianYu MF47 can be had from the usual Chinese suppliers for about £15, or $20. It’s not the most compact of multimeters, having a front panel slightly larger than and being about three times as thick as a DVD case. The upper half of the front panel is occupied by the large meter, while the selector switch takes up the bottom half. There is a handle that folds behind the meter to make a desk stand. It offers the usual multimeter voltage, current, and resistance ranges, plus a transistor tester, a battery tester, and a capacitance meter which I haven’t figured out how to use yet due to an all-Chinese manual.

On the back is a hatch for batteries, a C cell and a PP3, and another for a fuse. A really nice touch is a spare fuse clipped to the inside. The Chinese text below the meter face extols the virtues of its protection circuitry and gold-plated contacts according to Google Translate. If I wanted to describe it in a sentence, I’d put it as a semi-decent budget workhorse multimeter, its relatively low price is a function of its coming direct from Chinese suppliers rather than indicative of deficient quality.

I can demonstrate immediately the effect of the relatively lower internal resistance of the MF47 when compared to a digital multimeter by looking at a simple potential divider. Connecting two 22 kΩ resistors in series across a 12 V supply, you would expect the resulting voltage at the meeting of the two resistors to be 6 V, or half the supply voltage. This is confirmed with a digital multimeter, my run-of-the-mill Uni-T UT33D. However, the MF47 measures 5.8 V, as it has a low enough internal resistance to affect the lower half of the potential divider and pull the voltage down by 0.2 V. The MF47 documentation has a table of specifications in which the internal resistance is quoted as 20 kΩ/V, and indeed measuring the MF47’s resistance on its 10 V range with the Uni-T returns a figure of 200 kΩ. Adding this extra resistor into the voltage divider circuit gets us to 5.8 V.

When you are dealing with lower impedance circuits there should be no issues when using a moving-coil meter such as the MF47. These meters make rugged and cheap instruments for automotive electrics and (subject to the claims of protection circuitry being true!) household wiring, in addition to the previously mentioned use for finding current peaks and troughs in analogue circuits. The MF47 will stay on my bench as a much lighter alternative to the Avo, but it won’t replace my Uni-T by any means.

It’s been an interesting reconnection with the past to refamiliarise myself with a moving-coil multimeter, and to remember why we all use digital meters today. There is a temptation to look back through rose-tinted glasses, but in the case of multimeters they aren’t really justified. Still, it’s not entirely a waste of time to have a moving-coil meter on your bench, so if one comes your way don’t necessarily pass up on it. If nothing else, you’ll never have to worry about whether you have a spare 9 V battery on hand again.

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