Editing Category:Common Issues (section)

==== Multimeter Probes ====
You need 2 probes to do any measurements, in any mode, period. One is for the positive side of the signal, and the other is for the COM ("common") side of the signal. The reason that it is COM and not ground is that the DMM is a floating reference{{---}}voltage measurements always need to be taken with respect to some point.  The two probes set the point that you are trying to measure against.

This is a really important thing to understand. Suppose that you want to measure a 6{{nbsp}}V signal. If you probe with the red probe on a 6{{nbsp}}V test point and the black one on ground, you will find a reading of 6{{nbsp}}V. However, if you were to measure with the red probe on a 18{{nbsp}}V signal and the black on a 12{{nbsp}}V signal, you would find that this also reads 6{{nbsp}}V (18{{nbsp}}V {{minus}} 12{{nbsp}}V = 6{{nbsp}}V). So, the meter is not always referenced to ground, and this is why you need to pay attention to where you are probing. The probes will also have different functions based on what type of thing you are trying to measure. For voltage measurements, the probes are in parallel with the circuit under test. For current measurements, the probes are '''in series'''. So, if you don't pay much attention when setting up your measurements, it's very easy to end up with useless information about what you are looking at, and even damage your circuit beyond the state it came to you in.