High-Voltage Probe

An accessory probe can be used with a multimeter to measure dc voltages up to

30 kV. This probe is often referred to as a high-voltage probe. One application is
measuring the high voltage of 20 to 30 kV at the anode of the color picture tube in
a television receiver. The probe is just an external multiplier resistance for the dc
voltmeter. The required R for a 30-kV probe is 580 M with a 20-k / V meter on
the 1000-V range.

■ 8–6 Self-Review
Answers at end of chapter.
a. How much is RV on the 1-V range for a VOM with a sensitivity of
20 k⍀ / V?
b. If RV is 10 M⍀ for a DMM on the 100-V range, how much is RV on
the 200-mV range?
c. The low-power ohm (LP⍀) function does not require an internal
battery. (True/False)

Figure 8–16 Typical digital multimeter

(DMM).
8–7 Digital Multimeter (DMM)
The digital multimeter has become a very popular test instrument because the digital
value of the measurement is displayed automatically with decimal point, polarity, and
the unit for V, A, or . Digital meters are generally easier to use because they elimi-
nate the human error that often occurs in reading different scales on an analog meter
with a pointer. Examples of the portable DMM are shown in Figs. 8–14 and 8–16.
The basis of the DMM operation is an analog-to-digital (A兾D) converter cir-
cuit. It converts analog voltage values at the input to an equivalent binary form.
These values are processed by digital circuits to be shown on a liquid-crystal display
(LCD) as decimal values.

Voltage Measurements
The A兾D converter requires a specific range of voltage input; typical values are
−200 mV to 200 mV. For DMM input voltages that are higher, the voltage is
divided down. When the input voltage is too low, it is increased by a dc amplifier
circuit. The measured voltage can then be compared to a fixed reference voltage in
the meter by a comparator circuit. Actually, all functions in the DMM depend on the
voltage measurements by the converter and comparator circuits.
The input resistance of the DMM is in the range of 10 to 20 M, shunted by
50 pF of capacitance. This R is high enough to eliminate the problem of voltmeter
loading in most transistor circuits. Not only does the DMM have high input resis-
tance, but the R is the same on all ranges.
With ac measurements, the ac input is converted to dc voltage for the A兾D con-
verter. The DMM has an internal diode rectifier that serves as an ac converter.

R Measurement
As an ohmmeter, the internal battery supplies I through the measured R for an IR
drop measured by the DMM. The battery is usually the small 9-V type commonly
used in portable equipment. A wide range of R values can be measured from a frac-
tion of an ohm to more than 30 M. Remember that power must be off in the circuit
being tested with an ohmmeter.
A DMM ohmmeter usually has an open-circuit voltage across the meter leads,
which is much too low to turn on a semiconductor junction. The result is low-power
ohms operation.

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 I Measurements
To measure current, internal resistors provide a proportional IR voltage. The display
shows the I values. Note that the DMM still must be connected as a series compo-
nent in the circuit when current is measured.

Diode Test
The DMM usually has a setting for testing semiconductor diodes, either silicon or
germanium. Current is supplied by the DMM for the diode to test the voltage across
its junction. Normal values are 0.7 V for silicon and 0.3 V for germanium. A short-
circuited junction will read 0 V. The voltage across an open diode reads much too
high. Most diodes are silicon.

Resolution
This term for a DMM specifies how many places can be used to display the digits
0 to 9, regardless of the decimal point. For example, 9.99 V is a three-digit display;
9.999 V would be a four-digit display. Most portable units, however, compromise
with a 3½-digit display. This means that the fourth digit at the left for the most sig-
nificant place can only be a 1. If not, then the display has three digits. As examples,
a 3½-digit display can show 19.99 V, but 29.99 V would be read as 30.0 V. Note
that better resolution with more digits can be obtained with more expensive meters,
especially the larger DMM units for bench mounting. Actually, though, 3½-digit
resolution is enough for practically all measurements made in troubleshooting elec-
tronic equipment.

Range Overload
The DMM selector switch has specific ranges. Any value higher than the selected
range is an overload. An indicator on the display warns that the value shown is not
correct. Then a higher range is selected. Some units have an autorange function that
shifts the meter automatically to a higher range as soon as an overload is indicated.

Typical DMM
The unit in Fig. 8–16 can be used as an example. On the front panel, the two jacks
at the bottom right are for the test leads. The lower jack is the common lead, used
for all measurements. Above is the jack for the “hot” lead, usually red, used for the
measurements of V and R either dc or ac values. The two jacks at the bottom left side
are for the red lead when measuring either dc or ac I.
Consider each function of the large selector switch at the center in Fig. 8–16. The
first position at the top, after the switch is turned clockwise from the off position, is
used to measure ac volts, as indicated by the sine wave. No ranges are given as this
meter has an autorange function. In operation, the meter has the ranges of 600 mV,
6 V, 60 V, 600 V, and, as a maximum, 1000 V.
If the autorange function is not desired, press the range button below the display
to hold the range. Each touch of the button will change the range. Hold the button
down to return to autorange operation.
The next two positions on the function switch are for dc volts. Polarity can be
either positive or negative as indicated by the solid and dashed lines above the V.
The ranges of dc voltages that can be measured are 6, 60, 600, and 1000 V as a
maximum. For very low dc voltages, the mV switch setting should be used. Values
below 600 mV can be measured on this range.
For an ohmmeter, the function switch is set to the position with the  symbol.
The ohm values are from 0 to 50 M in six ranges. Remember that power must
be off in the circuit being measured, or the ohmmeter will read the wrong value.
(Worse yet, the meter could be damaged.)

