How to Measure Current in a Circuit
The last video demonstrated the basics of how to use manual and auto-ranging multimeters. The video below will demonstrate how to test voltage, current, and resistance in a circuit. For the purpose of covering one topic at a time, I will cover some information on measuring current in this blog, with separate blogs covering voltage and resistance to follow.
First, plug the black probe into the “COM” port. There will likely be two options for the red probe, a higher amperage rating and a lower either milli- or micro-amperage rating. If you are unsure, start with the higher option and move it to the lower one for a more accurate reading. Verify the max amperage of these functions on your meter before testing circuits. This will help to prevent blowing a fuse (if the meter is fused).
Next, turn the dial to the closest amperage option to what you will be measuring. If you have no idea, again you’ll want to start higher and move lower as you go. Some meters can only read DC current, and some can read both AC and DC. The three images below are examples of different meters.
Now that the meter is ready, we can start the measurement. Current measurements require the circuit to be on and physically interrupted so the multimeter can be placed in-line with the circuit. The easiest way to do this is to remove the VCC connection (voltage source), then place the positive probe on it, while placing the negative probe where the VCC connection initially was. The meter is now acting as a component in the circuit, and current is running through it. This will give you a measurement of the total amperage consumed by the circuit. If you want to measure a portion of a circuit, you’ll have to make a break in the circuit at that point and follow the same procedure.
It is best practice to leave the meter sit this way for a few seconds or minutes, just to monitor what’s happening. This way you will notice any spikes or drops in current if something is malfunctioning.
Remember, current is additive between each line in a parallel circuit, and constant through a series circuit. This will help you determine the most beneficial and easiest location to find the needed measurement.