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Transistor tester identifies terminals

It also lets you determine if a transistor is an NPN or PNP type.

Raju R Baddi, Tata Institute of Fundamental Research, Maharashtra, India; Edited by Martin Rowe and Fran Granville -- EDN, March 17, 2011  

The simple transistor tester in Figure 1 lets you identify the type of transistor, and it helps in detecting a transistor’s emitter, collector, and base. It checks all possible combinations of directions of current flow between the test transistor’s three terminals, T1, T2, and T3.

Transistor tester identifies terminals
Figure 1. Counters and transistors let you detect a transistor's terminals and determine whether the transistor under test is working.

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 through G3. A pair of LEDs connects in series to each test terminal to indicate the direction of current flow. The color of the LEDs directly reveals the junction side of the transistor.

Transistor tester identifies terminals
Figure 2. A reference indicates the health of a transistor under test for an NPN with normal supply voltage (a), an NPN with increased supply voltage (b), a PNP with normal supply voltage (c), and a PNP with increased supply voltage (d).

Figure 2 provides an easy reference for understanding the test procedure. A pair of NPN transistors, Q1 and Q3, and PNP transistors, Q4 and Q6, for each terminal connects the terminals to either −V or +V, which sets up the required potential difference between the terminals. The circuit generates all of the possible or required combinations of +V and −V between the terminals to establish the junction relations. Q7 and Q8 act as voltage translators, whereas G1 to G3 are inhibitors, which prevent T1 to T3 from clashing by being at +V and −V at the same time.

When you plug a functioning transistor into the test terminals, it restricts current flow in certain directions only. The series LEDs reveal these directions and, hence, indicate the type of transistor—for example, the LEDs glow red-green-red for an NPN transistor and glow green-red-green for a PNP transistor.

With this knowledge, you can easily choose the base of the transistor. To differentiate between emitter and collector you must understand the property that, under reverse bias, base-emitter junction breaks down more easily than does the base-collector junction, which is reverse-biased for normal operation.

Transistor tester identifies terminals
Figure 3. A common handle ensures symmetric variation of supply voltage about the ground.

Because transistors have different base-emitter reverse-breakdown voltages, the circuit provides a way to easily change the supply voltage (Figure 3). Under increased voltage, both LEDs connected with the emitter glow brightly, whereas only one LED glows for the collector (Figure 2b and d). A basic voltage of ±4V seems sufficient for detecting the base or type of transistor. By gradually increasing the supply voltage from ±4 to ±15V, you can test a variety of transistors for the emitter. This range provides a maximum reverse-breakdown voltage of greater than 26V for the base-emitter junction, taking into account the voltage drop of the series LEDs.

This circuit underwent testing and works. However, the testing employed CD4520 counters and CD4028 decoders because the CD4022/CD4017 ICs were unavailable. This replacement shouldn’t cause problems. Only the voltage levels matter, which for CMOS devices is more or less the same for logic one or logic zero. You can also use only two supply voltages: ±5V for detecting the base and ±15V for detecting the emitter.


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