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 Next on the function switch is the position for testing semiconductor diodes, as
shown by the diode symbol. The lines next to the symbol indicate that the meter
produces a beep tone. Maximum diode test voltage is 2.4 V.
The last two positions on the function switch are for current measurements. The
jacks at the lower left are used for larger or smaller current values.
In measuring ac values, either for V or I, the frequency range of the meter is
limited to 45 to 1000 Hz, approximately. For amplitudes at higher frequencies, such
as rf measurements, special meters are necessary. However, this meter can be used
for V and I at the 60-Hz power-line frequency and the 400-Hz test frequency often
used for audio equipment.

Analog Display
The bar at the bottom of the display in Fig. 8–16 is used only to show the rela-
tive magnitude of the input compared to the full-scale value of the range in use.
This function is convenient when adjusting a circuit for a peak value or a minimum
(null). The operation is comparable to watching the needle on a VOM for either a
maximum or a null adjustment.

■ 8–7 Self-Review
Answers at end of chapter.
a. The typical resistance of a DMM voltmeter is 10 M⍀. (True/False)
b. The ohmmeter on a portable DMM does not need an internal battery.
(True/False)
c. A DMM voltmeter with 3½-digit resolution can display the value
of 14.59 V. (True/False)

8–8 Meter Applications

Table 8–4 summarizes the main points to remember when using a voltmeter,
ohmmeter, or milliammeter. These rules apply whether the meter is a single unit or
one function on a multimeter. The voltage and current tests also apply to either dc
or ac circuits.
To avoid excessive current through the meter, it is good practice to start on a high
range when measuring an unknown value of voltage or current. It is very important
not to make the mistake of connecting a current meter in parallel, because usually
this mistake ruins the meter. The mistake of connecting a voltmeter in series does
not damage the meter, but the reading will be wrong.

Table 8–4 Direct-Current Meters

Milliammeter
Voltmeter or Ammeter Ohmmeter
Power on in circuit Power on in circuit Power off in circuit
Connect in parallel Connect in series Connect in parallel
High internal R Low internal R Has internal battery
Has internal series Has internal shunts; Higher battery
multipliers; higher lower resistance for voltage and more
R for higher ranges higher current sensitive meter for
ranges higher ohm ranges

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 Figure 8–17 How to insert a current meter in different parts of a series-parallel circuit to
read the desired current I. At point A, B, or C, the meter reads IT; at D or E, the meter reads I2; at
F or G, the meter reads I3.

mA mA
R1 
5 B F
D mA

A mA

R2  R3 
I2 10  I3 10 

10 V

E mA
IT
C G

mA mA

If the ohmmeter is connected to a circuit in which power is on, the meter can
be damaged, beside giving the wrong reading. An ohmmeter has its own internal
battery, and the power must be off in the circuit being tested. When R is tested with
an ohmmeter, it may be necessary to disconnect one end of R from the circuit to
eliminate parallel paths.

Connecting a Current Meter in the Circuit

In a series-parallel circuit, the current meter must be inserted in a branch to read
branch current. In the main line, the meter reads the total current. These different
connections are illustrated in Fig. 8–17. The meters are shown by dashed lines to
illustrate the different points at which a meter could be connected to read the respec-
tive currents.
If the circuit is opened at point A to insert the meter in series in the main line
here, the meter will read total line current IT through R1. A meter at B or C will read
the same line current.
To read the branch current through R2, this R must be disconnected from its junc-
tion with the main line at either end. A meter inserted at D or E, therefore, will read the
R2 branch current I2. Similarly, a meter at F or G will read the R3 branch current I3.

Calculating I from Measured Voltage

The inconvenience of opening the circuit to measure current can often be eliminated
Figure 8–18 With 15 V measured by the use of Ohm’s law. The voltage and resistance can be measured without open-
across a known R of 15 , the I can be ing the circuit, and the current calculated as V兾R. In the example in Fig. 8–18, when
calculated as V/R or V/15   1 A. the voltage across R2 is 15 V and its resistance is 15 , the current through R2 must
be 1 A. When values are checked during troubleshooting, if the voltage and resis-
tance are normal, so is the current.
R1  This technique can also be convenient for determining I in low-resistance circuits
5 where the resistance of a microammeter may be too high. Instead of measuring I,

measure V and R and calculate I as V兾R.
20 V
 Furthermore, if necessary, we can insert a known resistance RS in series in the
R2  circuit, temporarily, just to measure VS. Then I is calculated as VS 兾RS. The resistance
15 V V
15  of RS, however, must be small enough to have little effect on RT and I in the series
I1A circuit.
This technique is often used with oscilloscopes to produce a voltage waveform of
IR which has the same waveform as the current in a resistor. The oscilloscope must
be connected as a voltmeter because of its high input resistance.

