Voltmeters, Ammeters, and Ohmmeters

Meters are the simplest, easiest to use instrument for measuring voltage, current, and resistance. A voltmeter measures voltage; an ammeter measures current; and an ohmmeter measures resistance. Any particular meter might provide one or more of these functions. Convenience features such as auto-ranging (automatic selection of measurement range) combine with very high input impedance and digital display to provide uncomplicated meter measurements. For many applications, meter measurements have become as simple as selecting the meter function and connecting to the circuit under test. However, as the capability of a meter is stretched, more attention must be paid to the measurement at hand.

Analog and Digital Meters

Meters can be classified not only by the parameter that they measure, but also by the type of display or readout used. Meter displays fall into two categories: analog and digital.

Analog Meters

Analog meters generally use some sort of electromechanical mechanism to cause a small arm to move, depending on the voltage or current applied to the meter (Figure 2-1). A graduated measurement scale is imprinted behind the mechanism such that the moving arm points to the value of the meter reading. The actual mechanism involved is not critical; the important point is that analog meters provide a continuously varying readout, without any discrete jumps in meter reading. The mechanism does not have to be mechanical. It can be implemented using other technologies as long as the result is a continuous “analog” type of display.

Figure 2-1. An analog meter has a continuous scale.

Analog meters require care in observing meter readings. Different people watching the same measurement may disagree slightly on the correct interpretation of the meter reading. The resolution of the meter (the ability to measure small changes) will depend on the physical layout of the meter, but generally will be no smaller than a few percent of the full-scale meter reading. The biggest advantage that analog meters have when compared to digital meters is the ability to track changes in meter readings. Sometimes the ability to spot a trend in a meter reading (whether it's increasing or decreasing) is just as important as the absolute reading. For example, if a circuit is to be adjusted for maximum output voltage, an analog meter gives the user visual feedback for making the adjustment, and the output can be peaked very quickly.

Digital Meters

Digital meters, on the other hand, don't provide continuously variable meter readings. The meter reading is converted into decimal digits and is displayed as a number (Figure 2-2). This results in a reading that's very easy to interpret. Several different people viewing the same meter will usually record the same reading (except for cases where the number is changing due to electrical noise or measurement drift). Digital meters are usually specified according to the number of digits in the display. A 3-digit voltmeter has three digits in its display. A 3 1/2-digit voltmeter has three normal decimal digits plus a leading digit which can have only the value 1 or 0 (the maximum reading for this type of display would be 1999). Digital meters lend themselves nicely to interfacing to computers, since both use digital numbers internally.

Figure 2-2. A digital meter displays the meter reading as a decimal number.

Adjusting a circuit for maximum voltage is much more difficult with a digital meter. The user must read the number, decide whether the new reading is larger or smaller than the previous reading, and then tweak the adjustment in the proper direction. For meters with three or four digits, this can be difficult. Some digital meters have included a simple analog- like display in addition to the high resolution digital display to provide the best of both technologies. The digital readout can be used for precise absolute measurements while the analog display can be used to adjust for maximum or minimum value.

Do not assume that the presence of many display digits automatically means very high accuracy. Often, a meter will provide more resolution than its accuracy specification supports. This ensures that the resolution of the meter does not limit its accuracy, but may mislead some users. The amount of accuracy vs. resolution may vary with the different operating modes of the instrument, so careful examination of the specification sheet is in order.

DC Voltmeters

The ideal voltmeter (Figure 2-3) senses the voltage across its terminals without drawing any current because it has an infinite internal resistance. Therefore, it does not load the circuit under test (and, therefore, it's impossible to implement an ideal voltmeter). In other words, the ideal voltmeter looks like an open circuit to the circuit under test. This is exactly what we want: a meter that can be connected to a circuit without disturbing it in any way. The internal implementation of the meter is not a concern as the circuit model is valid for both analog and digital meters.

Figure 2-3. The ideal voltmeter senses the voltage across its terminals without drawing any current.

A more realistic model of a real voltmeter (Figure 2-4) includes the internal resistance of the meter. If the internal resistance of the meter is high enough compared with the resistance of the circuit under test then there will be only minimal loading and the meter will approximate the ideal case. (See Section 1 for a discussion of the loading effect.)

Figure 2-4. The real voltmeter has a finite internal resistance which draws some current from the circuit under test.

