Find the Right Test Leads for Your Hand-Held DMM

Hand-held digital multimeters (DMMs) go every place there is a need to check electrical circuits and electronic devices—no place is too cramped, too small or too difficult to access. A DMM is the tool of choice for field service, equipment test and factory maintenance professionals because it makes the mundane voltage and current measurements as well as some not-so-ordinary checks, such as frequency and capacitance.

Although you may spend hours deciding which hand-held DMM is best, remember the importance of the test leads. Choosing the right lead is not difficult as long as you consider all the technical issues before you make a purchase.

The first factor is the mechanical properties such as connector and tip style, cable length, color and material type. For example, not all banana-plug leads fit all DMM inputs, said Mark Hilton of Tektronix. They must have the proper plug length to assure good electrical contact. Also, many newer UL-certified test leads have shrouded banana plugs for added safety, but these connectors do not fit all DMMs.

The length and diameter of the tip should match the application, continued Mr. Hilton. If you probe tight spaces on a circuit board, a narrow probe tip is best. But if you need high tip force, a larger-diameter tip will usually provide the strength and durability required.

Don’t forget to check the availability and compatibility of tip accessories such as hook and alligator clips. Some tip accessories attach directly to the test lead via threads or a slip-on interface; others have banana-plug inputs and require a dual banana-plug cable.

Cable and boot material is another differentiator to examine. Some PVC leads are less flexible over a broad temperature range and melt at a relatively low temperature, said Mr. Hilton. Silicone leads are preferable in extreme conditions and resist cracking from mechanical fatigue.

Other mechanical characteristics, such as color, are important for easy identification, while cable length is important for test-point access. Finally, a relief boot at the cable/connector interface protects the test lead and reduces broken or damaged wires.

Most standard leads supplied with DMMs are suitable for probing around a circuit, either electrical or electronic, said Cliff Asbill, Senior Product Specialist for Fluke. But additional probe flexibility can be more convenient when you are working on equipment that requires hands-free operation. For example, industrial test clips with alligator-style ends and a long handle help keep your hands away from potentially dangerous circuit voltages.

The quality of the test probes, however, is best guaranteed if they are certified by a nationally recognized testing laboratory, such as UL, said Mr. Asbill. A laboratory certification symbol on the probes indicates the leads meet industry safety standards.

Voltage and current limits are the key electrical characteristics to consider for basic test probes. The bandwidth of these test leads is usually not an issue since the DMM is limited to a relatively low AC frequency, said Mr. Hilton. For applications with frequencies higher than 10 kHz, use an RF probe.

The key specifications of an RF probe, in addition to maximum voltage and current, are bandwidth, accuracy and true rms capability, added Mr. Hilton. Some RF probes assume all waveforms are sine waves and determine rms values based on this parameter. True rms probes do not follow this assumption and yield more accurate readings for a wider range of waveforms.

To select a high-voltage probe, ensure that the input impedance of the probe is appropriate for the DMM, said Mr. Hilton. Remember that the voltage rating of the high-voltage probe decreases as frequency increases. The attenuation of the probe also is important because it prevents a signal from overloading the DMM.

The current safety certifications for DMM accessories include UL 1244 and IEC 1010. These specifications indicate certain safety factors above the advertised electrical limits. They also require safe mechanical design, such as hand stops, to prevent your hand from slipping down the probe and being exposed to shock hazard. Make sure the DMM test probes you purchase comply with these specifications.

Most hand-held DMM manufacturers offer a choice of leads, but help is also available from companies such as ITT Pomona that specialize in test probes and leads. For your hand-held DMM needs, consult the comparison chart that accompanies this article.

Copyright 1996 Nelson Publishing Inc.

February 1996

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