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?
Simple way to measure amps with a volt meter
current measurement
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Looks like a good idea at first glance. No immediately obvious "gotchas.
Won't compete with the best test instruments but should be viable.
I'd guess there would be some learning required along the way for a practical device.
These two Burr Brown application notes were cited in your reference. Don't seem to specifically address the test issue BUT cover the high impedance fron end issues well enough that they will provide useful material.
The opamp version of a TIA looks like this, (one of my fav. circuits.)
simulate this circuit – Schematic created using CircuitLab
Best Answer
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.