Voltage is a difference in energy level between two different points in a circuit. Voltage has not only a value, but also a polarity—one of the points will have a higher energy level (or a higher voltage) than the other. The link to the right provides more information about voltage.
The voltage polarity must be accounted for when making a measurement—when you connect your DMM to a circuit, you will be assuming a particular polarity. The sign of the reading displayed on the DMM reflects this assumption —if the actual and assumed polarities are the same, the displayed number will be positive. Conversely, if the number displayed is negative, the actual polarity is opposite to your assumption.
Voltages are measured simply by connecting the DMM probes to the points (or terminals) in the circuit between which the voltage difference is to be measured. The way in which the probes are connected to your circuit defines the assumed polarity of the voltage measurement:
To set up your DMM to measure a constant (or DC) voltage level, perform the following steps. Figure 1 displays the physical appearance of the DMM in the correct mode for measuring constant voltage resulting from these steps:
Next, connect the DMM probes to your circuit at the points between which the voltage is to be measured. You will place the DMM leads across the component(s) whose voltage difference you want to measure. The “V- Ω” terminal is connected to the assumed positive voltage terminal in your circuit, and the “COM” terminal is connected to the assumed negative voltage terminal in your circuit1. For example, in Fig. 2(a), we want to measure the voltage difference between terminals A and B of the circuit. If we define the polarity of the voltage difference to be such that we assume node A is at a higher voltage than node B, we will connect the V-Ω probe of the DMM to point A and the COM probe of the DMM to point B, as shown in Fig. 2(b).
When measuring voltage using a DMM:
a) -3V (negative)
b) 5V (positive)c) 2V (positive)
1If your assumption is incorrect, the displayed voltage will be a negative number.