# The Basics of Digital Multimeters

## Introduction

Digital multimeters (DMMs) are multipurpose devices used to measure several different circuit parameters. Commonly available DMMs measure the following quantities:

• DC (constant) voltages
• AC (sinusoidally varying) voltages
• DC current
• AC current
• Resistance
• Capacitance

Many DMMs are capable of measuring additional quantities, such as frequency, conductance, and inductance. In this exercise, we will only concern ourselves with the measurement of constant voltages, constant currents, and resistance. Some brief definitions relative to these parameters are provided below; somewhat more in-depth background information is provided in the links to the right.

## Terminology:

• A device to measure voltage is called a voltmeter.
• A device that measures current is called an ammeter.
• A device to measure resistance is called an ohmmeter.
• A multimeter will perform all of these functions, depending on how the meter is used.
• Voltages and currents which do not vary with time are often referred to as DC voltage or DC current. DC stands for direct current.
• Voltages and current which vary sinusoidally with time (as do typical household voltages and currents) are called AC voltages and currents. AC stands for alternating current.

DMMs are extremely common devices, and a variety of them are readily available. They can be broadly categorized as either laboratory/bench systems or hand-held devices. Bench systems will generally have greater capabilities than hand-held DMMs and typically better accuracy, but are not easily portable. Hand-held DMMs are battery operated, making them readily portable and correspondingly popular. The basic operation of the bench and hand-held DMM models are similar.

The operation descriptions provided in this project are for the hand-held DMM, which is available for purchase on Digilent's website. This is a relatively inexpensive DMM, and has all the capabilities necessary for the labs (which are also available on Digilent's website). The instructions provided here should be helpful regardless of the DMM model you have.

All DMMs provide the same basic components:

• An LCD or LED screen to display the value being measured.
• A dial (or buttons) to select the desired measurement.
• Ports into which the probes are plugged.
• Probes which provide the interface between the DMM and the circuit.

These components are indicated on Fig. 1. Since DMMs are intended to perform a number of functions, there are a variety of combinations of configurations for both the dial, which selects the function; and the ports, which are used to connect the DMM to the circuit. The dial position must be consistent with the ports being used.

## Important Points:

• Digital multimeters (DMMs) can be used to make a variety of measurements. Most DMMs will measure voltage, current, resistance, as well as a number of other parameters.
• The measurement being performed is set by the positions of a dial or buttons on the front of the DMM. In addition, the DMM probes which connect to the circuit must be plugged into the appropriate ports in the front of the DMM.
• Configuring the DMM to make a measurement involves two things:
1. The desired measurement must be selected using the dial (or buttons) on the DMM.
2. The probes must be connected to the ports corresponding to the desired measurement.