Transistors made in Japan are different from those made in China. Generally, the third letter is A, C is PNP, and B and D are NPN. For example, 2SD 1942NPN is sometimes omitted as D 1942, and a10/5 is PNP.
If the transistor is not clearly marked, a multimeter can be used to judge the polarity of the transistor, determine whether it is a silicon tube or a germanium tube, and at the same time distinguish its pins. For ordinary low-power tubes, generally only Rx 1K file is used to judge. The steps are as follows:
1. Normal measurement and reverse measurement Measure the resistance of any two legs of the transistor with red and black probes, and then exchange the red and black probes and still measure the resistance of these two legs. The two measurements have different resistance readings. We call the measurement with small resistance reading as normal measurement and the measurement with large resistance reading as reverse measurement.
2. Determine the base of transistor 1.2.3 and number the three pins. The multimeter makes three kinds of measurements, namely 1-2, 2-3, 3- 1, and each measurement is divided into positive measurement and negative measurement. Of the six measurements, three were positive and the resistance readings were different. Find the pin with the largest positive resistance, such as 1-2, and the other pin 3 is the base. This is because the two lamps are connected by two diodes in reverse (as shown in the attached figure). The positive resistance between emitter, collector and base, that is, the forward resistance of general diodes, is very small. When two probes are connected to the collector and emitter, their resistance is much greater than the forward resistance of a general diode.
3. The black stylus for judging polarity is connected to the determined base, and the red stylus is connected to another arbitrary pole. If it is a positive measurement, it is an NPN tube; If the measurement is negative, it is PNP tube. This is because the black stylus is connected to the positive battery of the multimeter. If it is a positive test, the black stylus is connected to the P terminal, and the transistor belongs to NPN type. In the reverse test, the black probe is connected to the N terminal, and the transistor is PNP type.
4. Determine the collector and emitter of positive electrode measurement. In forward measurement, the black contact pin of NPN tube is connected to the collector, and the black contact pin of PNP tube is connected to the emitter. This is because there is a PN junction in both positive and negative directions, and most of the battery voltage falls on the reverse PN junction. When the emitter junction is forward biased, the current flowing through the collector circuit is large and the resistance is small. Therefore, for NPN tubes, when the collector-emitter resistance is small, the collector is connected to the positive electrode of the battery, that is, to the black contact pin. For PNP tube, when the resistance between collector and emitter is small, the emitter is connected with a black contact pin.
5. Determine whether the silicon tube or the germanium tube is used for the positive measurement of the emitter base. If the pointer deflects 1/2-3/5, it is a silicon tube. If the pointer deflects more than 4/5, it is a germanium tube. This is because the voltage applied between the base emitter and the base emitter is Ube=( 1-n/N)E, E= 1.5v is the battery voltage, n is the total grid number of DC voltage with a linear scale, and n is the grid number of pointer deflection. Generally, silicon tube U=0.6~0.7v, and germanium tube UBE = 0.2 ~ 0.3V Therefore, for silicon tube, n/N is about1/2-3/5; For germanium tube, n/N is greater than 4/5. In addition, judging the general low power, do not use Rx 10 or Rx 1 for the multimeter. Take the 500 multimeter to measure the silicon tube as an example. The internal resistance of the meter at Rx 10 is100Ω, and the positive measurement value of the b.e pole of the silicon tube is that the current reaches IBE = (1.5v-0.7v)/100Ω = 8ma. When measuring germanium tube, the current is larger, and when using Rx 1, it is even larger, which may damage the transistor. As for the Rx 1k block, the battery voltage of this block is relatively high, such as 1V, 12V, 15V, 22.5V, etc. Reverse measurement may lead to PN junction breakdown, so this module should also be used with caution.