Elenco Diode/Transistor Tester - Manual (Page 5)

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THEORY OF OPERATION
Note the schematic diagram on page 6. The test transistor in this
circuit is an NPN. Adjusting the variable resistor will cause the NPN
LED to light up indicating that the base current is flowing. The output
of the test transistor is fed to amplifiers Q2 and Q3. The output of
Q2 is feedback 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
Ω.
When measuring PNP transistors, the power supplied to the test
transistor is reversed via the NPN/PNP switch. Therefore, the PNP
LED will light up.
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 PNP 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.