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Transistor Testing
The DT-100 can measure transistors in or out of circuit. It will identify NPN or PNP by a simple adjustment.
Transistor testing - Out of Circuit
1. Place switch in transistor position.
2. Place transistor in socket or attach to C,B,E leads. If collector C, base B and emitter E are not known, assume
B is the center lead on small plastic transistors and C the metal case or tab on power transistors.
3. Push in test button. Adjust the base current control so that OK LED lights. This indicates a good transistor.
4. If OK lamp does not light, then adjust the base current control co that either NP or PN LED lights. This
happens at minimum or maximum position of the control setting. This will indicate transistor type. Place
switch to NPN if the NP lamp lights, to PNP if PN lamp is lit.
5. If no lamps light, the transistor is open or we have not identified the base lead. Repeat assuming another
lead as base.
6. When the transistor is shown to be OK, the base current control gives an indication of transistor beta. The
lower the setting relative to another transistor, the higher the beta. Lamps NP and PN measure base current.
Higher base current results in a brighter LED. It also indicates if current is entering or leaving the base, thus
NP or PN respectively will light.
Transistor Testing - In Circuit
The DT-100 will test transistors in circuit provided the base biasing resistance is greater than 100 ohms. Simply
follow the previous procedure for testing out of circuit transistors. Do not apply power to circuit of transistor or
diode under test. The DT-100 will supply the necessary power.
THEORY OF OPERATION
Note the circuit diagram on page 10. The test transistor in this circuit is a NPN. Adjusting the variable resistor
will cause the NPN LED to light, indicating that base current is flowing. The output of the test transistor is fed
to amplifier Q2 and Q3. The output of Q2 is fed back in phase to the base of the test transistor causing the
circuit to oscillate. Part of the oscillations are fed to a power rectifier Q5 which switches on the OK LED
indicator.
The design configuration is such that in-circuit transistors can be measured provided that the base and collector
resistors are greater than 100 ohms.
When measuring PNP transistors, the power supplied to the test transistor is reversed via the NPN/PNP switch,
therefore the PN LED will light.
Varying the base current control will reduce the base current. The lower the base current, the higher the gain
of the transistor under test. Comparative tests of two transistors gain (beta) can be made by observing the dial
setting or the intensity of the base LED diode. The lower the setting with the PN LED lit, the higher the beta of
that transistor.
On diode operation, power is applied to IC1. This causes the circuit to oscillate at about a 1Hz rate. Placing a
diode in series with the LED indicators will cause a current to flow depending on the direction of the diode. Thus,
the red test lead will identify the cathode or anode of the diode via the LED readout. Transistor Q4 reverses the
current flow in this circuit.
All types of diodes may be tested: Silicon, germanium, LEDs or zeners over 6 volts. Zener diodes under 6V
causes the second LED to glow at lower intensity, indicating that zener breakdown has occurred.
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