Functions of Multimeters

Introduction

One of the most popular, versatile, and useful electrical testing instruments is the digital multimeter, also known as a DMM. DMMs are considered the best overall meter available for use with energized as well as de-energized electrical circuits.

They offer a variety of uses for both the professional and amateur users, and can be used for the measurements of AC and or DC voltages, resistance and continuity, AC and or DC current, Ohms law and power formula, frequency and duty cycle, diodes, and capacitance.

Applications include electrical engineering design, maintenance, quality control, and inspection, and any application requiring electrical servicing and diagnostic testing.

As with any involvement with electricity, safety is mandatory, and the user needs to learn safe practices and tips when using a DMM instrument. Learn what to look for when selecting DMMs, and determine weather or not the DMM has extra protection features such as overload protection, high energy fuses, etc. Also there are new international safety standards for low voltage (1000 volt and less) test equipment and how they apply to digital multimeters.

Selection

Choosing a DMM should be made based upon the electrical measurements needed.

For example: A precision DMM is required if you need more exact small value measurements in resistance and current and the capability to check transistors and diodes.

Digital meters are generally accurate within 0.1% to 0.5% of the reading.

Autoranging, or dialing, is also a useful feature to look for in a DMM. It allows the user to quickly move from the instruments various measuring scales while maintaining accurate readings.

Another important feature to consider when selecting a DMM is whether its a True RMS (root mean square) meter. A RMS value is also called the effective or heating value of an AC signal. The RMS value is equivalent to DC voltage that provides the same amount of heat generated by a resistor as AC voltage would if applied to that same resistor. Since an AC signals voltage rises and falls with time, it takes more AC voltage to produce a given RMS voltage. A handy formula to remember is: Peak Volts AC x .707 = VRMS (volts root mean square). A power grid that must produce 169 volts peak AC turns out to be 120 volts RMS (.707 X 169). What does all this mean?

The typical DMM is not a True RMS (root mean square) meter and as a result it will produce misleading voltage readings when used to measure anything other than a DC signal or sine wave.

DMMs featuring True RMS Measurement are for nonlinear voltage and current loads. This feature is necessary when taking AC voltage and current measurements.

There are several methods to incorporate the RMS feature into a DMM. Each handles AC differently. The following are three basic types:

1. A rectifier type multimeter indicates RMS values for sinewaves only, (sine wave is also called cycle and is measured in Hertz-i.e. 60 Hertz or 60 cycle) It does this by measuring average voltage and multiplying by 1.11 to find RMS. Trying to use this type of meter with any wave form other than a sine wave will result in erroneous RMS.
2. Average reading digital volt meters are just that, they measure average voltage for an AC.
3. A True RMS meter uses a complex RMS converter to read RMS for any type of AC waveform.

Unfortunately, True RMS reading meters are very expensive; thus careful consideration should be given as to what type DMM will best suit your specific needs.

Summary

The most important factor to keep in mind is that electrical test equipment should be used with respect and maintained in accordance with the manufacturers guidelines. As with all electrical test equipment, individuals using a DMM should receive training in its proper operation.

Commonly Asked Questions