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Determining the type and pins of a transistor 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, transistors of two different conductivity types can be divided into NPN type and PNP type. Figure 1 shows their circuit symbols and equivalent circuits. To test the transistor, use the ohmmeter of the multimeter and select the R×100 or R×1k position. Figure 2 shows the equivalent circuit of the ohmmeter of the multimeter. As can be seen from the figure, 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 that the transistor being tested is an NPN transistor. Judging the tube type and pins is a basic skill for beginners of electronic technology. In order to help readers quickly master the test and judgment methods, the author summarizes four sentences: "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 them sentence by sentence. 2. PN junction, fixed tube type After finding the base of the transistor, we can determine the conductivity type of the tube according to the direction of the PN junction between the base and the other two electrodes (Figure 1). Touch the black probe of the multimeter to the base and the red probe to any of the other two electrodes. If the meter pointer deflects a large angle, it means that the transistor being tested is an NPN type tube; if the meter pointer deflects a small angle, the tube being tested is a PNP type. 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 NPN transistors, the measurement circuit of the penetration current is shown in Figure 3. According to this principle, the black and red test leads of a multimeter are used to measure the forward and reverse resistances Rce and Rec between the two poles in reverse. Although the deflection angles of the multimeter pointer are very small in the two measurements, if you observe carefully, there will always be a slightly larger deflection angle. At this time, the current flow direction must be: black test lead → pole c → pole b → pole e → red test lead. The current flow direction is exactly the same as the arrow direction in the transistor symbol ("follow the arrow"), so at this time, 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 flows in the following direction: black test lead → e pole → b pole → c pole → red test lead. The current flows in the same direction as 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 (see Figures 1 and 3). 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 judgment method of "follow the arrow, large deflection" 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. , and it is unclear what 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 the 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 (refer to Figures 1 and 2 for easy understanding of the reason).
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