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Maintenance: Using a multimeter

Maintenance: Using a multimeter
A multimeter being used to test a 'wall wart'

Maintenance: Using a multimeter

For routine maintenance and for troubleshooting, shares Frank Wells, one of a tech’s best friends can be a multimeter

A multimeter (or VOM or volt-ohmmeter) is an electrical measurement device used to test voltage, current and resistance, and a worthy addition to every house of worship’s tech team’s toolkit. Multimeters are easy to learn how to use and can be a great aid in troubleshooting wiring, electrical power and DC voltage power supply problems.

While there are analogue multimeters with a needle meter display, a digital meter with a direct readout of measured value is simpler and recommended.

Digital multimeters feature an LCD display of measured value. The selection of function is most commonly made with a dial switch. Within a given function, there are often a range of measurements that can be made, or ‘auto-ranging’ meters that simplify use. ‘Banana jacks’ are fitted for interface to probes (male banana plug on one end and a metal tip on the other) for extending the inputs to what’s being tested. Often, handy ‘alligator clip’ tip adaptors are provided for making hands-free measurement. Basic and adequate digital multimeters can be purchased for between US$20 and $50.

Voltage measurements

A multimeter can measure the available voltage at a wall outlet simply by selecting an AC voltage mode on the meter and plugging the two probes into the hot and common of an outlet. No voltage? Run check the circuit breakers.

A Fluke digital multimeter
A Fluke digital multimeter

Voltage measurements can also tell you if you have ‘brown-out’ issues (lower than expected voltage). I once had a client with a cassette tape copy system (yes, well before digital file swapping), and the decks were all dropping out of record seemingly randomly. Measuring the voltage where they were plugged in, we observed it dropping from 120VAC to 95VAC at the same time the decks stopped. We found that a worker had a space heater under her desk, plugged into the same circuit. Every time the heater kicked on, the voltage dropped at all the connected outlets.

Voltage measurements can also be made of external power supplies (‘wall warts’) simply by using the probes to touch the contacts on the output connector and reading the value of the voltage. Similarly, batteries can be tested quickly with a multimeter. A new AA or AAA will measure 1.5VDC or above. Measured voltage just below 1.5VDC can be considered suspect or only suitable for non-critical use (put them in the wall clock, not the wireless mic). A battery measuring 1.3VDC or below would be considered spent. Rechargeable batteries will measure lower by their nature.

Current measurements

Measuring current is likely to be the least used function on a HOW’s multimeter. Current measurements are made in series, requiring breaking a circuit and putting the meter inline so the current being measured flows through the meter. For electrical power circuits, this is not safe and shouldn’t be attempted.

Current tests can be used in battery testing. I recommend against just touching the probes of a multimeter across a battery in current mode. This effectively short circuits the battery and, while the large current reading can be used to tell if the battery is good, it also wastes battery life. A better method would be to place a current-limiting resistor in series with the battery and meter and getting reference measurements from known good and depleted batteries. The electrically handy could build a simple test jig with a battery holder and resistor, connect a multimeter with alligator clips and pop batteries in and out to test.

An 'old-school' analogue multimeter
An 'old-school' analogue multimeter

Resistance measurement

HOW techs won’t have a lot of use for measuring resistance in ohms, except for measuring the lack of resistance, or continuity. Microphone or other signal cables can be tested with an ohmmeter just by touching the probe to, say, Pin 1 on one end of a mic cable and the other probe to Pin 1 on the other end and looking for a very low resistance (most multimeters have a continuity test mode that sounds an audible tone when a path has continuity, which saves you having to look at the display). Also measure from Pin 2 to Pin 2 and 3 to 3 to test those wires. Then measure from each pin to the others (1 to 2, 1 to 3, etc.) where a continuity ‘beep’ would indicate shorted or cross-wired connections. Other types of connections can be similarly tested. A continuity test can tell if a fuse is blown or good.

For testing installed wiring where the ends are far apart, I would have a helper place a resistor of 100–5,000Ω across two contacts of a connector and measure for that value on the other end of the cable.

Continuity or resistance measurements should never be made on cables plugged into devices, or on any device plugged into a wall.

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