Understanding multimeters

Sections: VOMs How VOMs work DMMs A/D converter Making measurements with multimeters Meter comparison

Measurement of electrical parameters has evolved considerably during the past century. However the parameters we measure do not change. Measuring volts, amperes, and ohms can be done with an analog volt-ohm-milliammeter (VOM) (Fig. 1.) or a digital multimeter (DMM) (Fig. 2.).

VOMs

VOMs are electromechanical. They use switches to select among electrical parameters and ranges. As the name implies, VOMs measure volts, amperes, or ohms, whereas some DMMs can measure these parameters in addition to frequency and temperature.

Fig. 1. The volt-ohm-milliammeter (VOM) is an analog meter used to measure voltage, current and resistance using analog technology. (Courtesy of Simpson Electric Co.)

Fig. 2. The digital multimeter (DMM) measures voltage, current, and resistance using digital technology. It features high input impedance and a high degree of readability. (Courtesy of Fluke Corp.)

The heart of the VOM is the D’Arsonval movement (Fig. 3). It is a meter movement consisting of a small, lightweight coil of wire supported on jeweled bearings between poles of a permanent magnet. When the dc to be measured passes through the coil, its magnetic field interacts with that of the permanent magnet and causes the coil attached to a pointer to rotate in direct proportion to the current passing through it.

By design, it is necessary to pass dc through the D’Arsonval movement coil. The coil would not respond if ac were applied to it. VOMs measure ac voltage using a rectifier arrangement to convert the ac to a proportional dc so the movement will operate.

The VOM controls allow users to measure dc or ac voltage or current, as well as resistance. Figure 4 illustrates the front panel controls, jacks, and indictors of a typical VOM. These are described in the following list.

Fig. 4. The front panel controls, jacks, and indicators of a typical VOM allow users to select the appropriate measurement type and range to fit a specific application. (Courtesy of Simpson Electric Co.)

1. Front panel — Includes the meter indication, controls, and jacks.

2. Range switch — May be rotated in either direction to select any one of the available voltage, current, or resistance ranges.

3. Function switch — Selects among off, +dc, -dc, and ac V on a typical VOM. To measure dc current or voltage, set the function switch to the -dc or +dc position, depending on the polarity of the input signal. AC voltage is measured with the switch in the ac position. For resistance measurements, the function switch may be set in either the +dc or -dc position. The polarity of the test voltage will be as marked at the jacks when the switch is in the +dc position and reversed in the -dc position. Set the switch to off when not using the meter to take measurements.

4. Zero ohms — Used when measuring resistance to calibrate the selected ohms range to read 0 with the test leads shorted.

5. Circuit jacks — Provide the electrical connections to the test leads. The number of jacks on the front panel depends on the VOM manufacturer. Some VOMs have as many as eight jacks. The common (-) jack is the reference point for the measurement of all the functions in most cases.

6. Pointer adjustment for zero — Used to mechanically zero the D’Arsonval movement of the instrument. With the function switch set to a voltage or current measuring position and no applied input, the pointer should indicate 0 on the meter scale. If it does not, use a screwdriver to turn this adjustment until it does. Once this adjustment is made, back off slightly so that the pointer rests freely over the zero mark. Most meters should be in the horizontal position, with the face pointing upward, to do this adjustment.

How VOMs work

A voltmeter is a current measuring instrument. No, this is not a mistake or misprint. As stated earlier, the D’Arsonval movement is a current measuring device. Period. A voltmeter is designed to indicate voltage by measuring the current flow through a resistance of known value.

Stretching the explanation a bit further, adding several resistances with known values and a switch to connect them to a measurement circuit results in a multirange voltmeter. Figure 5 illustrates a simplified diagram of a dc voltmeter.

Fig. 5. A multirange voltmeter uses a switch and several known resistances to connect to a measurement circuit as illustrated in this simplified diagram.

An ammeter is designed to indicate current flowing within a circuit. The D’Arsonval movement is arranged with the appropriate resistors of known value in series and parallel to provide an accurate reading. Using a VOM to measure current, the circuit under test must be opened and the VOM inserted in series with the rest of the circuit. However, there are clamp-on ammeter attachments available for VOMs as well as stand-alone clamp-on ammeters.

Resistance measurement is accomplished by supplying a known current at a known voltage to a device, usually a resistor, under test. The VOM supplies a voltage, which causes current to flow through the device being tested. The reading indicates the value of resistance in ohms of the device being tested. Resistance measurements are also used to indicate continuity within a circuit.

DMMs

DMMs are electronic. Instead of a mechanical meter movement with pointer and scale, the DMM has a display, usually a liquid crystal display (LCD), with numerals that indicate the measured parameter.

In a VOM, the D’Arsonval movement does mechanical work to move the needle. This uses some of the energy being measured. However, in a DMM, the circuits send information by switching the current on and off.

