Transistor tester fits into your pocket

It can be helpful to rapidly and easily determine the polarity (npn or pnp) and function of a transistor. The pocket-transistor tester in Figure 1 is ideal for quickly testing without regard to such parameters as gain and frequency response. You connect the transistor, or device under test, between the collectors, T, of an astable multivibrator. Thus, the V_CE voltage of the device under test is alternately positive and negative. Two LEDs connected in an antiparallel configuration to the device alternately light as long as the device is not conducting. The frequency of the multivibrator is a function of the values of C and R_B. If the device under test conducts in only one direction, then only one LED turns off. If the device conducts in both directions, then both LEDs turn off. You can leave the base of the device unconnected to check for excessive leakage current or short circuits between base and collector or base and emitter. Using the switch, S₁, you can connect the base to the collector to inject current into the base of the device under test. Table 1 sums up the behavior of the tester.

Figure 1.	This simple tester allows you to test the polarity and function of a transistor.

Table 1.	Testing results
Test D1 D2 S1 Comments 1 On Off Off Wrong connection? Invert C and B. 2 Off On Off Wrong connection? Invert C and B. 3 Off Off Off Device under test shorted (bad). 4 On On Off Device under test is OK if test 5 or 6 is OK. 5 On Off On Device under test is pnp. 6 Off On On Device under test is npn.

Figure 2.	A DIN connector (a) allows you to easily connect transistors; a DPDT switch provides various testing options (b).

You can also test diodes connected between C and E, FETs, small thyristors, and triacs. You can mount the entire circuit inside a small housing, such as one measuring 20 × 30 × 60 mm. You can effect the external connections to the device under test with wires terminated in alligator clips or by using a connector. It is practical and economical to use a five-pole DIN plug with the pinout shown in Figure 2a. This pinout allows you to easily connect any transistor, regardless of the arrangement of the CBE connections. Figure 2b shows the S₁ switch connections. S₁ is a DPDT switch with three positions:

• Position 1 is On, with no base current (S₁ open).
• Position 2 is Off (middle position).
• Position 3 is On, with base current (S₁ closed).