DMMs Do More Than Measure

Today’s DMMs offer many “cool” features that enhance their measurement capabilities. Besides serial ports, you’ll find graphical displays, dual numerical displays, backlit displays, power calculations, and test signals. Table 1 shows the “cool” features currently available in handheld DMMs. Bench meters, which now offer features such as scanners and specialized measurement functions, are listed in Table 2.

Serial-port DMMs such as the Tektronix Model TX3 (Fig. 1) have software that lets you view and record the meter’s measurements. You can display the current reading or plot the measurements on a PC, too. The serial cable and software are usually available as options, but with the HC Protek 506, the serial cable is standard, and you can download the PC software from the company’s Web site.

Figure 1. DMMs with serial ports let you get measurements from your meter. Software lets you store measurements in data files and display measurements in graphs. (Courtesy of Tektronix.)

Some meters have their own graphical displays so you don’t need a computer to view waveforms. The Extech Instruments 381270 Multiscope (Fig. 2) and the Fluke 860 series graphical DMMs let you view waveforms right on the meter. Both meters have serial ports and optional PC software. Extech’s meter has a waveform 100 kHz display bandwidth while the Fluke meters reach 1 MHz. Fluke’s 860 series meters also measure and display waveforms and perform calculations. Figure 3 shows the meter displaying minimum and maximum DC current, plus the average measurement.

Figure 2. Graphical displays, which tell you more about a waveform than numeric displays do, now appear in DMMs. (Courtesy of Extech Instruments.)	Figure 3. DMMs can calculate min, max, and average readings. (Courtesy of Fluke.)

When you make AC measurements, you often need to see voltage and frequency simultaneously. Or you may need to simultaneously view voltage and current. For voltage and frequency measurements, you can use the B&K Precision Models 2880A or 2890. The second digital readout can measure frequencies up to 200 kHz.

AC measurements often require you to measure power as well as voltage and current. Before DMMs had their own processors, you had to measure voltage and current separately and then calculate the power, taking power factor into account. Now, you can use a single meter to take all three measure- ments. One meter that performs these tasks is the MetraHit 29S from GMC Instruments (Fig. 4 ).

Meters Offer Safety Features
To measure the AC power, set the meter’s dial to the watts setting. The meter will open its safety shutter and give you access to its 10-A probe jack. That’s a safety feature of the 29S and other meters—you can’t connect probes to the current input unless you set the meter to measure current. When measuring power, connect the voltage probes to your voltage source and connect current probes inline with your current flow, respectively. Or, you can use a current clamp to measure current. The 29S meter will then display either active power, reactive power, or apparent power. You can change power readings by pressing a function button.

Although GMC’s MetraHit 29S measures voltage and current and calculates power, you still need to connect probes to measure the power consumption of an AC-powered device. If you want to make the connection more easily, consider the Extech 382860. This meter has an attachment that you plug into a standard AC receptacle, then you connect your device’s AC power cord to the meter’s attachment. Next, plug the attachment’s other end into the meter and take measurements.

While some meters are designed for electrical power measurements, other are designed for taking measurements on circuit assemblies. Sure, you can use almost any DMM to measure voltage on a board, but how many times has your hand slipped off the test point when you turn your head to look at the meter? A handy way around that problem is to have the probe and display in one unit.

Two companies that offer pen-style DMMs are Extech and HC Protek. Figure 5 shows the HC Protek Model D-935A. Besides letting you see your measurement without turning away from the test point, both pen-style meters have one-hand operation, which lets you change settings without removing your hand from the meter—and without losing the test point.

Today’s handheld DMMs not only measure signals, they generate them. Meters like the Extech 381270 and HC Protek 506 generate TTL-level square waves. Extech’s CMM-15 also produces analog outputs for calibrating process-control equipment. The meter can produce up to 15-VDC or 25-mA DC square wave outputs ranging in frequencies from 0.5 Hz to 4800 Hz.

Bench Models Offer Options
Handheld DMMs aren’t alone in having “cool” features. Bench models have them, too. Models from Hewlett-Packard and Keithley can scan multiple channels making them, effectively, small data-acquisition or test systems. HP’s 34970A uses the same 61/2-digit analog-to-digital converter (ADC) found in the company’s popular 34401A DMM. The 34970A base model comes with 20 voltage input channels, and you can add two more modules to increase channel count. You can also get measurement modules for other parameters such as temperature.

Keithley’s Model 2000, also a 61/2-digit DMM, has space for one switch card, with 10- or 20-channel voltage models available. A nine-channel thermocouple card is also available.

HP’s 34420A is a nanovoltmeter that adds temperature measurements for standard platinum resistance thermometers (SPRTs). It works as a laboratory temperature standard for when you need to compare other temperature probes to your SPRT reference probe.

Keithley has introduced specialized DMMs such as the Model 2015 total harmonic distortion (THD) DMM. This meter performs audio-frequency tests in both the time and frequency domain. Under computer control, the meter produces frequency spectrum displays of harmonics and frequency response. Fixed and programmable digital filters let you measure the response of telecom audio equipment such as cellular phones and transducers. To stimulate a UUT, the 2015 has a sine wave audio signal source. T&MW