Answer: A multimeter can be used to judge the polarity of a transistor, determine whether it is a silicon tube or a germanium tube, and distinguish its pins at the same time. For general low-power tubes, generally only R×1K gear should be used for judgment. The steps are as follows:
(1) positive and negative measurement:
(2) determine the base:
Number 1, 2 and 3 on the three pins of the transistor. The multimeter makes three kinds of measurements, namely, 1-2, 2-3 and 3-1, each of which is divided into positive measurement and negative measurement. Of these 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. Because the semiconductor triodes are all made of two diodes connected in reverse. The positive resistance between emitter, collector and base, that is, the forward resistance of general diode, is very small. When the two probes are connected to the collector electrode and emitter, their resistance is much greater than the forward resistance of the general diode.
(3) Discrimination of polarity:
(4) determine the collector and emitter:
Make a positive measurement of the base. For NPN tube, the black stylus is connected to the collector, and for PNP tube, the black stylus is connected to the emitter. This is because there is a PN junction in the reverse direction regardless of the forward or reverse measurement, 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, when the collector-emitter resistance of NPN tube is small, the collector is connected to the positive electrode of the battery, that is, the black stylus is connected. For PNP tube, when the resistance between collector and emitter is small, the emitter is connected with a black stylus.
(5) judging whether it is a silicon tube or a germanium tube;
Make a positive measurement of the emitter and base. If the pointer deflects by 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 emitters is UBE=(1-N/N) E, E=1.5 V is the battery voltage, N is the total number of grids of a DC voltage with linear scale, and N is the number of grids that the hands deflect on the scale line. Generally, silicon tube U {{10}}.6 ~ 0.7 V, and germanium tube UBE=0.2 ~ 0.3 V.. Therefore, when testing, for silicon tubes, n/N is 1/2 ~ 3/5; For germanium tube, n/N is more than 4/5. In addition, for the discrimination of general low power, the universal meter should not use R×10 or R×1. Take the 500 multimeter to measure the silicon tube as an example, the internal resistance of the meter is 100 Ω at R×10, and the current reaches IBE=(1.5-0.7)/100=8 Ma when measuring the germanium tube, and the current at R×1 is even greater, which may damage the transistor. As for the R×1 K block, the battery voltage of the block is relatively high, such as 1 V, 12 V, 15 V, 22.5 V, etc., which may cause PN junction breakdown during reverse measurement, so the block should also be used with caution.