EXAMPLE 2-1. A voltmeter with an internal resistance of 14 kohms is used to measure a 10-volt source having an internal resistance of 2 kohms (Figure 2-5). What is the error in the measurement (due to loading)?

Figure 2-5. A real voltmeter is used to measure the voltage source with an internal resistance.

If the voltmeter were ideal and did not load the circuit (Figure 2-6A), the measured voltage would simply be the value of the voltage source, 10 volts. But with a real voltmeter, the loading effect must be taken into account (Figure 2-6B). Since the circuit is a voltage divider:

VM = 10 x 14k / (2k + 14k) = 8.75 volts (real voltmeter)

Error = real - ideal = 8.75 - 10 = -1.25 volts

The error is 12.5% of the ideal value.

Figure 2-6. Measuring with an ideal voltmeter and a real voltmeter. (A) With an ideal meter, no current is drawn and no voltage drop appears across the resistor. The meter reads 10 volts. (B) With a real meter, some current is drawn and the loading effect reduces the measured voltage.

AC Voltmeters

The AC voltmeter is very similar to the DC voltmeter except that it measures the voltage of AC waveforms. (In some cases, it measures the DC and the AC portions of a waveform.) The same ideal voltmeter and real voltmeter circuit models shown in Figure 2-3 and Figure 2-4 apply to the AC voltmeter. Again, the ideal voltmeter does not load the circuit under test, due to its infinite internal resistance. The loading effect of the real AC voltmeter is modeled by the resistance in Figure 2-4. A capacitor may be added in parallel with the resistance in the circuit model of the real meter to account for the input capacitance of the AC meter. (This is not a concern for DC meters since a capacitor acts like an open circuit for DC and does not load the circuit.)

Average Responding Meter

Unless otherwise specified, AC voltmeters are usually calibrated to read out in RMS volts. Not all meters, however, actually measure the RMS value of the signal. Many low cost meters use a circuit that responds to the average value of the waveform. (Here, average value refers to the full-wave rectified average value discussed in Section 1.) The meter readout is still calibrated to read RMS volts as long as the waveform is a sine wave. This is possible because the ratio of the average value and RMS value for a particular waveform can be determined. This is a perfectly valid technique as long as it's understood that the meter read out is only correct for sine waves. If, for example, a square wave were measured using an average responding meter, the meter reading would be invalid. Some average responding meters respond to the average value of the half-wave rectified waveform (instead of the full-wave rectified wave form). These meters will also be in error when used on anything but sine waves.

Some manufacturers supply correction factors with their average responding meters such that other waveforms can be measured. This obviously requires that the shape of the waveform be known, since each waveform will have a different correction factor. If waveforms other than sine waves must be measured using an average responding meter and the manufacturer has not supplied the correction factors, they may be deter mined experimentally by connecting the appropriate waveform with a known RMS voltage. This is a fairly reliable technique as long as the shape and frequency of the waveform are not varied.

Peak-Reading Meters

Some AC voltmeters use a peak-detecting circuit that responds to the peak-to-peak value of the waveform. The meter display may be in terms of peak-to-peak volts or RMS volts. If the meter reads in peak-to-peak volts, the measured value should be accurate regardless of the shape of the waveform. However, if the meter reads RMS volts, then the reading is valid only for a sine wave. In a manner very similar to the average responding meter, the peak-reading meter internally scales its measured voltage to obtain an RMS reading. This is done for only one wave form—the sine wave. So the peak-to-peak reading will be accurate independent of waveform, but the RMS reading is valid only for a sine wave.

True RMS Meters

Meters which actually respond to the RMS value of the waveform are called true RMS meters. (The term “true” RMS is used to distinguish this type of meter from average responding meters which are calibrated to read RMS voltage.) This simplifies the measurement, since the meter reads the correct RMS voltage regardless of the type of waveform. The root-mean-square function described in Section 1 is not simple to implement. Historically, these meters have been rather expensive to build, especially if a wide bandwidth was required. As integrated circuit technology has advanced in recent years, the price of true RMS meters has been decreasing dramatically and this feature is now appearing in low cost meters.

Even true RMS meters have some limitation on the shape of the waveform that can be measured. This limitation is usually specified as a maximum crest factor (peak-to-RMS ratio) that can be tolerated, perhaps with some specified accuracy. As a waveform’s crest factor increases, a meter has to handle a larger peak voltage while measuring a relatively smaller RMS value. For large crest factors, the meter runs out of head room and can no longer measure the waveform accurately.

