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Analogue meters: Maximising your meter

13 May 2015  | David Ashton

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Here is the third instalment of this series on analogue meters that offers everything you want to know about these gems:

Part 1: Experiencing the thrill of movement in analogue meters

Part 2: Experiencing the thrill of movement in analogue meters

Part 3: You are here!


Testing meters
If you buy old meters or take them out of old equipment, you can't assume that the markings on the scale on the meter is what it takes to actually deflect it. Max and I were discussing one he purchased that had a scale of 0-2000 V, yet had no internal resistors to make it indicate that—the multiplier resistance must have been external. So you'd need to measure it to find out the full scale deflection (FSD), and if possible its resistance as well, though this is not essential.

The easiest way to do this is with a variable power supply. Let's say you have at your disposal a 0-20V DC power supply. The smallest current a meter will measure is likely to be 50µA. To get a current of 50µA at 10V DC you'll need a resistance of 200 KΩ. So hook up your variable power supply in series with the meter and a 220 KΩ or 180 KΩ resistor, and your best DC current DMM like this:



Slowly turn up the voltage on the power supply and see what the meter does. If the meter does not deflect to FSD, reduce the resistor by a decade (to 22 KΩ) or even to 2 KΩ or 220 Ω if the initial deflection was very small. Once you get it to FSD, read the current off your DMM.

If you don't have a variable supply, use for example a 5V supply and a 100 KΩ potentiometer in series with a 10 KΩ resistor, and then a 10K pot +1K resistor and so on. Make sure the pot is initially at maximum resistance, and decrease the resistance until you get FSD. This is basically what Max has been doing in his recent blog Oh no! The demise of my analogue meter. He made up a nifty test box with 3 pots and a polarity reversal switch, driving it from his Arduino with PWM at full duty cycle. Although this does not tell Max directly what his meter's FSD is, it tells him what series (multiplier) resistor he needs to get FSD—and seeing as he only wants to drive his meters from Arduinos, this is precisely what he needs to know. For more general purposes, you could make the same sort of thing using a couple of sockets or clips to connect your DVM(s) as shown above.

You now know your meter's FSD current. It's also handy to know the meter's resistance and/or the voltage across it at FSD—it will simplify working out shunts and multipliers later. Remove your DMM from the meter current path (or use a second one—all engineers have got at least two DMMs, right?), set it to voltage and measure the voltage across the meter at FSD—it will usually be less than a volt. From this you can work out the meter resistance (RMETER = VFSD / IFSD).

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