The good or bad judgment of FET
Time：2020.10.27
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The good or bad judgment of FET
First, qualitatively judge the quality of MOS field effect transistors
First use a multimeter with R×10kΩ block (built-in 9V or 15V battery), connect the negative test lead (black) to the grid (G), and the positive test lead (red) to the source (S). Charge between the gate and the source, the pointer of the multimeter is slightly deflected. Then switch to the R×1Ω block of the multimeter, connect the negative test lead to the drain (D), and the positive lead to the source (S). If the multimeter indicates a few ohms, it means that the FET is good.
Second, qualitatively judge the electrode of the junction field effect tube
Set the multimeter to the R×100 range, connect the red test lead to one pin, and the black test lead to another pin, leaving the third pin floating. If you find that the hands of the watch have a slight swing, it proves that the third pin is the grid. To obtain a more obvious observation effect, you can also use the human body to approach or touch the dangling foot with your fingers. As long as you see that the hands of the watch make a large deflection, it means that the dangling foot is the grid, and the remaining two feet are the source and drain respectively.
Judgment reason: The input resistance of JFET is greater than 100MΩ, and the 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 tube tend to cut off or turn on. If the human body induced voltage is directly applied to the grid, the above phenomenon will be more obvious due to the strong input interference signal. If the watch needle deflects to the left by a large margin, it means that the tube tends to be cut off, the resistance RDS between the drain and the source increases, and the current between the drain and the source decreases IDS. On the contrary, the hand of the watch deflects to the right by a large margin, indicating that the tube tends to conduct, RDS↓, IDS↑. However, the direction in which the hands of the watch deflects depends on the polarity of the induced voltage (forward voltage or reverse voltage) and the operating point of the tube.
Precautions:
(1) The test shows that when both hands are insulated from the D and S poles and only touch the grid, the hands of the watch generally deflect to the left. However, if you touch the D and S poles with your hands and touch the grid with your fingers, you may observe that the hands are deflected to the right. The reason is that several parts of the human body and resistors bias the field effect tube and make it enter the saturation zone. (2) You can also lick the grid with the tip of your tongue, the phenomenon is the same as above.
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Third, the pin identification of the transistor
The triode is composed of a tube core (two PN junctions), three electrodes and a tube case. The three electrodes are called collector c, emitter e and base b respectively. At present, the common triode is a silicon planar tube, which is divided into PNP and There are two types of NPN type. Germanium alloy tubes are now rare.
Here is a simple method of how to measure the three pins of a triode with a multimeter.
1. Find the base and determine the tube type (NPN or PNP)
For PNP transistors, the C and E poles are the positive poles of the two internal PN junctions, and the B poles are their common negative poles. For NPN transistors, the opposite is true: C and E poles are the two PN junctions. The negative pole, and the B pole is their shared positive pole. According to the characteristics of the PN junction with small forward resistance and large reverse resistance, it is easy to determine the type of base and tube. The specific method is as follows:
Set the multimeter to the R×100 or R×1K gear. The red pen touches a certain pin, and the black test pen is used to connect the other two pins respectively. In this way, three sets of readings (two times for each set) can be obtained. When one set of secondary measurements is a low resistance value of several hundred ohms When the common pin is a red test lead, the contact is the base, and the transistor type is PNP; if the common pin is a black test lead, the contact is also the base, and the transistor has an NPN type.
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2. Distinguish the emitter and collector
Since 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 a strong amplification ability. On the contrary, if the emitter and collector are used interchangeably, Then 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 following methods can be used to distinguish the collector and emitter.
Set the multimeter to the R×1K position. Pinch the base electrode and the other pin together by hand (be careful not to let the electrode directly touch). To make the measurement obvious, moisten your finger and connect the red test lead to the pin pinched together with the base electrode. , Connect the black test lead to the other pin, and observe the amplitude of the multimeter pointer swinging to the right. Then swap 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 swing amplitude. For the PNP type transistor, connect the black test lead to the pin pinched together with the base electrode, repeat the above experiment, and find out the one with a large swing of the test needle. For the NPN type, the black test lead is connected to the collector and the red test lead is connected. Is the emitter. For PNP type, the red test lead is connected to the collector, and the black test lead is connected to the emitter.
The principle of this distinguishing electrode method is to use the battery inside the multimeter to apply voltage to the collector and emitter of the triode, so that it has the ability to amplify. When there is a hand to pinch its base and collector, it is equivalent to adding a forward bias current to the triode through the resistance of the hand to turn it on. At this time, the amplitude of the hand swing to the right reflects the magnitude of its amplification ability, so it can be correctly judged Come out the emitter and collector.