Bandwidth

When making AC measurements, the bandwidth of the meter must be considered. Sometimes the bandwidth of the meter is specified directly, but more often the manufacturer provides an accuracy specification that depends on the frequency. Either way, the useful frequency range of the meter is defined.

For sine waves, the bandwidth must be at least as high as the frequency of the waveform. For other waveforms that contain harmonics, the harmonics must be included (see the discussion of bandwidth in Section 1).

AC and DC Coupling

As discussed in Section 1, some waveforms contain both DC and AC. The AC voltage may be thought of as riding on top of the DC level. II an AC voltmeter is used to measure this type of voltage, the resulting meter reading will greatly depend on the design of the meter. Some manufacturers have included a coupling capacitor on the input of their voltmeters so that the meter is said to be AC coupled. This AC coupling capacitor blocks any DC voltage that's present, but lets the AC portion of the waveform go through to the meter. So if a meter is AC coupled, only the AC portion of the voltage is measured. (This concept of AC and DC coupling also applies to other instruments, particularly oscilloscopes.)

Some instruments don't have a coupling capacitor and will respond to the DC (and AC) voltage present. This is referred to as DC coupling. It is important to understand how a particular meter behaves, if mixed DC and AC voltages are to be measured. Suppose the waveform in Figure 2-7 is to be measured by both types of meter. If the meter is AC-coupled, then only the 2 volt peak-to-peak sine wave will be measured, resulting in V = 0.707 volt. On the other hand, if the meter is DC coupled then both the AC and DC will be measured, resulting in a much higher reading. One might expect that the RMS reading in this case is just the sum of the DC and AC (RMS) voltages. This is not the case, since the RMS (root-mean-square) function of the voltmeter does not simply add the two voltages together. Assuming that the meter is a true RMS meter, it will read

Vrms = √( Vdc² + Vac² ) = √ (10² + 0.707²) = 10.025 volts

If a meter is DC coupled, adding an external capacitor will make it AC coupled, as shown in Figure 2-8. The capacitor will cause the meter response to roll off at low frequencies. The 3-dB point will be given by:

f (3dB) = 1/(2 π Rint C)

where

Rint is the internal resistance of the voltmeter.

The value of the capacitor should be chosen such that the 3-dB, frequency is about ten times smaller than the frequency of the waveform being measured. This ensures that the frequency being measured is well within the bandwidth of the meter/capacitor combination.

Figure 2-7. The measured value of a waveform containing both AC and DC will depend on the design of the meter.

Figure 2-8. A DC coupled instrument can be AC coupled by adding a capacitor in series with the input.

RF Probes

Since it's difficult to design an AC voltmeter that has very wide band width, an RF (radio frequency) probe is often used to allow a DC voltmeter to make high frequency AC measurements. An RF probe usually detects the peak of the AC waveform and converts it into a DC voltage which is then measured by a DC voltmeter. The probe is adjusted so that the DC voltmeter reads RMS volts, even though it's really responding to the peak value. Therefore, the probe is calibrated only for sine waves. Measuring any other waveform will result in errors (similar to the errors resulting from using a peak-reading AC meter). The RF probe will have some finite frequency range over which it will work (100 kHz to 500 MHz, for example). The frequency of the waveform being measured must, of course, fall within this bandwidth. Figure 2-9 shows a typical RF probe circuit.

Figure 2-9. A typical RF probe circuit converts a radio frequency AC signal to a DC level readable by a DC voltmeter.

EXAMPLE 2-2. The RMS values of three waveforms are to be measured: 1. A 60-Hz sine wave; 2. A 100-Hz triangle wave; 3. A 2-MHz sine wave.

The following AC voltmeters are available:

A. An average responding meter with 20-Hz to 10-MHz bandwidth.

B. A true RMS meter with 5-Hz to 100-kHz bandwidth.

Which meters will accurately measure which waveforms? Meter A can accurately measure only sine waves, since it's an average responding meter (assuming that no correction factors are available). Both sine waves are well within the bandwidth of the meter, so waveforms 1 and 3 can be measured.

Meter B can measure any waveform shape, since it's a true RMS meter, but the 2-MHz sine wave is outside the bandwidth of the meter. So Meter B can measure waveforms 1 and 2.

