Basic electrician practice: identification and measurement of transistors

Fundamental Skills Training in Marine Electronics and Electrical Systems

I. Basic Concepts of Transistors

A transistor is one of the most fundamental semiconductor devices in electronic circuits, capable of amplifying signals, switching control, and modulating signals. It is a core component of modern electronic devices, having replaced early vacuum tubes, and offers advantages such as small size, low power consumption, and high reliability.

Transistors are divided into two main categories:Bipolar Junction Transistor (BJT) 和Field-Effect Transistor (FET). A bipolar transistor consists of three layers of semiconductor material (NPN or PNP) and is classified intoEmitter (E)、Base (B) 和Collector (C)。

Figure 12-6 Symbol for a BJT

II. Practical Transistor Measurement (Analog Multimeter Method)

1. Tube Type and Base Identification Process

(1) Gear:Resistor Range Selection R×1k Block.

(2) Test A:Touch the red probe to one pin and the black probe to each of the other two pins to obtain three sets of readings (two readings per set).

(3) Test B:Touch the black probe to one pin, and touch the red probe to each of the other two pins, obtaining three sets of readings (two readings per set).

Determining the Type of an NPN Transistor

When one of the groups has two readings that are both low (with a large needle deflection), ifThe common pin is the black test lead., then it is an NPN transistor; in this case, the pin touched by the black probe isBase (B)。

PNP Tube Type Identification

When one of the groups has two readings that are both low (with a large needle deflection), ifThe common pin is the red probe., then it is a PNP transistor, and in this case, the pin touched by the red probe isBase (B)。

2. Distinguishing Between the Emitter (E) and Collector (C) (Finger-Through Method)

Discrimination Principle:Determine this based on the principle that the gain is high when the transistor is used in the forward direction and low when used in the reverse direction.

Operating procedures using an NPN transistor as an example:

Of the remaining two pins, let’s assume that one of them is the collector (C) and the other is the emitter (E).
用Connect the black probe to C, and the red probe to E.。
Key Moves:Using two dampened fingers, simultaneously touch the C and E terminals (be careful not to short-circuit the terminals directly; the goal is to use the body’s resistance to provide a bias current to the base), and note the angle of the meter needle’s deflection.
Assume that another pin is the collector (C), and repeat the measurement using the same method.

Conclusion: Comparing the results of the two measurements, the one with the larger needle deflection angle indicates that the transistor has a higher current gain. This confirms the original hypothesis, namely, that the black probe corresponds to the collector (C).

💡 Summary of Tips and Tricks:

When identifying transistors, remember: “Three inversions—find the base; two correspondences—find the collector.” When using an analog multimeter, the black probe is connected to the positive terminal of the internal battery; this is crucial for determining whether a transistor is NPN or PNP.