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 Figure 8–19 Voltage tests to localize an open circuit. (a) Normal circuit with voltages to chassis ground. (b) Reading of 0 V at point D
shows R3 is open.

100 V 75 V 50 V 25 V 100 V 100 V 100 V 0V

A B C D A B C D

R1  R2  R3  R4  R1  R2  R3, R4 
10  10  10  10  10  10  open 10 
100 V 100 V
V

G G G G

(a ) (b )

Checking Fuses
Turn the power off or remove the fuse from the circuit to check with an ohmmeter. A
good fuse reads 0 . A blown fuse is open, which reads infinity on the ohmmeter.
A fuse can also be checked with the power on in the circuit by using a voltmeter.
Connect the voltmeter across the two terminals of the fuse. A good fuse reads 0 V
because there is practically no IR drop. With an open fuse, though, the voltmeter
reading is equal to the full value of the applied voltage. Having the full applied volt-
age seems to be a good idea, but it should not be across the fuse.

Voltage Tests for an Open Circuit

Figure 8–19 shows four equal resistors in series with a 100-V source. A ground
return is shown here because voltage measurements are usually made with respect
to chassis or earth ground. Normally, each resistor would have an IR drop of 25 V.
Then, at point B, the voltmeter to ground should read 100  25  75 V. Also, the
voltage at C should be 50 V, with 25 V at D, as shown in Fig. 8–19a.
However, the circuit in Fig. 8–19b has an open in R3 toward the end of the series
string of voltages to ground. Now when you measure at B, the reading is 100 V,
equal to the applied voltage. This full voltage at B shows that the series circuit is
open without any IR drop across R1. The question is, however, which R has the
open? Continue the voltage measurements to ground until you find 0 V. In this ex-
ample, the open is in R3 between the 100 V at C and 0 V at D.
The points that read the full applied voltage have a path back to the source of
voltage. The first point that reads 0 V has no path back to the high side of the source.
Therefore, the open circuit must be between points C and D in Fig. 8–19b.

■ 8–8 Self-Review
Answers at end of chapter.
a. Which type of meter requires an internal battery?
b. How much is the normal voltage across a good fuse?
c. How much is the voltage across R1 in Fig. 8–19a?
d. How much is the voltage across R1 in Fig. 8–19b?

8–9 Checking Continuity with

the Ohmmeter
A wire conductor that is continuous without a break has practically zero ohms of
resistance. Therefore, the ohmmeter can be useful in testing for continuity. This
test should be done on the lowest ohm range. There are many applications. A wire

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 Figure 8–20 Continuity testing from point A to wire 3 shows that this wire is connected.
GOOD TO KNOW 5
4
Most DMMs have a switch setting 3 5-wire cable
for testing the continuity 2
1 3
between two points. If the
resistance between two points is Point
A
less than about 200 , an audible
tone will be heard. The audible
 0
tone provides immediate feedback
to indicate whether or not there
is continuity between the two
Ohmmeter
points being measured. reads zero

conductor can have an internal break which is not visible because of the insulated
cover, or the wire can have a bad connection at the terminal. Checking for zero ohms
between any two points along the conductor tests continuity. A break in the conduct-
ing path is evident from a reading of infinite resistance, showing an open circuit.
As another application of checking continuity, suppose that a cable of wires is
harnessed together, as illustrated in Fig. 8–20, where the individual wires cannot
be seen, but it is desired to find the conductor that connects to terminal A. This is
done by checking continuity for each conductor to point A. The wire that has zero
ohms to A is the one connected to this terminal. Often the individual wires are color-
coded, but it may be necessary to check the continuity of each lead.
An additional technique that can be helpful is illustrated in Fig. 8–21. Here
it is desired to check the continuity of the two-wire line, but its ends are too far
apart for the ohmmeter leads to reach. The two conductors are temporarily short-
circuited at one end, however, so that the continuity of both wires can be checked
at the other end.
In summary, the ohmmeter is helpful in checking the continuity of any wire con-
ductor. This includes resistance-wire heating elements, such as the wires in a toaster
or the filament of an incandescent bulb. Their cold resistance is normally just a few
ohms. Infinite resistance means that the wire element is open. Similarly, a good fuse
has practically zero resistance. A burned-out fuse has infinite resistance; that is, it is
open. Any coil for a transformer, solenoid, or motor will also have infinite resistance
if the winding is open.

■ 8–9 Self-Review
Answers at end of chapter.
a. On a back-off ohmmeter, is zero ohms at the left or the right end of
the scale?
b. What is the ohmmeter reading for an open circuit?

Figure 8–21 Temporary short circuit at one end of a long two-wire line to check
continuity from the opposite end.

Ohmmeter
reads zero

 0

Temporary
short circuit

Two-wire cable

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