Ammeters

The ideal ammeter (Figure 2-10) senses the current going through it while maintaining zero volts across its terminals. This implies that the meter must have zero internal resistance. In other words, the meter acts like a short circuit when connected to the circuit under test. Recall from Section 1 that current measurements are made by breaking the circuit at the point of interest and inserting the instrument such that the current being measured flows through the meter. Since it's connected in series (and has zero ohms), it does not affect the circuit under test. An ideal ammeter can't be achieved in practice, but meters that have very low internal resistance can come close.

Figure 2-10. The ideal ammeter has zero internal resistance.

A more realistic model of a real ammeter (Figure 2-11) includes the internal resistance of the meter. If the internal resistance of the meter is small enough compared with the resistance of the circuit under test, there will be only a minimal effect on the circuit being measured and the meter will approximate the ideal case.

EXAMPLE 2-3. An ammeter with an internal resistance of 100 ohms is used to measure the current shown in Figure 2-1 2A. What is the error in the measurement (due to the internal resistance of the ammeter)?

If the ammeter were ideal and had zero internal resistance (Figure 2- 12B) the measured current would simply be the value of the voltage source, 10 volts divided by the 2-kohm resistance.

I = 10V / 2k = 5 mA

Figure 2-11. A real ammeter has a nonzero (but usually small) internal resistance.

Figure 2-12. An example of current measurement using an ideal ammeter and a real ammeter. (A) Circuit to be measured. (B) Measurement using an ideal ammeter. (C) Measurement using a real ammeter.

But with a real ammeter, the internal resistance of the ammeter must be taken into account. Since the meter is in series with the circuit, the resistances add together.

I = 10 V/(2k + 100) = 4.76 mA

Error = real - ideal = 5 - 4.76 = 0.24 mA

The error is 4.8% of the ideal value.

Ammeter Used as a Voltmeter

An ammeter can be configured such that it measures voltage. Many meters use this technique internally to implement the voltmeter function, while the actual metering mechanism responds to current. Figure 2-13 shows an ideal ammeter with a series resistance connected. (This series resistor should not be confused with the internal resistance of a real ammeter. If a real ammeter were being considered here, its internal resistance would add in series with R The current through the meter, I, is given by Ohm’s Law:

I=Vm/Rs

Figure 2-13. An ammeter can be configured to measure voltage by adding a resistor in series.

The current that the meter will read is proportional to the voltage being measured. For simplicity, consider the case where R = 1 kohm.

I = Vm / 1k

if the ammeter was originally calibrated to read milliamperes, then in this configuration it will display VM in volts. The values in this example were chosen to illustrate the concept easily; in practice, other values for Rs may be used as long as the meter scale is chosen to display the voltage properly. Note that Rs is now the internal résistance of the simulated voltmeter. Since a large (ideally infinite) internal resistance is desired for a voltmeter, R is usually chosen to be fairly large and is limited by the sensitivity of the ammeter being used. (If R is chosen too large, then very little current will flow through the ammeter, requiring an ammeter capable of measuring very small currents.)

Voltmeter Used as an Ammeter

A voltmeter can be configured such that it measures current. This technique can be used to make a current measurement even though only a voltmeter is available (or is more convenient). Figure 2-14 shows an ideal voltmeter connected in parallel with Rp. (This parallel resistor should not be confused with the internal resistance of a real voltmeter, If a real voltmeter was shown instead of an ideal voltmeter, its internal resistance would add in parallel with Rp.) Since the ideal voltmeter has no current flowing through it, all of the current, Im, must be flowing through Rp. Therefore, the voltage across the ideal voltmeter is:

V=Im x Rp

The voltage displayed by the voltmeter is directly proportional to the current being measured. Suppose that R is equal to 1 ohm, then one amp of current through the new ammeter will correspond to a one volt reading on the voltmeter. Other values can be used for R as long as the scale of the voltmeter is modified to provide the correct current reading. Rp is the internal resistance of the simulated ammeter, so it's desirable to make Rp as small as possible. The value of Rp is limited by the sensitivity of the voltmeter and the amount of current being measured. For very small values of Rp very little voltage will appear across it, requiring a very sensitive voltmeter.

