What is a Digital Multimeter & How to Use One

Digital multimeters (DMMs) collectively form a range of versatile tools, designed for measuring and displaying various important parameters relating to electronics and electrical circuitry.

This will typically include voltage (V), current (A), and resistance (in Ohms). In short, a DMM combines the functions of a voltmeter, an ammeter, and an ohmmeter into a single handy unit.

• Voltage is the difference in electrical potential between two points, expressed as the potential energy of an electric field to cause current flow through a conductor or circuit
• Current is the active rate of electrical charge flow through a component or circuit, measured as it passes a designated point or region on the circuit (for most circuit types, this charge flow is typically carried by electrons moving through a wire)
• Resistance is a measurement of the extent to which the properties of a specific device or material limit the rate of electrical current flowing through it

These devices are also known as digital multitesters. All of these tools generally require a variety of probes and test leads to be plugged into them, to transfer signals to and from the components, contacts, wires or traces being tested.

Multimeters can be used for a wide range of monitoring, detection, analysis and troubleshooting applications. This will depend on what aspect and stage of electronic prototyping or troubleshooting you are concerned with, and the specific details of the electrical circuit, component or fault being tested.

In short, they can be used for any job requiring accurate readouts of device voltages and resistances as an easily measurable output value. Specific examples might include:

• Electrical testing of plug sockets and lighting setups
• During wiring installation
• General analysis and measurement of devices and components such as batteries, electric motors, and power supplies
• Fault-finding and troubleshooting of electronic circuits

There are numerous methods for using a DMM to give a meaningful readout on any given type of circuit or component. However, it will largely depend on what you are measuring.

Common uses include:

• Device or circuit voltage measurement
• Component resistance measurements
• Measuring the current flowing through a circuit
• Measuring continuity on components such as switches, connections and fuses
• Testing diodes
• Measuring a capacitor's value
• Measuring the frequency of an AC signal
• Temperature measurement (provided that the particular multimeter has the correct input)

Each of these functions will involve using slightly different methodologies, functions and settings on your digital multimeter.

You can use the image below as a generalised digital multimeter user guide to familiarise yourself with the tool’s basic settings, probes and functions:

Using a digital multimeter safely is essential. As with all electrical test equipment, you should always ensure your multimeter is in proper working order and safe to use before taking measurements.

Safety checks include:

1. Ensure you are wearing the appropriate PPE
2. Perform a visual inspection, checking for any damage, cracks, and other potential hazards
3. Verify that the multimeter is operating as it should, using a proving unit or known voltage source
4. Monitor the instrument carefully during use, in case warnings or irregularities are detected

Other important safety considerations include:

Fused vs Non-Fused Multimeter Test Leads

Fused test leads provide an additional level of protection to users. However, there are also some negatives to using fused test leads that you should keep in mind. For instance, a blown fuse may suggest that a measured circuit is dead when it is actually live. This can be addressed with safe working practices such as using a proving unit.

These leads can also introduce additional resistance into the test lead, potentially affecting reading accuracy for resistance, for example. It is recommended to carry out a risk assessment to identify whether fused or non-fused leads are the most suitable for a given application.

Built-In Fuses

Any multimeter that measures current must have an internal fuse to protect both the user and the meter from excessive currents. This is essential if the meter is connected to a live voltage source, where it will apply a short circuit. It is critical that such a fuse is replaced with another of the same specification to ensure ongoing protection against short circuits. Failure to do so could lead to significant injury.

Measurement Categories

It is important to note that levelled categories of digital multimeters are available for use in different locations, voltages and circuit types. You will need to choose the appropriate category for your specific application.

• Measurement Category I: Best used for testing circuits that are not directly connected to the mains or specially-protected internal mains-derived circuits.
• Measurement Category II: Best used for testing circuits that are directly connected to low-voltage installations. Examples include domestic appliances and smaller, portable tools.
• Measurement Category III: Best used for testing circuits involved with building installation, such as wiring, circuit breakers, and distribution boards, as well as industrial equipment.
• Measurement Category IV: Best used for testing at the source of low-voltage installations, including electricity meters, ripple control units, and overcurrent protection devices.

