I’ve got a simple voltmeter that I’d like to use to measure the power load of some components that I have no idea of. Some are simple (like an LED) and some are more complex sub-assemblies. I’m not sure how to use my voltmeter to measure the mA drawn by the component. I don’t seem to be able to just touch the contacts on either side of the component and ground like I do when measuring resistance or voltage levels. Do I need to break the circuit and then use my volt-meter to connect the source to the load? That seems like a lot of trouble and quite difficult to do with soldered-on components and complex wiring harnesses. There must be an easier way. What do I not know?
2 Answers
Yes, in general, you need to put an ammeter inline with the circuit to measure current. If your “simple voltmeter” is actually a multimeter, then it likely includes an ammeter which will do the job. Multimeters are very inexpensive.
It’s possible in some cases to deduce current in very simple circuits if you know the total resistance and the voltage using Ohm’s law.
Additionally it’s possible to measure current (usually in AC lines) by measuring the resulting magnetic fields produced. This is typically used in very high current measurements.
How you use a voltmeter to measure current is by inserting a current sensing resistor into the circuit, and measuring the voltage across that resistor. The resistor has some small value that does not disturb the quantity of current (at least, not very much). you have to take care that it has enough power dissipation (\$I^2R\$) for the expected maximum value of \$I\$.
The scale of the meter will be sensitive to the value of the resistor, so you need to be able to calibrate your instrument. For instance, use a known, accurate current source and mark that off on the dial. You could set up a potentiometer to calibrate it. The nice thing is that the current sensing resistor is a low-impedance voltage source. It can feed some some reasonably valued potentiometer.
There are ways to measure current without breaking circuits. That is done with a current clamp. That’s a separate instrument from your volt meter. You could build one yourself, using the volt meter for the instrumentation.
