Calculator & Measurement

# Transistor Tester Schematic Circuit Diagram

#### Transistor Tester for Quick Checks

This tester serves the purpose of swiftly determining the functionality of a transistor and potentially selecting two or more transistors with approximately equal gains. It represents one of the most basic test circuits, offering limited precision. The design focuses on enabling a quick check when there is neither sufficient time nor specialized equipment for a comprehensive test.

#### Battery Test Mode (S2 Closed)

In this mode, when S2 is closed and set to ‘battery test’, a 10 mA moving coil meter (M1) is connected in series with a 600 Ω resistor (R4 + R5) to a 6 V battery. This setup causes a current of 10 mA to flow, resulting in a full-scale deflection of the meter, indicating the battery’s condition.

#### Transistor Testing Mode (S2 Open, S3 in Positions 2 or 3)

During transistor testing, with S2 open and S3 in positions 2 or 3, a current passes through the base-emitter junction of the transistor under examination. This current can be calculated by dividing the voltage across R1 or R2 by its resistance. When S3 is in position 2, the current is (6 V – 0.6 V) / 560 kΩ, which equals approximately 10 µA. If the transistor has a gain of 1000, it results in a collector current of 10 mA, causing full-scale deflection of the moving coil instrument. Therefore, the meter reading, when S3 is in position 2, needs to be multiplied by a factor of 100 to obtain the transistor’s gain. In position 3, the base resistor is ten times lower (R1 = 56 kΩ), requiring the reading to be multiplied by 10 to determine the gain.

#### Selecting Gains and Polarity

Position 2 of S3 is designed for high gains up to 1000, while position 3 is for gains from 0 to 100. S1 allows the polarity to be reversed: the upper position is for NPN transistors, and the lower position is for PNP types. If a moving coil instrument is unavailable, it is possible to substitute M1 with a digital meter.

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