In terms of knowing how to read digital multimeters properly, you will need to be familiar with the basic symbols found on standard electronic schematics to fully understand all of the various output readings from your DMM.

Digital Multimeter How-to Guide

Digital multimeters (DMMs) are handheld tools that can measure capacitance, voltage, electrical current and resistance with diode and continuity check.

Watch our handy video to find out more!

Explore RS Pro Digital Multimeters

Take a look at this handy video for an in-depth look at one of our RS Pro digital multimeters. With a wide range of features, multiple functions, and powerful testing capabilities, the benefits of choosing an RS Pro digital meter are clear to see.

Watch the video to learn more, or click below to browse our full range of RS Pro digital multimeters.

Digital multimeters are useful tools that are used daily across a broad spectrum of professional scenarios. They are widely available in various models and form factors.

Many new DMMs also provide a range of enhanced features and modes to cover an even broader array of electronics tasks. This also allows them to offer full compatibility with a greater number of electrical devices and components. Importantly, the ability to switch between AC and DC current measuring capabilities is largely standard these days.

Various types might also offer additional settings for diode and continuity test functions, often indicating the status of certain readings via an audible alarm. Depending on the exact model, further optional modes and advanced features might include the ability to test for capacitance and inductance on relevant electrical components.

Multimeters can be designed either as highly portable handheld devices, ideal for fieldwork and for accessing harder-to-reach circuits and machinery, or else built for semi-permanent installation on a work surface.

The latter type, known as bench meters or bench testers, tend to be bulkier and less easy to move around. However, they will typically offer a greater degree of accuracy and detail in their measurements and digital readout displays than their more lightweight handheld counterparts. The main difference between handheld and bench-top models tends to be in their maximum achievable accuracy. Broadly speaking, the smaller a device is, the less sensitive (and therefore pinpoint accurate) it will be overall. Bench multimeters typically also offer the ability to be remote-controlled and to make large numbers of measurements per second. These functions are often used in production test applications.

Spotlight: True-RMS Digital Multimeters

A true-RMS (TRMS) digital multimeter is a specific type of multimeter. RMS stands for root mean square, and these devices are often preferred to their standard counterparts. This is because true-RMS multimeters are the only type that can accurately measure both sinusoidal and non-sinusoidal AC waveforms.

RMS devices achieve this by calculating the equivalent DC current value of an AC waveform. Therefore, these sophisticated instruments can measure both types of waveforms to a high degree of accuracy. To put this into perspective, standard averaging multimeters may lose up to 40% accuracy when attempting to measure non-sinusoidal waveforms.

The potential for non-sinusoidal waves in circuits has increased significantly over recent years. This means that true-RMS has become a much greater necessity, and these types of multimeters have become more widely used. Some examples where measuring true-RMS could be useful include electronic ballasts, HVAC systems, variable-speed motor drives, and solid-state environments.

Analogue multimeters perform many of the same sorts of functions as their digital counterparts, but use traditional dials to zero in on voltage, current and resistance settings. While you can still buy analogue multimeters, you are more likely to use digital nowadays. As a result, analogue multimeters tend to be much less widely available than modern digital versions, which are generally more efficient, accurate and cost-effective.

Analogue meters can be useful on rapidly fluctuating circuits as the needle can be observed more easily than a rapidly changing digital display. In addition, analogue meters tend to have a lower input impedance (DMMs are typically 10 Megohms) which is useful for measuring voltages in certain types of circuits. Although, digital multimeters help to address these needs. Some offer an analogue bar graph to simulate an analogue dial. Certain models also include a low impedance (low Z) mode to reduce the possibility of false readings due to ghost voltages and improve the accuracy of measurements.