Module 16—Introduction to Test Equipment
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If the transistor is to be tested out of the circuit, plug it into the test jack located on the right-hand side
below the meter. If the transistor is to be tested in the circuit, at least 300 ohms must exist between E- B
(emitter to base), C-B (collector to base), and C-E (collector to emitter) for accurate measurement. Initial
setting of the test set controls is performed as follows:
1. Set the function switch to BETA.
2. Set the POLARITY switch to PNP or NPN (depending on the type of transistor under test).
3. Set the RANGE switch to X10.
4. Adjust METER ZERO for zero meter indication (transistor
5. The POLARITY switch should remain OFF while the transistor is connected to or
disconnected from the test set; it should then be set to PNP or NPN, as in step 2 above.
If the beta
reading is less than 10, perform the following steps:
1. Reset the RANGE switch to X1 and reset the
meter to zero.
2. After connecting the yellow test lead to the emitter, the green test lead to the
base, and the blue test lead to the collector, plug the test probe (not shown) into the jack located at the lower
right- hand corner of the test set.
3. When testing grounded equipment, unplug the 115-volt line cord
and use battery operation. A beta reading is attained by multiplying the meter reading times the RANGE switch
setting. Refer to the transistor characteristics book provided with the tester to determine if the reading is
normal for the type of transistor under test.
Adjust the METER ZERO control for a zero meter indication. Plug the transistor
to be tested into the jack, or connect the test leads to the device. Set the PNP/NPN switch to correspond with the
type of transistor under test. Set the function switch to ICO and the RANGE switch to X0.1, X1.0, or X10, as
specified by the transistor data book for allowable leakage. Read leakage on the bottom scale and multiply by the
range setting figure as required.
ELECTRODE RESISTANCE MEASUREMENTS
in-circuit probe test leads to the transistor with the yellow lead to the emitter, the green lead to the base, and
the blue lead to the collector. Set the function switch to the OHMS E-B position and read the resistance between
the emitter and base electrode on the center scale of the meter marked OHMS.
To read the resistance
between the collector and base and the collector and emitter, set the function switch to OHMS C-B and OHMS C-E,
respectively. These in-circuit electrode resistance measurements are used to correctly interpret the in-circuit
beta measurements. The accuracy of beta times 1 and 10 range is ±15 percent only when the emitter-to-base load is
equal to or greater than 300 ohms.
Diode in-circuit quality measurements are made by connecting the green
test lead to the cathode and the yellow test lead to the anode. Set the function switch to DIODE IN/CKT and the
RANGE switch to
times 1 position. Ensure that the meter has been properly zeroed on this scale. If the meter reads
down- scale, reverse the polarity switch. If the meter reads less than midscale, the diode under test is either
open or shorted. The related circuit impedance of this test is less than 25 ohms.
RESISTANCE-CAPACITANCE-INDUCTANCE (RCL) BRIDGES
Resistance, capacitance, and inductance can be measured with precise accuracy by alternating- current
bridges. These bridges are composed of capacitors, inductors, and resistors in a wide variety of combinations.
These bridges operate on the principle of the Wheatstone bridge; that is, an unknown resistance is balanced
against known resistances and, after the bridge has been balanced, the unknown resistance is calculated in terms
of the known resistance.
The universal Impedance Bridge, Model 250DE (shown in figure 4-18) is used to measure resistance, capacitance, and
inductance (RCL) values. It is also used to make other special tests, such as determining the turns ratio of
transformers and capacitor quality tests. This instrument is self-contained, except for a source of line power,
and has an approximate 500-hour battery life expectancy. It has its own source of 1,000-hertz bridge current with
a sensitive bridge balance indicator and an adjustable source of direct current for electrolytic capacitor and
resistance testing. The bridge also contains a meter with suitable ranges to test for current leakage on
Figure 4-18.—Resistance-capacitance-inductance bridge.
Figure 4-18 is a panel view of the model 250DE bridge switches,
dials, controls, and connections. Refer to the figure as we briefly discuss some of the switches and dials below.
