Identifying the base of a transistor: According to the transistor's structural diagram, we know that the base of a transistor is the common terminal of the two PN junctions. Therefore, to identify the base, simply find the common terminal of the two PN junctions. The specific method is to set a multimeter to the R×1k range. First, place the red probe on one leg of the transistor and touch the other two legs with the black probe. If there is continuity both times, the leg where the red probe was placed is the base. If not found the first time, move the red probe to the other leg and test twice more. If still not found, move the red probe again and test twice more. If still not found, move the black probe to one leg and test twice with the red probe to see if there is continuity. If not found the first time, try again. Repeat this process up to 12 times until the base is found.
Identifying the type of transistor: There are only two types of transistors: PNP and NPN. To determine the transistor type, you only need to know whether the base is P-type or N-type. When using a multimeter on the R×1k range, the black probe represents the positive terminal of the power supply. If the transistor conducts when the black probe is connected to the base, the base is P-type, and the transistor is NPN. If the transistor conducts when the red probe is connected to the base, the base is N-type, and the transistor is PNP.
Identifying the transistor type and pinout is a fundamental skill for beginners in electronics. To help readers quickly master the testing method, I have summarized a four-line mnemonic: "Reverse the three steps to find the base; PN junction determines the type; Follow the arrow for a larger deflection; If unsure, speak." Let's explain each line below.
1: Reverse the three steps to find the base. As we know, a transistor is a semiconductor device containing two PN junctions. Based on the different connections of the two PN junctions, transistors can be divided into two different conductivity types: NPN and PNP.
Testing a transistor requires using the ohmmeter function of a multimeter, selecting the R×100 or R×1k range. The equivalent circuit for the ohmmeter function is shown below. The red probe is connected to the negative terminal of the internal battery, and the black probe is connected to the positive terminal.
Assuming we don't know whether the transistor being tested is NPN or PNP, or what the terminals are, the first step is to determine which terminal is the base. We randomly select two terminals (e.g., terminals 1 and 2), and measure their forward and reverse resistance using the multimeter probes in reverse, observing the needle deflection. Next, we select terminals 1 and 3, and terminals 2 and 3, and measure their forward and reverse resistance again, observing the needle deflection. In these three inverted measurements, there will inevitably be two measurements with similar results: one with a large deflection and the other with a small deflection; the remaining measurement will have a small deflection angle before and after the inversion. The pin that was not measured in this measurement is the base we are looking for.
2: PN Junction, Determining Transistor Type
After finding the base of the transistor, we can determine the conductivity type of the transistor based on the direction of the PN junction between the base and the other two electrodes. Connect the black probe of the multimeter to the base and the red probe to either of the other two electrodes. If the meter pointer deflects significantly, the transistor is an NPN type; if the deflection is small, the transistor is a PNP type.
3: Forward Arrow, Large Deflection
Having found the base (b), which of the other two electrodes is the collector (c) and which is the emitter (e)? We can determine the collector (c) and emitter (e) by measuring the leakage current (ICEO).
(1) For NPN transistors, the leakage current measurement circuit. Based on this principle, when measuring the forward and reverse resistances Rce and Rec between the two terminals using a multimeter with the black and red probes reversed, although the multimeter pointer deflection angle is small in both measurements, careful observation will reveal that there will always be one slightly larger deflection angle. At this point, the current flow direction is: black probe → collector (c) → base (b) → emitter (e) → red probe. This current flow direction is exactly the same as the arrow direction in the transistor symbol. Therefore, the black probe must be connected to the collector (c), and the red probe must be connected to the emitter (e).
(2) For PNP transistors, the principle is similar to NPN transistors. The current flow direction is: black probe → emitter (e) → base (b) → collector (c) → red probe. This current flow direction is also consistent with the arrow direction in the transistor symbol. Therefore, the black probe must be connected to the emitter (e), and the red probe must be connected to the collector (c).
4. If you can't distinguish the difference due to small pointer deflections in both measurements (forward arrow, large deflection), you need to use your mouth. The specific method is as follows: In the two measurements with the "forward arrow, large deflection" rule, hold the two probes and their terminals with both hands, and hold (or press against) the base electrode b in your mouth. Then, use the "forward arrow, large deflection" method to distinguish the collector c and emitter e. The human body acts as a DC bias resistor to make the effect more obvious.
