Measuring Voltage Using Digital Multimeters

Measuring Voltage Using Digital Multimeters

Introduction

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:

  • The V-Ω port is connected to the point which is assumed to be at the higher voltage.
  • The COM port is connected to the point which is assumed to be at the lower voltage.

DMM Settings

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:

  • Plug one of the DMM leads to the COM port of the DMM. The other lead should be plugged into the port labeled V-Ω (the V-Ω port may have additional letters next to it, depending on the capabilities of your DMM).
  • Select the setting on the DMM. This may be either a push-button or a dial setting, depending on the type of DMM you are using. (The straight bars next to or above the V indicate that you will be measuring a constant value.)
Figure 1. Voltage measurement settings.

Probe Connections

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).

Figure 2. DMM used to measure voltage between nodes A and B.

Important Points

When measuring voltage using a DMM:

  • The DMM function selection should be set at .
  • The V-Ω and COM ports are used.
  • The V-Ω port is connected to the terminal in the circuit which is assumed to be at the higher voltage.
  • The COM port is connected to the terminal in the circuit which is assumed to be at the lower voltage.

Test Your Knowledge!

  1. We want to measure the voltage V1 with the indicated polarity the circuit below. Which of the diagrams shows the correct measurement?
  2. The actual voltage differences across the circuit elements are shown on the left of each figure. The DMM connections used to measure these voltages are shown on the right of each figure. What is the displayed voltage for each case?
  3. Selected Answers

    1. (c)
    2. a) -3V (negative)

      b) 5V (positive)

      c) 2V (positive)


1If your assumption is incorrect, the displayed voltage will be a negative number.

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