Pithy formula for measuring and judging triode

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Pithy formula for determining triode

judging the tube type and pin of triode is a basic skill for beginners of electronic technology. In order to help readers quickly master the measurement method, the author summarizes four pithy formulas: "three reverses, find the base; PN junction, determine the tube type; arrow, large deflection; uncertainty, move your mouth." Let's explain it sentence by sentence

turn one and three upside down and find the base

as we all know, triodes are semiconductor devices containing two PN junctions. According to the different connection modes of the two PN junctions, they can be divided into two types of triodes: NPN type and PNP type

use the ohm gear of the multimeter to test the triode, and select R × 100 or R × 1K gear. For the pointer multimeter, the red probe is connected to the negative pole of the battery in the meter, and the black probe is connected to the positive pole of the battery in the meter. Suppose we don't know whether the tested triode is NPN type or PNP type, and we can't tell what electrode each pin is. The first step of the test is to determine which pin is the base. At this time, we take any two electrodes (for example, the two electrodes are 1 and 2), measure its forward and reverse resistance with the two probes of the multimeter upside down, and observe the deflection angle of the probe; Then, take two electrodes 1 and 3 and two electrodes 2 and 3, and measure their forward and reverse resistances respectively. The plastic products industry is the origin of waste plastics, and observe the deflection angle of the watch needle. In the three reverse measurements, there must be two similar measurement results: that is, in the reverse measurement, the first deflection of the gauge needle is large and the first deflection is small; The other time, the deflection angle of the pointer before and after the reverse measurement must be very small. The unmeasured pin this time is the base we are looking for

II. PN junction, fixed tube type

after finding out the base of the triode, we can determine the conductive type of the tube according to the direction of the PN junction between the base and the other two electrodes. Contact the black probe of the multimeter with the base electrode and the red probe with any of the other two electrodes. If the deflection angle of the meter pointer is large, it indicates that the measured triode is an NPN type tube; If the deflection angle of the meter pointer is very small, the tested tube is PNP type

III. The arrow is straight and the deflection is large.

the base B is found. Which of the other two electrodes is the collector C and which is the emitter e? At this time, we can determine the collector C and emitter e by measuring the penetration current ICEO

(1) for NPN type triode, according to the flow direction principle of penetration current of NPN type triode, use the black and red probes of the multimeter to reverse the measurement of the forward and reverse resistances rce and rec between the two poles. Although the deflection angle of the multimeter pointer is very small in both measurements, careful observation shows that there will always be a slightly larger deflection angle once, and the output of ports each year will reach about 5billion. At this time, the flow direction of current must be: Black probe → pole C → pole B → pole e → red probe, The current flow direction is exactly the same as the arrow direction in the triode symbol, so at this time, the black probe must be connected to the collector C, and the red probe must be connected to the emitter E

(2) for the PNP type triode, the principle is similar to that of the NPN type. The current flow direction must be: Black probe → e pole → B pole → C pole → red probe. The current flow direction is also consistent with the arrow direction in the triode symbol. Therefore, if the tested part is a slender axis, the black probe must be connected to the emitter e, and the red probe must be connected to the collector C

IV. it can not be measured. Move your mouth

if during the measurement process of "following the arrow, the deflection is large", if the deflection of the two measurement pointers before and after the reversal is too small to distinguish, it is necessary to "move your mouth". The specific method is as follows: in the two measurements of "arrow, large deflection", hold the joint between the two probes and the pin with two hands, hold (or hold) the base electrode B with your mouth, and still use the discrimination method of "arrow, large deflection" to distinguish the open collector electrode C from the emitter electrode E. The human body acts as a DC bias resistor to make the effect more obvious. (end)


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