Electronics are everywhere. They permeate every conceivable aspect of our lives and perform all kinds of functions, from controlling vehicle engine performance to fine- tuning the amplified frequencies of hearing aids. As a result, test instruments are needed to verify that these products are operating at peak performance.
The hand-held digital multimeter (DMM) fits the bill, in many cases, because it is a versatile instrument that measures electrical operation and gives a quick health check of these products. Typically, it has a generous assortment of test features, some that are basic and others that are job specific. So how do you select the right one?
The minimum functions are those required for basic, fundamental measurements. These measurements include voltage, resistance, and current. For AC and DC volts, the meter should measure to 500 V with a resolution of 1 mV, and current functions should provide a resolution of 1 mA with a measurement range to 10 A. The meter should measure resistance from 1 W to 10 MW .
Of course, make resistance measurements with the circuit power off. A DMM that has an ohms-mode protective circuit will prevent damage if contact is accidentally made with a voltage.
For accurate low-ohms measurements, resistance in the test leads must be subtracted from the total resistance measured. Typical test-lead resistance is from 0.2 W to 0.5 W . You should replace the test leads if the resistance is above 1 W .
Other essential measurement capabilities include continuity checks and diode tests. For continuity checks, the meter should have a beeper to allow you to make many checks easily and quickly without looking at the meter.
You need a meter with a specific function for checking diodes because they can be turned on if the meter measuring voltage is more than a specific value, such as 0.6 V. This mode measures the voltage drop across a junction. For example, a silicon junction should have a forward voltage drop <0.7 V when measured in the forward direction and an open circuit when measured in the reverse direction.1
Other important measuring capabilities include capacitance and temperature functions, while significant features to look for are total rms (AC + DC) and minimum, maximum, and average recordings, said Mike Engbretson at Tektronix. The reason you want a total root mean square capability is to get an accurate measurement of the current or voltage.
For example, it is important if you want to calculate the power in a 1-W resistor in a circuit that has a DC bias current of 2 A and an AC signal current of 5 Arms. A meter without the AC + DC feature measures a voltage of 5 Vrms. You conclude that the power in the resistor isTo determine if there is a DC bias current, toggle between the AC and AC + DC measurement modes on the meter. If the AC + DC is greater than the AC value, then there is a DC bias current.
The minimum, maximum, and average recording features help capture sags and swells in the rms value, continued Mr. Engbretson. Subcycle events are captured by putting the meter into the 1-ms peak-hold function. A meter with a time-stamp feature is useful because it indicates when the event occurred. For example, a facility maintenance technician may find electrical equipment experiencing a periodic failure due to a power fluctuation. A meter with a time-stamp feature can determine when the disturbance occurs and link the failure time to certain operations in the plant.
There is no minimum function and feature set everyone should have in a DMM, according to Randy Mather at Fluke. You determine them based on what you need to test. For example, manufacturers make some meters with current measurement capability and others without it because customers do not want to pay for features they do not need.
Meter and User Safety
To protect you from electrical shock and prevent electrical damage to the meter, it is important to know the maximum voltage that can be applied to the input of the meter and the environment in which it will be used. For example, if you are making measurements on a 480-V distribution transformer, you must use a DMM that is rated for at least 600 Vrms CAT III. The 600 Vrms refers to the maximum voltage that will be applied to the meter, and the CAT III indicates the environment you are working in, such as single-phase commercial or three-phase distribution equipment. The over-voltage installation category, or CAT number, is broken into four classes.
A higher CAT number refers to an electrical environment with higher power available and higher-energy transients than an environment designed to a lower number, such as CAT II (see “Choosing the Right Meter for Safety”). Within a category, a higher voltage rating means the instrument has a higher transient-withstand rating. For example, a CAT III 1,000-V meter rating has superior protection compared to a meter with a CAT III 600-V rating.2
Automatic input blocking also prevents damage to the instrument or shock to the user, said Andy Zeidler, marketing manager at GMC Instruments. It helps match the measuring function to the correct input terminals by preventing you from connecting the test leads to the wrong meter terminals when you select a measurement function.
The best meter for you offers all the appropriate measurement functions, ranges, accuracies, and resolutions for your job. It also should have built-in features that protect you and the meter from dangerous energy levels. Before you buy, check the chart that accompanies this article to find the best one for you.
Reference
1. ABCs of DMMs, Multimeter Features and Functions Explained, Application Note, Fluke, August 1997.
2. ABCs of Multimeter Safety, Multimeter Safety and You, Company Application Note, Fluke, December 1996.
Copyright 1998 Nelson Publishing Inc.
February 1998