TM 11-6625-539-14-3
b. With the switch at BETA CAL, 1-kilohertz
(kHz) rnultivibrator Q1, Q2, and Q3 applies a
A simplified schematic diagram of the circuit
symmetrical square wave to the input of the test
used to check 1CO (direct current (de) collector
device. The magnitude of the square wave is
current when the collector junction is reverse-
adjustable by CAL resistor R5. The output of the
biased and the emitter is open-circuited) is il-
square wave generator is coupled to the test
lustrated in figure 54. Figure 54 illustrates
device by transformer T1. With the switch in the
a PNP transistor. With the function switch at
BETA CAL position, the amplitude of the square
ICO X1, the test device is reverse-biased by the
wave is adjusted by CAL resistor R5 until a 1-
battery. The test device emitter is open-circuited.
milliampere (ma) current flows in the collector
Meter M1 indicates the collector current. With
of the test device. Setting the function switch to
function switch at ICO X10, shunt resistor R28
BETA X1 connects meter M1 to indicate the base
and series resistor R29 are connected to the
current. Meter M1 is calibrated directly in beta
meter. The sensitivity of the meter is decreased
from the relationship beta equals Ic/Ib, and beta
by 10.
is read directly. Setting the function switch to
BETA X10 removes shunt resistor R27 from the
C L.
A simplified schematic diagram of the cir-
meter, and the meter is 10 times as sensitive in
cuit used to check diodes for a 10:1, front-to-
the BETA X1 position.
back ratio is shown on figure 55. A, figure 55,
illustrates the circuit used to obtain the forward,
Checks
or reference, reading and B, figure 55, illustrates
the circuit used to obtain the reverse reading.
a. Figure 53 is a schematic diagram of the
circuit used to measure resistance of transistor
b. The 10:1 ratio measurement is obtained in
junctions. Figure 53 illustrates the emitter-to-
the following manner. The test device is placed
in series with the meter and its shunt and cur-
base resistance measurements; however, the
same principle applies to collector-to-base and
rent limiting resistors in both forward and re-
collector-to-emitter resistance measurements.
v e r s e positions. With the polarity switch at
FWD PNP, the diode is forward-biased and con-
b. Resistor R10 and 6.8-volt Zener diode VR1
ducts heavily. Resistors R9, R5, meter shunt R27,
serve as a voltage divider. The Zener diode es-
and the forward resistance of the test device
tablishes constant voltage source, eliminating the
limit the current through meter M1. CAL poten-
need for a meter zero potentiometer. Resistors
tiometer R5 is adjusted to set the current through
R11 and R12 limit the maximum current through
M1 to 100 ma which corresponds to a full-scale
the meter to 100 ua. Resistor R11 is variable to
meter reading. After CAL potentiometer R5 has
compensate for tolerance variations of the meter
b e e n set to provide a full-scale reading, the
and resistor R13, which is in series with the meter.
polarity switch is set to NPN REV as shown in
Without the test device in the circuit, the full
B, figure 55. The circuit is essentially the same
100 ma flows through the meter, causing full-
as the circuit used to obtain the reference read-
scale deflection, which corresponds to a reading
ing; the only change is the reversal of the applied
of infinity. With the test device in the circuit,
voltage and reversal of the meter. The test device
current flow through the test device, which is
is now back-biased and offers a high resistance.
inversely proportional to resistance, results in a
Circuit current is proportionally decreased be-
decrease in current flow through the meter. Meter
cause of the increased resistance of the test
deflection is some amount less than full scale,
device, and consequently, meter deflection is pro-
depending on the resistance of the test device,
portionally reduced. The forward-to-reverse cur-
and indicates, directly in ohms, the resistance
rent ratio is read directly on the BETA scale of
of the element under test. Because the test set
the meter.
measures semiconductor resistance at very low
current and voltage levels, resistance at the semi-
conductor electrodes may be measured without
a. A schematic diagram of the circuit used to
polarizing the leads. The voltage used, 0.01 volt,
calibrate the test set for field effect transistor
is well below the knee of a voltage-current diode
( F . E. T.) transconductance measurement is
plot; consequently, the resistance of the diode is
shown in figure 56. Figure 56 shows an N-
extremely high. The correct value of resistance
channel F. E. T. transistor; however, the same
is read unless the diode is short-circuited or is
principle applies for a P-channel F. E. T, transis-
excessively leaky.
tor.