How DMMs work

A DMM uses both analog and digital methods to make measurements. Signals are received only in analog form.

A simplified block diagram of a typical DMM is illustrated in Fig. 6.

Fig. 6. A DMM measures dc voltage by applying it directly to the A/D converter. AC voltage and current, dc voltage, and resistance must be converted to dc volts before they can be applied to the A/D converter as illustrated in this simplified DMM block diagram.

From the inputs, signals are routed to the range switches, which determine the signal paths inside the DMM.

Signals that are already in dc V are routed directly to the analog-to-digital (A/D) converter.

All other signal types are routed to the signal conditioner, where they are converted to dc V. The signal conditioner consists of the ac converter, current shunt, and the ohms converter.

The dc V signal, either directly routed or converted, is sent to the A/D converter.

The signal is routed to the microcomputer, which converts the signal from the A/D into a signal that can be displayed.

A/D converter

Just as the D’Arsonval movement is the heart of the VOM, the A/D converter is the heart of the DMM. The microcomputer is important, but as with the D’Arsonval, the A/D is where the measurement takes place.

The signal applied to the A/D converter must be dc V. The converter uses the dual slope method to determine the magnitude of the signal applied to it (Fig. 7.).

Fig. 7. The A/D converter uses the dual slope method to determine the magnitude of the signal applied to it. The dual slope method compares a capacitor’s charging time (known) with its discharge time (unknown), which is proportional to the original analog signal.

The dual slope method uses a charging slope and a discharge slope. The capacitor inside the A/D circuitry charges to a magnitude proportional to the dc V applied to it. A known voltage produced inside the A/D converter discharges the capacitor. The higher the magnitude of dc voltage charge on the capacitor, the longer it takes for the capacitor to discharge. The discharge time is proportional to the original analog signal.

Making measurements with multimeters

Both VOMs and DMMs can be used to make electrical measurements. Depending on the type of circuit and the desired accuracy, either meter type is adequate. The input impedance of a VOM is expressed in ohms-per-volt (ohms/V). Typical voltmeters can range from 10 Kohms/V to 50 Kohms/V. A typical DMM has 10 Mohm input impedance across all ranges. The lower impedance of a VOM when measuring small signals can affect its accuracy.

(a)

(b)

(c)

Fig. 8. (a) Voltage measurements are made across, or in parallel with a load or device under test. (b) Current measurements are made in a series with a load. (c) Resistance measurements are typically made by isolating the component to be measured from its circuit. (Courtesy of Fluke Corp.)

Figure 8 illustrates techniques for measuring voltage, current, and resistance. Generally, voltage measurements are made across, or in parallel with a load or device under test. Current measurements are made in series with a load. Clamp on ammeters measure current by surrounding a conductor in the circuit to be measured with a pickup loop. Resistance measurements are typically made by isolating the component to be measured from its circuit. This is especially true when using VOMs. However, in some cases a DMM, with its inherently high input impedance, can be used to measure resistances without breaking the circuit.

This is because high-input-impedance DMMs allow a minute amount of current to flow, while maintaining a high degree of accuracy.

Meter comparison

VOMs and DMMs are both used in plants today. Both do the jobs for which they are intended. Both have advantages when used properly. The accompanying table lists and compares characteristics of VOMs and DMMs.

PLANT ENGINEERING magazine extends its appreciation to Fluke Corp., Ideal Industries, Inc., and Simpson Electric Co. for the use of their materials in the preparation of this article.

VOM vs. DMM

VOM
DMM

Display
Uses an analog scale. Operator identifies proper scale and interprets reading.
Uses digital display. Accurate reading from a distance and at an angle.

Readability
Scale division limits resolution. Readings must be interpreted. Typical resolution is 1%.
High resolution. Meter interprets signal through A/D operation. Typical resolution for 31/2 digit is 0.5% and for 41/2 digit is 0.01%.

Accuracy
Accuracy is a percentage of full scale. Accuracy lessens as the voltage becomes a smaller percentage of full scale. D’Arsonval movement draws power from the circuit. Typical accuracy is
Accuracy is percentage of the reading. Very little power drawn from the circuit. Typical accuracy is

Input impedance
Low input impedance. Input impedance is expressed in ohms/V. Typical input impedance is 10K ohm/V to 50K ohm/V.
High input impedance. Typical input impedance is 10M ohms in all ranges. Impedance is independent of parameter or range measured.

Polarity and ohms zeroing
Manually selected polarity can damage instrument if done improperly. Ohms zeroing must be done manually.
Automatic polarity and ohms zero adjustment.

Range changes
Manual ranging. Operator selects range and interprets readings.
Some DMMs have switched ranging, which must be selected manually. Some DMMs have high-speed autoranging functions.

Commonly used DMM symbols

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