CHAPTER 16 ■ TRANSISTOR SWITCHES
The transistor lead that is present in both conducting lead combinations is the base lead. Referring to
Table 16-2, for Sample PNP, the base lead is 2 (the middle lead) because it appears in test combinations 1 & 2
and 3 & 2. For Sample NPN, the base lead is also 2 (the middle lead) because it appears in test combinations
2 & 3 and 2 & 1. Although these examples both had base leads in the middle, that won’t be the case with all
transistors.
Furthermore, if the base lead conducted when it was connected to the red test probe, the transistor is
NPN. Otherwise it is PNP. For Sample PNP, the base lead conducted when it was connected to the black test
probe, so it is a PNP transistor. For Sample NPN, the base lead conducted when it was connected to the red
test probe, so it is an NPN transistor.
If you’d like to reproduce the test results, Sample PNP is a 2907A transistor (no surprise) and Sample
NPN is a 2222A transistor. The 2222A has very similar attributes to the 2907A, and is often used to replace the
2907A when a positive-power—activated transistor is needed.
The diode test can only determine the base lead. In order to determine the emitter and collector, a test
circuit needs to be made on a breadboard.
Bipolar Transistor Test Circuits
A couple of example circuits may be helpful in understanding how a transistor can act as a switch. The first
example that will be presented is for PNP transistors, which take negative power to turn on. The second
example is for NPN transistors, which take positive power to turn on.
You can place a transistor in these test circuits and determine:
- whether the transistor is NPN or PNP
- which lead is the emitter, base, and collector
- the transistor’s gain (hFE)
Examining the Schematic for the PNP Transistor Test Circuit
Figure 16-5 is a schematic of a PNP transistor test circuit. LED1 turns on when a PNP transistor is
correctly inserted for Q1. NPN transistors or incorrectly inserted PNP transistors won’t work, but no
harm will be done.