Replacement principle and good or bad judgment of FET
Time:2022-07-17
Views:1972
1、 Qualitatively judge the quality of MOS FET
Use multimeter r first × 10K Ω gear (built-in 9V or 15V battery), connect the negative probe (black) to the grid (g), and the positive probe (red) to the source (s). Charge between the grid and the source, and the multimeter pointer deflects slightly at this time. Then use multimeter R × At 1 Ω gear, connect the negative probe to the drain (d) and the positive probe to the source (s). If the indicated value of the multimeter is a few ohms, it indicates that the FET is good.
2、 Qualitatively judge the electrode of Junction FET
Set the multimeter to R × In gear 100, the red probe is connected to one foot tube arbitrarily, and the black probe is connected to another foot tube, so that the third foot is suspended. If the watch needle swings slightly, it proves that the third pin is the grid. To obtain more obvious observation effect, you can also use the human body to approach or touch the suspended foot with your fingers. As long as you see the watch needle deflect greatly, it means that the suspended foot is the grid, and the other two pins are the source and drain respectively.
Judgment reason: the input resistance of JFET is greater than 100m Ω, and the transconductance is very high. When the grid is open, the space electromagnetic field is easy to induce a voltage signal on the grid, making the tube tend to cut off or turn on. If the human induced voltage is directly applied to the grid, the above phenomenon will be more obvious due to the strong input interference signal. If the gauge needle deflects to the left, it means that the tube tends to cut off, the drain source resistance RDS increases, and the drain source current decreases IDs. On the contrary, the gauge needle deflects sharply to the right, indicating that the tube tends to be connected, RDS ↓, IDS ↑. However, the direction in which the meter needle deflects depends on the polarity of the induced voltage (forward voltage or reverse voltage) and the working point of the tube.
matters needing attention:
(1) The test shows that when two hands are insulated from D and S poles and only touch the grid, the watch needle generally deflects to the left. However, if both hands touch the D and S poles respectively, and touch the grid with your fingers, it is possible to observe the right deflection of the watch needle. The reason is that several parts and resistances of the human body bias the FET into the saturation region. (2) You can also lick the grid with the tip of your tongue. The phenomenon is the same as above.
3、 Crystal triode pin discrimination
Triode is composed of tube core (two PN junctions), three electrodes and tube shell. The three electrodes are called collector C, emitter E and base B respectively. At present, the common triode is silicon planar tube, which is divided into PNP and NPN types. Germanium alloy tubes are now rare.
Here is a simple way to measure the three pins of a triode with a multimeter.
1. Find out the base and determine the tube type (NPN or PNP)
For PNP type triode, pole C and E are the positive poles of the two PN junctions inside, and pole B is their common negative pole. For NPN type triode, it is just the opposite: pole C and E are the negative poles of the two PN junctions respectively, and pole B is their common positive pole. According to the characteristics of small forward resistance and large reverse resistance of PN junction, it is very convenient to judge the type of base and tube. The specific methods are as follows:
Set the multimeter to R × 100 or R × 1K gear. When the red probe contacts a certain pin, connect the other two pins with the black probe respectively, so that three groups of readings (two times in each group) can be obtained. When the secondary measurement of one group is a low resistance value of hundreds of ohms, if the common pin is a red probe, the contact is the base, and the tube type of the triode is PNP; If the common pin is a black probe, the contact is also the base, and the tube type of the triode is NPN.
2. Distinguish emitter and collector
Because the doping concentration of the two p regions or the two N regions is different when the triode is made, if the emitter and collector are used correctly, the triode has strong amplification ability. On the contrary, if the emitter and collector are used interchangeably, the amplification ability is very weak, so the emitter and collector of the tube can be distinguished.
After distinguishing the tube type and base B, the collector and emitter can be distinguished by the following methods. Set the multimeter to R × 1K gear. Pinch the base with the other pin by hand (be careful not to let the electrode directly touch). In order to make the measurement obvious, wet your fingers, connect the red probe to the pin pinched with the base, and connect the black probe to the other pin. Pay attention to the range of the multimeter pointer swinging to the right. Then adjust the two pins and repeat the above measurement steps. Compare the amplitude of the meter needle swinging to the right in the two measurements, and find out the one with a large swing amplitude. For PNP type triode, connect the black probe to the pin pinched with the base, repeat the above experiment, and find out the time when the pointer swings greatly. For NPN type, the black probe is connected to the collector, and the red probe is connected to the emitter. For PNP type, the red probe is connected to the collector, and the black probe is connected to the emitter.
The principle of this electrode discrimination method is to use the battery inside the multimeter to apply 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 positive bias current to the triode through the resistance of the hand, so that it can be turned on. At this time, the amplitude of the meter needle swinging to the right reflects the amplification ability. Therefore, the starting emitter and collector can be correctly identified.
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