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Analogue meters: High voltage and current levels

04 May 2015  | David Ashton

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In the first instalment of this series, I introduced a variety of analogue meters. Here, I continue this series on analogue meters with a look at shunts and multipliers as well as additional types of meters.

Shunts and multipliers
A bare meter (if it is of the moving coil type) has a fairly low DC resistance and a full scale deflection (FSD) that will be anywhere from 50µA to 10mA or higher. At FSD it will typically have a fair bit less than a volt across it. You can make it read a different value, which may be a higher current or a voltage, by using shunts, which are resistances used to divert—or shunt—current away from the meter, and multipliers, which are resistors used to drop voltage, or limit the current to FSD at a certain voltage.

Here are some values I measured on a few meters I had knocking around (don't take me literally, meters are fragile and you shouldn't knock them around!). Don't worry about how I got these measurements, I'll get to that later.

@ Note: No. 4 is included for comparison.

See "Multipliers" below for further explanation.

If a meter directly measures the current that you want it to measure, you're home free. If not, you have to adapt it to do so. You can't get a meter to measure less at FSD than its basic sensitivity without using some electronics − for example the 5A meter above cannot be adapted to measure 50 mA. You can get a meter to measure more than its base FSD current, or indicate voltage instead of current, by using shunts and multipliers. So let's look at those.

Let's say we have meter No. 2 in the table above with an FSD of 1 mA. Let's say we want this to give us an FSD of 1 Amp—1000 times as much. We can do this by putting a shunt across the meter terminals so that 999 mA flows through the shunt and only 1 mA flows through the meter as show below.

The shunt is obviously going to have a very low resistance—1/999 of the meter resistance in fact. With the 1mA meter above, this works out to around 0.19 Ω. You can probably just use a piece of copper or resistance wire as a shunt, though you'd have to experiment with the length and gauge of the wire. If you are doing this, you can measure a long piece of your shunt wire (a metre or two) and then guesstimate how much you are going to need for your shunt. Another trick is to make your shunt a bit longer than you need, and move one of the meter wires along the shunt until you get the correct reading (again, use a known good DMM to check). You can use a screw connector or a soldered connection to fix the meter wire in place. If your shunt winds up a bit too short, you can often file or saw your shunt a bit to increase its resistance and get the FSD just right. If you end up using just a low value resistor for your shunt, you can put a higher value preset pot across it to trim it to the right value (for example a 100Ω preset across a 1Ω resistor if your shunt needs to be about 0.9Ω), but shunts are generally too low a value to use standard resistors. You can get current sensing resistors down to 0.1Ω or less and these are sometimes ideal—see below.

You can work out the shunt resistance with the following formulae. You have to know either the meter resistance or its voltage at FSD for this, and how to find this is covered under "Testing Meters" in the final installment of this series.

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