Can you measure/work out current output with a voltmeter or multimeter?

Question

My brother wants a pocket multimeter for his birthday but I can't find one that has good ranges on it in a small size. Voltage and resistance ranges are not a problem but current ranges seem to stop at 200mA. Can you work out current output from other multimeter readings like voltage etc for AC and DC current upto about 15A. The main use would be for testing powersupplies onsite,

thanks guys.

Answer 1

For AC current, a clamp on meter is very convent, though perhaps a bit more than "pocket size". Generally the clamp ons don't work on DC unless they use Hall effect sensors, and then they are either quite expensive, or the accuracy is not so good. What one needs to measure current with a voltmeter is called a "shunt" which is, as stated above, a low value precision resistor.

Answer 2

You can measure any current by inserting a series resistor into the circuit and measuring the voltage across it. However, you need to be aware that the presence of the resistor can affect circuit operation and that if you want to measure large currents the resistor must be able to handle a lot of power.

Suppose, for example, you have a circuit drawing 15A from a 240V supply. The resistance of the circuit is 240 / 15 = 16 ohm, and the power dissipated 3600 W.

If you inserted a 1 ohm series resistor, the current would drop to 240 / 17 = 14A, which is what you would measure, so the resistor has introduced a 6% error.

Worse still, the power dissipated in the 1 ohm resistor would be 14^2 x 1 = 199W, so you would need a 200W, 1 ohm precision resistor – a huge, expensive thing.

It might be cheaper to buy a better test meter.

Answer 3

Sure. It's called Ohms Law ☺

Doug

Answer 4

yes, it is possible.
you can use a low value high watt resistor in parallal to mutimeter.