How to identify transistor pins using a multimeter

　　Identifying the transistor type and pins is a basic skill for beginners of electronic technology. In order to help readers quickly master the detection and judgment methods, the author summarizes four mnemonics: "Three inversions, find the base; PN junction, determine the tube type; follow the arrow, large deflection; if you can't measure accurately, move your mouth." Let us explain each sentence one by one.

　　1. Reverse the three and find the base

　　As we all know, a transistor is a semiconductor device containing two PN junctions. According to the different connection methods of the two PN junctions, it can be divided into two different conductive types of transistors: NPN type and PNP type.

　　To test the transistor, use the ohmmeter of the multimeter and select the R×100 or R×1k position. From the equivalent circuit of the ohmmeter of the multimeter, we can see that the red test lead is connected to the negative pole of the battery in the meter, and the black test lead is connected to the positive pole of the battery in the meter.

　　Assume that we do not know whether the transistor under test is NPN or PNP, and we cannot tell which electrode each pin is. The first step of the test is to determine which pin is the base. At this time, we randomly select two electrodes (such as electrodes 1 and 2), use the two probes of a multimeter to measure its forward and reverse resistance in reverse order, and observe the deflection angle of the needle; then, take electrodes 1 and 3 and electrodes 2 and 3, respectively, and measure their forward and reverse resistance in reverse order, and observe the deflection angle of the needle. In these three reverse measurements, there must be two similar measurement results: that is, the needle deflects a lot in one reverse measurement and a little in another; the remaining one must be that the deflection angle of the pointer is very small before and after the reverse measurement. The pin that was not measured this time is the base we are looking for.

　　2. PN junction, fixed tube 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.

　　For any of the two electrodes, if the meter pointer deflects at a large angle, it means that the transistor being measured is an NPN type tube; if the meter pointer deflects at a small angle, the tube being measured is a PNP type tube.

　　3. Follow the arrow, large deflection

　　After finding the base b, which of the other two electrodes is the collector c and which is the emitter e? At this time, we can use the method of measuring the penetration current ICEO to determine the collector c and emitter e.

　　(1) For an NPN transistor, use the black and red test leads of a multimeter to measure the forward and reverse resistances Rce and Rec between the two electrodes in reverse order. Although the deflection angles of the multimeter pointer are very small in both measurements, careful observation will reveal that there will always be one time when the deflection angle is slightly larger. At this time, the direction of the current must be: black test lead → pole c → pole b → pole e → red test lead. The direction of the current flow is exactly the same as the direction of the arrow in the transistor symbol ("follow the arrow"). Therefore, the black test lead must be connected to the collector c, and the red test lead must be connected to the emitter e.

　　(2) For PNP transistors, the principle is similar to that of NPN transistors. The current must flow in the following direction: black test lead → e pole → b pole → c pole → red test lead. The current flow direction is also consistent with the direction of the arrow in the transistor symbol. Therefore, the black test lead must be connected to the emitter e, and the red test lead must be connected to the collector c.

　　4. If you can’t measure it, use your mouth

　　If the deflection of the pointer is too small to distinguish in the two measurements before and after the reversal, you need to "use your mouth". The specific method is: in the two measurements of "follow the arrow, large deflection", pinch the joints of the two test leads and the pins with both hands, hold the base electrode b with your mouth (or press it with your tongue), and still use the "follow the arrow, large deflection" judgment method to distinguish the collector c and the emitter e. The human body plays the role of a DC bias resistor, the purpose is to make the effect more obvious for easy judgment.