· The FUNCTION switch selects the type of bridge circuit that will measure resistance, capacitance, or
· The RANGE switch selects the multiplier for each function.
· L-R-C decade dials are a DEKASTAT decade resistor that is the main balancing element of the bridge. The
setting of the dials after the bridge is balanced indicates the value of inductance, resistance, or capacitance.
· The D-Q dial is used to balance the phase of the capacitance or inductance of the bridge. The setting of the
dial after the bridge is balanced indicates the value of dissipation factor (D) or storage factor (Q).
· The GEN-DET switch selects bridge generator and detector connections, ac or dc, internal or external
generator. The switch also connects the internal batteries to the battery test circuit.
· The DET GAIN
control adjusts the sensitivity of the ac-dc detector and turns on power to the generator.
L, R, and C terminals 1, 2, and 3 are used to connect unknown resistors,
inductors, and capacitors to the bridge. Resistors and inductors are connected between terminals 1 and 2, and
capacitors are connected between terminals 2 and 3. EXT BIAS terminals are normally connected
with a shorting lug. They allow insertion of a dc voltage or current to bias capacitors or inductors. EXT
DET connector is a BNC coaxial socket that allows an external detector to be used with the instrument. It
is connected to the bridge at ALL TIMES.
EXT D-Q terminals are normally connected with a
shorting lug. They allow an external rheostat to extend the range of the D-Q dial. EXT GEN terminals provide a
connection to the bridge for an external generator. When the GEN-DET switch is in the AC EXT GEN position, the
terminals connect an isolation transformer so that a grounded external generator can be used. When the GEN-DET
switch is in the DC EXT GEN position, the terminals are connected directly to the bridge.
The model 250DE bridge has a battery supply consisting of four 1.5 V dc batteries with an expected life of
500 hours. The battery power supply should be checked before each day’s operation. Turn DET GAIN control to 1 and
set GEN-DET switch to BATT. TEST (battery test). If the meter deflects beyond the BAT OK mark, the battery is
Resistance is usually measured with direct current for maximum accuracy. The model 250DE bridge can be used to
measure resistance with alternating current, but external reactance compensation is usually required. On
high-resistance ranges, care should be taken to avoid leakage across a resistor under test. Insulation with a
resistance of 109
ohms, which is adequate for most purposes, will cause a measurement
error of 1 percent if it shunts a 10-megohm resistor. Using the following steps, you will be able to
measure dc resistance ONLY:
1. Turn the DET GAIN control to 2.
2. Set the FUNCTION switch to R x
1 or R x 10.
3. Set L-R-C decade dials to 3.000.
4. Connect the unknown resistor to R-L
terminals 1 and 2.
5. Set the GEN-DET switch to INT DC.
6. Adjust the RANGE switch for minimum
7. Adjust L-R-C decade dials for null, turning the DET GAIN control clockwise to
increase sensitivity as necessary.
8. The measured resistance is the product of the L-R-C decade dial
setting times the RANGE and FUNCTION switch settings.
Capacitance is measured in terms of a two-element equivalent
circuit consisting of a capacitor in series with a resistor. The internal ac generator and detector of the model
250DE bridge are tuned to 1 kilohertz. Other frequencies can be used, but an external generator and detector are
required. The D and Q ranges of the bridge can be extended by use of an external rheostat connected to the
terminals provided. The measured capacitance is the product of the L-R-C dial setting times the setting of the
RANGE switch. Using the following steps, you can make a standard capacitance measurement:
1. Turn the DET
GAIN control to 1.
2. Set the FUNCTION switch to C, D x 0.1 or D x 0.01 SERIES.
3. Set L-R-C
decade dials to 3.000 and D-Q dial to 0.
4. Connect the unknown capacitor to C terminals 2 and 3.
5. Set the GEN DET switch to INT 1 kHz.
6. Adjust the RANGE SWITCH for minimum detector deflection.
7. Adjust L-R-C decade dials and D-Q dial alternately for a minimum meter deflection, turning the DET GAIN
control clockwise to increase sensitivity as necessary.
