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# What is a Digital Multimeter?

By Josh Brown

If you are an engineer working with electronics on a daily basis, the question of “what is a digital multimeter” may come off as, well, a little silly. You use one every day and cannot imagine your work life without a multimeter. Perhaps, at an early age, you gave up your security blanket in favor of a multimeter. (Yes, you have issues, but this makes you an amazing person in the eyes of Keithley engineers.)

However, for those who are not engineers – home hobbyists, do-it-yourself’ers, those interested in electronics or electrical work for a future, and students presently studying electronics but also struggling to pay attention in class – a little explanation may be in order. Let us break “digital multi-meter” down to help us shape a definition:

• Meter – a device that is used to provide a measure of a certain phenomenon or stimulus (for example, voltage, current, resistance, etc.)
• Multi – more than one
• Digital – providing a digital readout with more values more easily displayed than an analog indicator (like a needle on a gauge)

Putting it all together: A digital multimeter (DMM) is a single device that can provide more than one type of measurement of a certain electrical stimulus using a more accurate and readable digital output than an analog solution.

NOTE: IF YOU DO NOT LIKE MATH, SKIP AHEAD TO THE NOTE THAT READS, “IT IS SAFE TO START READING AGAIN”

DMMs for Ohms’s Law

A DMM is primarily used to verify one of the three factors of Ohm’s Law which are: voltage, current and resistance. These are related by the following mathematical equation:

V=I*R

Where,

• V = voltage, which uses volts (V) as the unit of measure
• I = current, which uses amps (A) as the unit of measure
• R = resistance, which uses ohms (Ω) as the unit of measure

The beauty of this formula is that while it is hammered into the heads of electrical engineers with 4+ years of college education, it is also understandable by elementary school students who have an exposure to multiplication and division (I know – I tested with my own offspring). It is simple to find the single unknown if you have knowledge of the two other elements:

• If you know current and resistance, then you can figure out voltage
• If you know voltage and current, then you can figure out resistance
• If you know voltage and resistance, then you can figure out current

For example, you may have a theoretical setup having a 1k resistor with a 5V signal across it. However, your resistor might truly measure at 995.6 Ω and your voltage source might be 5.05 V.

Current for the theoretical setup would be determined as:

Current for the real setup would be:

NOTE: IT IS SAFE TO START READING AGAIN

Why a DMM is Necessary

Let’s move away from the theoretical values and instead focus on real ones – those that are less than perfect and ones we truly work with. A digital multimeter shows us numerically-based signal levels and builds our confidence in the state of things. The DMM helps to provide immediate proof of live voltage and current by way of measurement. An important reason to get real, immediate readings from your DMM is for the verification of the state of a system (or circuit) for safety purposes:

Voltages in excess of 42V or currents in excess of 10 mA can provide painful shocks; beyond this can be lethal.

Additionally, resistances (which usually have insulated body material) are rated for certain powers (watts) and can get hot when driving higher currents and voltages.

Who Uses a DMM and for What

Digital multimeters have a wide variety of users, each with their own sets of needs:

• Home hobbyists and those with a “do-it-yourself” approach to electrical maintenance
• Why?: To make measurements and troubleshoot simple circuits: voltage, resistance, continuity, current
• What Type?: Generic handheld DMM is adequate, don’t need a high degree of accuracy, just a general idea of what is working. Simplicity is the key here: the controls, connections, and readings should be nearly fool-proof.
• Electricians, Electrical Contractors, and HVAC Specialists
• Why? To make measurements and troubleshoot circuits or systems that have hazardous levels present: voltage, current, continuity, resistance, temperature
• What Type?: Handheld DMM is appropriate but needs to be rated for the levels that are to be measured and meet safety standards for the job at hand. Simplicity is highly important here as well; so is mobility.
• Students, Instructors and Professors of Electrical Engineering and Electronics Engineering Technologies programs
• What and Why?: Handhelds and benchtop DMMs are both used but the degree of precision becomes more important, and the ranges over which the measurements can be made have more variety.
• Additional functions: continuity, temperature, capacitance, diode, frequency, period
• Resistance: > 1 GΩ and < 1 Ω
• Current: > 10 A and < 1 uA
• Voltages: > 1000 V and < 100mV
• Professional Electrical/Electronic Engineers
• What and Why?: Benchtop/system DMMs are the primary tool where the same sets of functions/precision used by students and instructors apply but additional features are leveraged to help them perform more advanced measurements in their daily work, providing greater confidence in their designs.
• Digital communication options for interfacing and automating with a PC and programming software
• Tools for interfacing with and triggering other instruments for measurement timing and synchronization
• Tools for slowing measurements to observe very low-level signal behavior
• Tools for speeding measurements to observe fast (transient) signal behavior

If you fancy yourself as a handheld DMM connoisseur, follow this link for some decent supplemental info on handheld digital multimeters.Did you get all that? It was a little wordy for me and I started nodding off while proof-reading, so I don’t blame you if you felt the same. (What does a person have to do to get some more pictures thrown in here?)

If you see yourself as a benchtop or system DMM user, follow this link for some decent supplemental info on bench/system digital multimeters.

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