Current-Sense Resistor

Another interpretation of Figure 2-14 is that Rp is being used as a current-sense resistor. Again, Rp is chosen to be very small so that the circuit being measured will not be disturbed. For example, suppose the current, Im in Figure 2-15A is to be measured. Figure 2-15B shows a small 1-ohm current-sense resistor placed in series where the current is to be measured. Since the 1-ohm resistor is very small compared to the 20-kilohm resistor, it will not affect the value of Im significantly. It does, however, provide a handy place to connect a voltmeter or other voltage measuring instrument so that the current, Im can be determined. The current is calculated using the measured voltage (across the current-sense resistor) divided by the value of the current-sense resistor. In this particular example, the voltage across the 20-kilohm resistor could have just as easily been measured and used to determine the current. In many cases, particularly solid-state circuits, a resistor is either not available or not conveniently located for current measurement. In those cases, a small current-sense resistor can be added without affecting the operation of the circuit.

Figure 2-14. A voltmeter can be used to measure current by adding a resistor in parallel.

Figure 2-15. The use of a current-sense resistor is shown. (A) The current to be measured flows through the 20-kilohm resistor. (B) A small current-sense resistor is inserted in series and the voltage across it's measured.

EXAMPLE 2-4. A current-sense resistor is to be used to measure a current that varies between 4 and 6 mA. If a voltmeter with full scale equal to 0.3 volt is used, what value of resistor will provide maximum sensitivity?

For maximum sensitivity, the voltmeter should read full scale when the current is 6 mA.

R = V/I = 0.3/0.006 = 50 ohms

The resulting voltmeter reading in volts must be multiplied by 1/50 = .02 to determine the current in amps. (Multiply the voltage by 20 to get the current in milliamps.) The circuit under test should be evaluated to see if the current-sense resistor will disturb it significantly.

AC Ammeter

The AC ammeter is very similar to the DC ammeter except that it measures the current of AC (not DC) waveforms. The same ideal am meter and real ammeter circuit models shown in Figure 2-10 and Figure 2-11 apply to the AC ammeter. Again, the ideal ammeter does not affect the circuit under test, due to its zero internal resistance, but a real ammeter will have some small resistance.

All of the considerations discussed under AC voltmeters (average vs true RMS responding meters, bandwidth, AC vs. DC coupling) apply to AC ammeters as well. The reader simply needs to mentally substitute “current” for every occurrence of “voltage.”

Ohmmeters

Voltmeter-Ammeter Method

The most obvious way to measure resistance is to connect a voltmeter and ammeter as shown in Figure 2-16. V and R cause a current to flow through the resistance being measured (R The current through and voltage across Rx are measured by the voltmeter and ammeter and the value of Rx can be computed using Ohm’s law, Rx = Vm / Im

This method is useful for making in-circuit tests when a voltmeter and ammeter are available. In such a case, V and R are not used since the circuit operation presumably causes some current to flow through Rx, allowing the measurement to be performed. This method is not normally used in ohmmeters since it requires both an ammeter and a voltmeter to be implemented inside the ohmmeter.

Figure 2-16. The voltmeter-ammeter method measures the voltage and current across the unknown resistance. The value of the resistance is then computed using Ohm’s law.

Series Ohmmeter

An ohmmeter implementation requiring only one meter is shown in Figure 2-17. R is the resistance being measured, while R and Vs are known values internal to the ohmmeter. The resulting current through R can be computed as follows:

Ix = Vs / (R1 + Rx)

Note that Ix is not directly proportional to the value of Rx There fore, since the ammeter reading is used to indicate resistance, the am meter will not have a simple linear scale but will look something like Figure 2-18 (assuming an analog meter is used). A full scale current reading indicates zero resistance while a zero current reading indicates infinite resistance. (The previous statement should be confirmed with a quick review of the circuit in Figure 2-17.)

Figure 2-17. The series ohmmeter measures the unknown resistance using only one ammeter.

Figure 2-18. The meter scale for the series ohmmeter. Notice that it's not a simple linear scale.

R1 is typically made variable to allow for precise calibration at the Rx = 0 point. Normally, the test leads of the ohmmeter are shorted together and R1 is adjusted until the meter reads zero ohms. This adjustment compensates for changes in Vs (which may be a battery) and for test lead resistance.

