A Complete Guide to Digital Multimeters

A multimeter, also known as a digital multimeter (DMM) or multitester, is a versatile tool used for measuring and displaying important parameters in electronics and electrical circuits. It combines the functionalities of a voltmeter, ammeter, and ohmmeter into a single convenient unit. A multimeter allows you to measure voltage (V), current (A), and resistance (in Ohms), which are crucial in understanding the behaviour of electronic components and circuits.

• Voltage refers to the difference in electrical potential between two points, representing the potential energy of an electric field to drive current through a conductor or circuit
• Current, on the other hand, measures the active rate of electrical charge flow through a component or circuit, usually carried by electrons moving through a wire
• Resistance quantifies the extent to which a device or material limits the flow of electrical current

To use a multimeter effectively, probes and test leads are typically connected to the device that facilitate the transfer of 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. They are versatile tools used extensively across professional fields and offer a wide range of features, with true-RMS (Root Means Square) multimeters being particularly valuable due to their ability to accurately measure both sinusoidal and non-sinusoidal AC waveforms. Their practicality spans from fieldwork to production testing, making them indispensable in today's electronics industry.

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 multimeter has the correct input)

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

In summary, a multimeter, or digital multimeter, is a valuable tool that enables precise measurement and display of voltage, current, and resistance in electronics and electrical circuits. It offers versatility and a compact design making it an essential instrument for professionals and enthusiasts alike.

Multimeters are essential measurement tools used daily and are important within various professional applications. Available in different models and form factors, they offer a range of different features and modes to suit your specific needs and electrical work. With the ability to measure both AC and DC currents, modern multimeters ensure compatibility with a wide range of electrical devices and components.

In addition to this, certain types of multimeters can also go beyond the basics by providing additional functions such as diode and continuity tests, accompanied by audible alarms to indicate readings being taken. More advanced models offer capabilities to test capacitance and inductance in relevant electrical components.

Two main multimeter designs

• Handheld multimeters are portable and ideal for fieldwork, allowing access to hard-to-reach circuits and machinery
• Bench meters, also known as bench testers, are intended for semi-permanent installation on work surfaces. While bulkier and less portable, they often offer higher accuracy and detailed digital readout displays compared to their handheld counterparts. They can also be remote-controlled while being capable of making many measurements per second which makes them suitable for production test applications

True-RMS (TRMS) digital multimeter

One notable type of multimeter is the True-RMS (TRMS) digital multimer, which is preferred over standard multimeters. True-RMS devices accurately measure both sinusoidal and non-sinusoidal AC waveforms by calculating the equivalent DC current value, whereas standard averaging multimeters can lose up to 40% accuracy when measuring non-sinusoidal waveforms. As non-sinusoidal waves have become more prevalent in circuits, the need for true-RMS multimeters has increased as the preferred device for testing. True-RMS provide vital and excellent results in various applications such as electronic ballasts, HVAC systems, variable-speed motor drives, and solid-state environments.

To use a digital multimeter (DMM) safely and effectively you must understand the importance of safety checks, the pros and cons of fused and non-fused test leads, the significance of built-in fuses, and the measurement categories for different applications. Also important is that you understand what you are measuring and how to use and set up all the various functions.

You can use the image below as a generalised multimeter user guide to familiarise yourself with the 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:

• Wear appropriate personal protective equipment (PPE)
• Perform a visual inspection to identify any damage, cracks, or potential hazards
• Verify the correct operation of the multimeter using a proving unit or known voltage source
• Continuously monitor the instrument during use for any warnings or irregularities

Fused 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 or digital multimeter used to measure 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

Digital multimeters are categorised based on their suitability for different locations, voltages, and circuit types. You will need to select the appropriate category for your specific application:

• Measurement Category I: Ideal for testing circuits not directly connected to the mains or specially protected internal mains-derived circuits
• Measurement Category II: Perfect for testing circuits directly connected to low-voltage installations, such as domestic appliances and smaller, portable tools
• Measurement Category III: Recommended for testing building installations, including wiring, circuit breakers, distribution boards, and industrial equipment
• Measurement Category IV: Designed for testing at the source of low-voltage installations, like electricity meters, ripple control units, and overcurrent protection devices

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.

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.