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Model 415E
tion.
Use the transistor symbol on the schematic
diagram to determine the bias polarity required to
forward-bias the base-emitter junction.
The A part
of Figure 5-4 shows transistor symbols withterminals
labelled. Notice that the emitter arrow points toward
the type N material.
The other two columns of the
illustration compare the biasing required to cause con¬
duction and cut-off in transistors and vacuum tubes.
If the transistor base-emmiter diode (junction) is
forward-biased the transistor conducts. If the diode is
heavily
forward-biased,
the transistor
saturates.
However, if the base-emitter diode is reverse-biased
the transistor is cut off (open).
The voltage drop
across a forward-biased emitter-base diode varies
with transistor collector current.
For example, a
germanium transistor has a typical forward-bias,
base-emitter voltage of 0.2-0.3 volts when collector
current is 1-10 ma, and 0.4-0.5 volts when collector
current is 10-100 ma. In contrast, forward-bias volt¬
age for silicon transistors is about twice that for ger¬
manium types: about 0.5-0.6 volts when collector cur¬
rent is low, and about 0.8-0.9 volts when collector
current is high.
5-45. Figure 5-4, part B, shows simplified versions
of the three basic transistor circuits and gives the
operating characteristics of each. When examining a
transistor stage, first determine if the emitter-base
diode is biased for conduction (forward-biased) by
measuring the voltage difference between emitter and
base.
When using an electronic voltmeter, do not
measure directly between emitter and base: there may
be sufficient loop current between the voltmeter leads
Table 5-4.
Out-of-Circuit Transistor
Resistance Measurements
Transistor
Type
Connect Ohmmeter
Measure
Resistance
(ohms)
Pos.
lead to
Neg.
lead to
PNP
Ger¬
manium
Small
Signal
emitter
base*
200-250
emitter
collector
10K-100K
Power
emitter
base*
30-50
emitter
collector
several
hundred
NPN
Silicon
Small
Signal
base
emitter
1K-3K
collector
emitter
very high
(might
read open)
Power
base
emitter
collector
emitter
high, often
greater
than 1M
*To test for transistor action, add collector-base
short.
Measured resistance should decrease.
Section V
Paragraphs 5-38 to 5-47
to damage the transistor. Instead, measure each volt¬
age separately with respect to a voltage common point
(e.g., chassis). If the emitter-base diode is forward-
biased, check for amplifier action by short-circuiting
base to emitter while observing collector voltage. The
short circuit eliminates base-emitter bias and should
cause the transistor to stop conducting (cut off). Col¬
lector voltage should then , shift to near the supply volt¬
age. Any difference is due to leakage current through
the transistor and, in general, the smaller this current,
the better the transistor. If collector voltage does not
change the transistor has either an emitter-collector
short circuit or emitter-base open circuit.
5-46. OUT-OF-CIRCUIT TESTING.
5-47. The two common causes of transistor failure
are internal short- and open-circuits.
Remove the
transistor from the circuit and use an ohmmeter to
measure
internal resistance.
See Table 5-4 for
measurement data.
CAUTION
Most ohmmeters can supply enough current
or voltage to damage a transistor.
Before
using an ohmmeter to measure transistor
forward or reverse resistance, check its
open-circuit voltage and short-circuit
current output ON
THE
RANGE TO BE
USED. Open-circuit voltage must not exceed
1.5 volts and short-circuit current must be
less than 3 ma. See Table 5-5 for safe resis¬
tance ranges for some common ohmmeters.
Table 5-5.
Ohmmeter Ranges for Transistor
Resistance Measurements
Ohmmeter
Safe
Range (s)
Open
Ckt
Voltage
Short
Ckt
Current
Lead
Color Polarity
hp 412A
hp 427A
R x IK
Rx 10K
R x 100K
Rx 1M
R x 10M
1 ma
100 pa
lOpa
lpa
0. lpa
Red
Black
+
hp 410C
R x IK
R x 10K
R x 100K
Rx 1M
Rx 10M
1.3V
1.3V
1.3V
1.3 V
1.3V
0. 57 ma
57 pa
5. 7pa
0. 5pa
0. Q5pa
Red
Black
+
hp 410B
R x 100
R x IK
R x 10K
R x 100K
R x 1M
1. IV
1. IV
1. IV
1. IV
1. IV
1.1 ma
110 pa
llpa
1. lpa
0. llpa
Black
Red
+
Rx 100
1. 5V
1 ma
Red
Black
+
R x IK
1. 5V 0. 82 ma Black
Red
+
Triplett
630
R x 100
R x IK
3. 25 ma
325pa
Varies with
Serial
Number
Triplett
310
R x 10
R x 100
1.5V
1.5V
750pa
75pa
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