Current Source Method

Another method used to implement an ohmmeter is to pass a known current through the unknown resistance and measure the resulting voltage (Figure 2-19). The current source, Is, causes a fixed current to flow, all of which passes through the Rx, since an ideal voltmeter has infinite resistance. (If the concept of a current source is bothersome, consider it an adjustable voltage source which always adjusts itself to the voltage required to cause the desired, fixed current to flow.) Since Vm = Is Rx, the measured voltage is directly proportional to the unknown resistance, so this method results in a linear relationship between the meter reading and the unknown resistance.

Figure 2-19. The current source method for implementing an ohmmeter passes a known current through the resistor being measured.

General Ohmmeter Principles

Only a few of the most common ohmmeter implementations have been outlined thus far. It is usually not necessary to know the internal operation of the ohmmeter as long as the operating manual is followed correctly. What is important is that certain general principles be understood when making resistance measurements.

First of all, since resistance measurements are derived from voltage and current measurements, ohmmeters supply their own stimulus (voltage or current) to the device being measured. This allows individual resistors to be measured without being part of a functioning circuit. This also means that if the resistance is part of a circuit, that circuit must have other sources of voltage (or current) removed from it. Power supplies, batteries, etc. must be turned off or disconnected from the circuit. If not, currents or voltages induced will cause the ohmmeter reading to be incorrect and , if large enough, may damage the meter. (Compare this to voltage and current measurements, where the circuit MUST be powered up to get any meaningful readings.)

Ohmmeters operate using DC voltages and currents, therefore the DC resistance of the device is measured (not AC impedance). Attempts to measure AC impedance with an ohmmeter result in inaccurate and frustrating readings. The classic example is the person who tries to verify the impedance of a loudspeaker, specified as 8 ohms, using an ohmmeter. The ohmmeter will actually measure the DC resistance of the speaker which is generally just the resistance present in the voice coil winding. This reading could be most any value and does not indicate the AC impedance of the speaker. Also, measurements on components other than resistors, such as diodes, transistors, capacitors, etc. may be misleading (although diodes are discussed later).

As described earlier, a zero-resistance adjustment is often provided in an ohmmeter. This adjustment compensates for some of the measurement drift internal to the meter as well as things external to the meter, such as test lead resistance. The usual procedure is to short the test leads together and adjust the meter until it reads zero. This procedure compensates for the resistance of the test leads, removing their effect from the measure ment. If the test leads are removed or changed for a particular measurement, the meter should be zero’d again. In some meters it's also necessary to zero the meter on every different range setting. Instead of a zero adjustment some meters have an infinity adjustment. In that case, the test leads are held apart (open circuited) and the meter is adjusted to read infinity. Other meters have both zero and infinity adjustments which must be adjusted separately.

Resistance exists between two points, very similar to voltage. When an ohmmeter is connected to two points (for example, two ends of a resistor being measured) all of the circuit paths from one point to the other will be measured. If there are actually two resistors in parallel between the two points, the parallel combination will be measured. If there are two resistors in series between the two measurement points, then the series combination will be measured. This must be carefully considered when measuring resistors that are part of a circuit. Simply connecting an ohmmeter across a resistor in a circuit does not necessarily measure only that resistor since there may be other components connected in parallel. Either these components must be accounted for in the resulting measurement or one end of the resistor being measured must be disconnected to guarantee that no other device is connected to it. The human body has a resistance that varies considerably and will affect high resistance measurements. The conducting part of the test leads should not be touched by the user when making resistance measurements above about 10 k-ohms, otherwise body resistance will corrupt the measurement by appearing in parallel with the resistor being measured.

Multimeters

The most popular type of meter, available in both analog and digital technologies, is the multi-meter. A multi-meter is simply a voltmeter, ammeter, and ohmmeter combined into one instrument. Internally, either an ammeter or voltmeter is implemented, which is then used, to perform the other functions. Other names given to the multi-meter are volt-ohm milliammeter or VOM (usually an analog meter) and digital multi-meter or DMM (always a digital meter). An example of a digital multi-meter is shown in Figure 2-20.

MeterRange

Most techniques for implementing voltmeters, ammeters, and ohmmeters are fundamentally accurate over a limited measurement range. Circuit techniques are then used to expand or contract this measurement ability to supply the user with different measurement ranges. Range selection may be implemented by switching different resistor values into the metering circuit, or by switching in an amplifier at the input of the meter. A multimeter functioning as a voltmeter, for example, might have 100 mV, 1 volt, 10 volt, and 100 volt ranges. For maximum accuracy, the lowest range larger than the voltage to be measured is used. A larger range will work, but with reduced accuracy. A lower range will cause the meter to be overloaded, possibly damaging it. If the approximate value of the voltage being measured is unknown, then starting with the maximum range is recommended. The range selection switches are clearly visible on the meter in Figure 2-20.

