Transistor tester for determining terminals

　　The simple transistor tester in Figure 1 can determine the type of transistor and help detect the emitter, collector, and base of the transistor by checking for all possible combinations of current directions flowing between the three terminals T1, T2, and T3 of the transistor under test.

　　

　　The circuit uses two CD4022 or CD4017 counters IC1 and IC2; a single-gate square wave oscillator G4; and a CD4011 quad-NAND gate, G1 to G3. Each test terminal is connected in series with a pair of LEDs to indicate the direction of the current. The color of the LED directly indicates the junction of the transistor.

　　Figure 2 is a simple schematic that helps understand the test process. Each terminal has a pair of NPN transistors Q1 and Q3 and PNP transistors Q4 and Q6, connected to either -V or +V, which establish the required potential difference between the terminals. The circuit generates all possible or required +V and -V combinations between the terminals to establish the junction relationship. Q7 and Q8 act as voltage translators, while G1 to G3 are suppressors that prevent T1 to T3 from experiencing a conflicting situation where they are both +V and -V at the same time.

　　

　　When a normal transistor is inserted between the test terminals, it limits the current to flow in certain directions. The LEDs in series indicate these directions, and therefore the type of transistor. For example, for an NPN transistor, the LEDs illuminate red-green-red; and for a PNP transistor, they illuminate green-red-green.

　　Knowing this, the base of the transistor can be easily selected. As for the difference between collector and emitter, it is important to understand that the base-emitter junction breaks down more easily when reverse biased than the base-collector junction, which is reverse biased under normal operation.

　　Since the base-collector reverse breakdown voltage of transistors varies, the circuit provides a simple way to vary the supply voltage (Figure 3). As the voltage is increased, both LEDs connected to the emitter light up, while only one LED at the collector lights up (Figure 2b and d). The basic voltage of ±4V is sufficient to detect the base or type of transistor. By increasing the supply voltage from ±4V to ±15V, the emitters of many transistors can be tested. Taking into account the voltage drop of the series LED, this range can provide a reverse breakdown voltage of up to 26V for the base-emitter junction.

　　

　　This circuit has been tested and works fine. However, it was tested with a CD4520 counter and a CD4028 decoder (because there were no CD4022/CD4017 ICs on hand), and this substitution should not be a problem. The key is the voltage levels, which are basically the same for a logic 1 or logic 0 of a CMOS device. Alternatively, it is possible to use only two supply voltages: ±5V for base detection and ±15V for emitter detection.