8. The measured capacitance is the product of the
L-R-C decade dial settings.
9. The measured dissipation factor (D) is the product of the D-Q setting
times the FUNCTION switch setting.
Inductance is measured in terms of a two-element equivalent circuit consisting of an inductance either in series
or in parallel with a resistance. The internal ac generator and detector of the model 250DE bridge are tuned to 1
kHz. Other frequencies can be used, but like capacitance measurements, an external
generator and detector are required. When inductance is being measured in ac or dc, it should be
realized that iron-core inductors are sensitive to current variations. Quantitative measurements of dc effects can
be made by supplying current to the unknown inductor through the EXT BIAS terminal. Use the following steps to
make inductance measurements:
1. Turn the DET GAIN control to 1.
2. Set the FUNCTION switch to L
PARALLEL if Q is greater than 10 to L SERIES if Q is less than 10.
3. Set L-R-C decade dials to 3.000 and
D-Q dial to maximum.
4. Connect the unknown inductor to the R-L terminals 1 and 2
5. Set the GEN DET switch to INT 1 kHz.
6. Adjust the RANGE switch for minimum detector deflection.
7. Adjust the L-R-C decade dials and D-Q dial alternately for a minimum meter deflection, turning the DET GAIN
control clockwise to increase sensitivity as necessary.
8. The measured inductance is the product of the
L-R-C decade dial setting times the RANGE switch setting.
9. The measured storage factor (Q) is read
directly from the D-Q dial, inner scale for parallel and outer scale for series inductance.
The important points of this chapter are summarized in the following paragraphs. You should be familiar
with these points before continuing with your studies of test equipment.
is a single meter that combines the functions of a dc ammeter, a dc voltmeter, an ac ammeter, an ac voltmeter, and
an ohmmeter. Observe the following safety precautions when using a multimeter:
· De-energize and
discharge the circuit completely before connecting a multimeter.
· Never apply power to the circuit
while you are measuring resistance with an ohmmeter.
· Connect the ammeter in series
for current measurements and in parallel for voltage measurements.
· Be certain the multimeter is switched to ac before attempting to measure ac circuits.
· Observe proper dc polarity when measuring dc circuits.
· Always start with the highest voltage or
· Select a final range that allows a reading near the middle of the scale.
Adjust the "0 ohms" reading after changing resistance ranges and before making a resistance measurement.
An ELECTRONIC DIGITAL MULTIMETER is used in sensitive electronic circuits where only
extremely small amounts of energy can be extracted without disturbing the circuits under test, or causing them to
The DIFFERENTIAL VOLTMETER is a precision piece of test equipment used to compare an
unknown voltage with an internal reference voltage and to indicate the difference in their values.
A SEMICONDUCTOR TEST SET is used to measure the beta of a transistor, the resistance
appearing at the electrodes, and the reverse current of a transistor or semiconductor diode. It also measures a
shorted or open condition of a diode, the forward transconductance of a field-effect transistor, and the condition
of its own batteries.
Resistance, capacitance, and inductance are measured for precise accuracy by RCL BRIDGES. They are
composed of capacitors, inductors, and resistors and operate on the principle of the Wheatstone bridge.
ANSWERS TO QUESTIONS Q1. THROUGH Q8.
A-1. No external power source is required.
A-5. Rechargeable batteries.
A-6. Simultaneous flashing of display readouts.
A-7. Light-emitting diodes.
A-8. To compare an unknown voltage with a known reference voltage and indicate the difference in their values.
Introduction to Matter, Energy, and Direct Current, Introduction
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading, Introduction to Generators and Motors,
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies,
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits, Introduction to Wave Propagation, Transmission
Lines, and Antennas, Microwave Principles,
Modulation Principles, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros,
Introduction to Test Equipment, Radio-Frequency
Communications Principles, Radar Principles, The Technician's Handbook,
Master Glossary, Test Methods and Practices, Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics