Judging transistor types and pins is a basic skill for beginners of electronic technology. In order to help readers master the measurement judgment method quickly, the author summarizes four formulas: "three inverses, finding the bottom;" PN junction, fixed tube type; In the direction of arrow, large deflection; Not sure, move your mouth. " Let's explain sentence by sentence. As we all know, triode is a semiconductor device with two PN junctions. According to the different connection modes of the two PN junctions, they can be divided into two transistors with different conductivity types: NPN type and PNP type. Figure 1 shows their circuit symbols and equivalent circuits. To test the triode, use the ohm range of the multimeter, and select the R× 100 or R× 1k range. Fig. 2 describes the equivalent circuit of ohm range of multimeter. As can be seen from the figure, the red stylus is connected to the negative electrode of the battery in the instrument, and the black stylus is connected to the positive electrode of the battery in the instrument. Suppose we don't know whether the transistor under test is NPN or PNP, and we can't tell which electrode each pin is. The first step in testing is to determine which pin is the base. At this time, we take any two electrodes (for example, these two electrodes are 1, 2), measure the positive and negative resistance with two probes of a universal ammeter upside down, and observe the deflection angle of the pointer; Then, take two electrodes of 1 and 3 and two electrodes of 2 and 3, measure their forward and reverse resistances respectively, and observe the deflection angle of the pointer. In these three back tests, there must be two similar results: that is, in the back test, the first deflection of the watch hand is large and the first deflection is small; The remaining time must be that the deflection angle of the pointer before and after reverse measurement is very small, and this time the unmeasured needle is the base we are looking for (see figure 1 and figure 2). Second, the PN junction, determine the type of the lamp After finding the base of the transistor, we can determine the conductivity type of the lamp according to the PN junction direction between the base and the other two electrodes (Figure 1). Touch the base with the black pen of the multimeter, and touch either of the other two electrodes with the red pen. If the deflection angle of the multimeter pointer is large, it means that the triode under test is an NPN tube. If the deflection angle of the instrument pointer is small, the tube under test is PNP type. Third, along the arrow, find the base b by deflection. Which of the other two electrodes is the collector C and the emitter E? At this point, we can determine the collector c and emitter e by measuring the penetration current ICEO. (1) For the NPN transistor, the circuit for measuring the penetration current is shown in Figure 3. According to this principle, the positive and negative resistances Rce and Rec between the two poles are measured with the black and red probes of the multimeter inverted. Although the deflection angle of the multimeter pointer measured twice is very small, there will always be a slightly larger deflection angle after careful observation. At this time, the current flow direction must be: black probe →c pole →b pole →e pole → red probe, and the current flow direction is completely consistent with the arrow direction (forward arrow) in the triode symbol. (2) For PNP triode, the reason is similar to NPN triode, and its current flow direction must be: black contact →e electrode →b electrode →c electrode → red contact, and its current flow direction is also consistent with the direction of the arrow in the triode symbol, so the black contact must be connected to the emitter E and the red contact must be connected to the collector C (see Figure 1 and Figure 3). 4. It can't be detected that in the process of "following the arrow, the deflection is large", and if the deflection of the measuring pointer before and after inversion is too small to distinguish, it is called "moving mouth". The specific method is: in the two measurements of "large deflection in the direction of arrow", hold the connection between the two pens and the pin with both hands, and hold the base B with the mouth (or tongue), and still use the discrimination method of "large deflection in the direction of arrow" to distinguish the collector C from the emitter E, in which the human body is used as a DC bias resistor, which makes the effect more obvious. Qualitative judgment of field effect transistor and triode 1. To determine the quality of FET, firstly, connect the negative stylus (black) to the grid (G) and the positive stylus (red) to the source (S) with a multimeter R×10Ω (built-in 15V battery). Charge between the gate and the source, at this time, the multimeter pointer slightly deflects. Then switch to the multimeter r× 1ω, connect the negative probe to the drain (d), and connect the positive probe to the source (s). If the multimeter indicates several ohms, the FET is good.

Qualitative judgment of quality of field effect transistor and triode-. Charge between the gate and the source, at this time, the multimeter pointer slightly deflects. Then switch to the multimeter r× 1ω, connect the negative probe to the drain (d), and connect the positive probe to the source (s). If the multimeter indicates several ohms, the FET is good. Secondly, the electrode of the junction field effect transistor is judged. Turn the multimeter to R× 100. The red stylus is randomly connected to one foot tube, and the black stylus is connected to the other foot tube, so that the third foot is suspended. If the watch hand is found to swing slightly, it proves that the third hand is a lattice. In order to get a more obvious observation effect, you can also use your human body to approach or touch your dangling feet with your fingers. As long as you see that the pointer of the watch has a large deflection, it means that the suspended foot is the grid, and the other two feet are the source and the drain respectively. Reason for judgment: JFET's input resistance is greater than100Ω, and its transconductance is very high. When the grid is open, the space electromagnetic field can easily induce a voltage signal on the grid, which makes the electron tube tend to turn off or on. If the artificially induced voltage is directly applied to the power grid, the above phenomenon will be more obvious because of the strong input interference signal. If the pointer deflects to the left, it means that the electron tube tends to turn off, the resistance RDS between the drain and the source increases, and the current IDS between the drain and the source decreases. On the contrary, the pointer of the watch deflects greatly to the right, indicating that the electron tube tends to conduct, RDS↓, IDS↑. However, which direction the hand deflects depends on the polarity of the induced voltage (DC voltage or reverse voltage) and the working point of the pipeline. Note: (1) test shows that the hands are insulated from the D and S poles, and when they only touch the grid, the hands generally deflect to the left. However, if you touch the D and S poles with your hands and touch the grid with your fingers, it is possible to observe the right deflection of your hands. The reason is that several parts of human body and resistors bias FET to saturation region. (2) You can also lick the grid with the tip of your tongue, as above. Third, the transistor pin identification triode consists of a tube core (two PN junctions), three electrodes and a tube shell. The three electrodes are called collector C, emitter E and base B. At present, there are two common triodes: silicon flat tube and germanium alloy tube, each of which has PNP and NPN types. Here is a simple method of measuring three pins of triode with multimeter. 1. Find out that for PNP triode, the C and E poles are the positive poles of two PN junctions, and the B pole is the same negative pole, but for NPN triode, the opposite is true: the C and E poles are the negative poles of two PN junctions, and the B pole is their positive poles. According to the characteristics of small forward resistance and large reverse resistance of PN junction, it is convenient to judge the base and tube. The specific method is as follows: set the multimeter at R× 100 or r×1k. The red pen touches one needle, and the other two needles are respectively connected with the black pen, so that three groups (twice in each group) of readings can be obtained. When a group of secondary measurements show a low resistance of several hundred ohms, the pin contacted by the red stylus is the base and the transistor is PNP type; If a group of high resistance values are measured by the above method, the pin contacted by the red stylus is the base and the transistor is the NPN. 2. Distinguish the emitter from the collector Because the doping concentrations of two P regions or two N regions are different when the triode is manufactured, if the emitter and collector are used correctly, the triode has strong amplification ability; On the other hand, if the emitter and collector are used interchangeably, the amplification ability is very weak, so that the emitter and collector of the electron tube can be distinguished. After identifying the tube type and base B, you can use one of the following methods to identify the collector and emitter. (1) Set the multimeter at R× 1. Hold the base with another needle by hand (be careful not to let the electrodes touch each other directly). In order to make the measurement phenomenon obvious, wet your fingers, connect the red stylus to the pin clamped by the base, and connect the black stylus to the other pin. Pay attention to the amplitude of the multimeter pointer swinging to the right. Then switch the two pins and repeat the above measurement steps. Compare the amplitude of the watch hand swinging to the right in the two measurements, and find the one with the larger amplitude. For PNP triode, connect the black stylus to the pin clamped by the base, and repeat the above experiment to find out that the stylus swings for a long time. At this time, the black contact pin is connected to the collector and the red contact pin is connected to the emitter. The principle of this electrode discrimination method is to use the battery inside the multimeter to add voltage to the collector and emitter of the triode, so that it has amplification ability. When the base and collector are pinched by hand, it is equivalent to adding a forward bias current to the transistor through the resistance of the hand to turn it on. At this time, the amplitude of the hand swinging to the right reflects its amplification ability, so the emitter and collector can be correctly distinguished. (2) Turn the multimeter to R× 1, connect the two probes of the multimeter to the other two pins of the pipe, lick the base with your tongue, look at the instructions of the probes, then switch the probes and repeat the above steps to find out the big pendulum. For PNP triode, the red contact pin is connected to the collector and the black contact pin is connected to the emitter; For NPN transistor, the black contact pin is connected to the collector and the red contact pin is connected to the emitter.