Figure 2-20. A typical 3 1/2 digit digital multimeter.

Autorange

Meters with an autorange function automatically choose the best range for a given measurement. Autoranging meters essentially duplicate the actions of an experienced instrument user. If the meter is currently being overloaded, a higher range is selected. If the measured value is small enough that a lower range could be used (with greater accuracy), then a lower range is selected. Autoranging meters usually allow the user to disable the autorange feature and instead select the range directly. This is convenient when a particular range is required or the correct range is already known. Although it takes a short amount of time for the meter to autorange, repetitive measurements near the same value may proceed faster if the range is selected directly.

Continuity Indicator

Other features may be included along with the basic voltage, current, and resistance metering. When tracing out wires in a circuit, an ohmmeter can be used to determine whether or not a connection exists between two points. If the resistance value is very low (typically less than a few ohms), there is circuit continuity. If the resistance value is high, then no connection exists. Since the actual resistance measured is not all that critical for many continuity checks, some meters have an audible indicator that can be set to beep when the resistance is less than some value (say, 10 ohms). Thus, the user does not have to turn and look at the meter, but instead can focus his or her attention on the circuit being tested.

Diode Test

Some meters provide a special diode test mode for checking out diodes. A known current is forced through the diode and the forward voltage drop across the diode is displayed. The current value is typically between 0.5 and 1 mA, large enough to switch most diodes on, but small enough so that sensitive diodes will not be damaged. Since a diode conducts in only one direction, proper polarity must be maintained when making the measurement. In some meters, the diode test mode is just a particular range of the ohmmeter function set up so the display reads out the voltage drop. Diode leakage (the flow of current in the reverse direction) may be tested by reversing the polarity of the leads.

Specifications

The exact method of specifying instrument performance varies with both instrument model and manufacturer, but generalities do exist. Voltmeter, ammeter and ohmmeter accuracy are usually specified as percent of full scale (i.e., percent of the range) or as percent of the reading, or both. Digital meters usually add to this an uncertainty of several digital counts.

Each different function of a meter (AC voltmeter, DC voltmeter, ohmmeter, etc.) as well as each range of each function may have separate specifications. It is important to examine the manufacturer’s specifications carefully.

EXAMPLE 2-5. A 3 1/2 digit digital multi-meter used on the 20-volt range (maximum reading is 19.99 volts) has an accuracy specification of ± (0.75% of reading + 2 counts). What is the maximum error if the input voltage is 12 volts?

One count is the smallest change that the meter can display (resolution), in this case, 0.01 volt. (With 3 1/2 digits the meter can read 12.00, 12.01, 12.02, etc.)

Error = ± ((0.75% x 12 volts)+( 2 x 0.01 volts)

Error = ± ( 0.09 + 0.02 )

Error = ± 0.11 volt

So the meter reading could be as low as 11.89 or as high as 12.11 volts. (This analysis does not include other sources of error, such as the loading effect described in Section 1.)

There exists quite a variety of multimeters in the marketplace, with a corresponding wide range of measurement capability as well as price. Generally, increased accuracy, resolution, and features cause a corresponding increase in cost. On the low end are small, portable analog VOMs such as the one shown in Figure 2-21. An abbreviated summary of its specifications is listed in Chart 2-1. Also small and portable, but with improved accuracy and a digital display, a compact digital multi- meter is shown in Figure 2-23, and its specifications are listed in Chart 2-2. Figure 2-25 shows a high performance bench top true RMS digital multimeter whose specifications are listed in Chart 2-3.

Chart 2-1. Abbreviated Specifications of a Compact Analog Volt-Ohm-Milliammeter

Figure 2-21. A compact analog multimeter with basic measurement

Figure 2-22. A compact 3 1/2 digit multi-meter including some advanced features, such as autoranging and audible continuity testing.

Chart 2-2. Abbreviated Specifications of a Portable Digital Multimeter

Figure 2-23. A bench top 5 1/2 digit multimeter with true RMS detection.

Chart 2-3. Abbreviated Specifications of a Bench Digital Multimeter

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