How to use a pointer multimeter to identify field effect tubes?

(1) Using the resistance measurement method to identify the electrodes of the junction field effect transistor
Based on the phenomenon that the forward and reverse resistance values ​​of the PN junction of the field effect transistor are different, the three electrodes of the junction field effect transistor can be identified. Specific method: Set the multimeter to the R×1k position, select any two electrodes, and measure their forward and reverse resistance values ​​respectively. When the forward and reverse resistance values ​​of two electrodes are equal and are several thousand ohms, the two electrodes are the drain D and the source S respectively. Because for the junction field effect transistor, the drain and the source are interchangeable, the remaining electrode must be the gate G. You can also touch the black probe (red probe) of the multimeter to any electrode, and touch the other probe to the remaining two electrodes in turn to measure their resistance values. When the resistance values ​​measured twice are approximately equal, the electrode touched by the black probe is the gate, and the other two electrodes are the drain and the source respectively. If the resistance values ​​measured twice are both very large, it means that it is the reverse direction of the PN junction, that is, both are reverse resistance, it can be determined that it is an N-channel field effect transistor, and the black test pen is connected to the gate; if the resistance values ​​measured twice are both very small, it means that it is a forward PN junction, that is, forward resistance, it is determined to be a P-channel field effect transistor, and the black test pen is also connected to the gate. If the above situation does not occur, you can swap the black and red test pens and test according to the above method until the gate is identified.

(2) Use the resistance measurement method to determine the quality of the field effect tube. The resistance measurement method is to use a multimeter to measure the resistance between the source and drain, gate and source, gate and drain, gate G1 and gate G2 of the field effect tube to determine whether it matches the resistance value indicated in the field effect tube manual. Specific method: First, set the multimeter to R×10 or R×100, and measure the resistance between the source S and the drain D, which is usually in the range of tens of ohms to thousands of ohms (it can be seen in the manual that the resistance values ​​of various types of tubes are different). If the measured resistance value is greater than the normal value, it may be due to poor internal contact; if the measured resistance value is infinite, it may be due to internal disconnection. Then set the multimeter to R×10k, and measure the resistance between the gate G1 and G2, the gate and source, and the gate and drain. When the measured resistance values ​​are all infinite, it means that the tube is normal; if the measured resistance values ​​are too small or are connected, it means that the tube is bad. It should be noted that if the two gates are disconnected inside the tube, the component replacement method can be used for detection.

(3) Use the induction signal input method to estimate the amplification capacity of the field effect tube. Specific method: Use the multimeter resistance R×100 position, connect the red test lead to the source S, and the black test lead to the drain D, and apply a 1.5V power supply voltage to the field effect tube. At this time, the needle indicates the resistance value between the drain and the source. Then pinch the gate G of the junction field effect tube with your hand and add the induced voltage signal of the human body to the gate. In this way, due to the amplification effect of the tube, the drain-source voltage VDS and the drain current Ib will change, that is, the resistance between the drain and the source has changed, and it can be observed that the needle has a large swing. If the needle swings less when the gate is pinched, it means that the tube has a poor amplification capacity; if the needle swings more, it means that the tube has a large amplification capacity; if the needle does not move, it means that the tube is broken.

According to the above method, we use the R×100 range of the multimeter to measure the junction field effect tube 3DJ2F. First, the G pole of the tube is open, and the drain-source resistance RDS is measured to be 600Ω. After pinching the G pole with your hand, the needle swings to the left, and the indicated resistance RDS is 12kΩ. The swing amplitude of the needle is large, indicating that the tube is good and has a large amplification capacity. There are a few points to note when using this method: First, when testing the field effect tube by pinching the gate with your hand, the multimeter needle may swing to the right (the resistance value decreases) or to the left (the resistance value increases). This is because the AC voltage induced by the human body is high, and different field effect tubes may have different working points when measured with the resistance range (either working in the saturation area or in the unsaturation area). The test shows that the RDS of most tubes increases, that is, the needle swings to the left; the RDS of a few tubes decreases, causing the needle to swing to the right. But no matter what the direction of the needle swings, as long as the needle swings a large amplitude, it means that the tube has a large amplification capacity. Second, this method is also applicable to MOS field effect tubes. But please note that the input resistance of MOS field effect tube is high, and the induced voltage allowed by the gate G should not be too high, so do not pinch the gate directly with your hands. You must use the insulated handle of the screwdriver to touch the gate with a metal rod to prevent the induced charge of the human body from being directly added to the gate and causing the gate to break down. Third, after each measurement, the GS poles should be short-circuited. This is because a small amount of charge will be charged on the GS junction capacitor, establishing the VGS voltage, causing the needle to not move when measuring again. Only by short-circuiting the charge between the GS poles can it be discharged.

(4) Using the resistance measurement method to identify unmarked field effect transistors First, use the resistance measurement method to find two pins with resistance values, namely the source S and the drain D. The remaining two pins are the first gate G1 and the second gate G2. Write down the resistance value between the source S and the drain D measured by the two test leads first, swap the test leads and measure again, and write down the resistance value measured. The one with the larger resistance value is the electrode connected to the black test lead as the drain D; the electrode connected to the red test lead is the source S. The S and D poles identified by this method can also be verified by estimating the amplification capacity of the tube, that is, the black test lead with a larger amplification capacity is connected to the D pole; the red test lead is connected to the ground pole, and the test results of the two methods should be the same. After determining the position of the drain D and the source S, install the circuit according to the corresponding position of D and S. Generally, G1 and G2 will also be aligned in sequence, which determines the position of the two gates G1 and G2, and thus determines the order of the D, S, G1, and G2 pins.

(5) Determine the size of transconductance by measuring the change in reverse resistance When measuring the transconductance performance of VMOS N-channel enhancement field effect tube, you can connect the red test lead to the source S and the black test lead to the drain D, which is equivalent to adding a reverse voltage between the source and drain. At this time, the gate is open and the reverse resistance value of the tube is very unstable. Set the ohm range of the multimeter to the high resistance range of R×10kΩ. At this time, the voltage in the meter is relatively high. When you touch the gate G with your hand, you will find that the reverse resistance value of the tube has changed significantly. The greater the change, the higher the transconductance value of the tube; if the transconductance of the tube being measured is very small, the reverse resistance value will not change much when measured by this method. 2. VMOS field effect tube VMOS field effect tube (VMOSFET) is abbreviated as VMOS tube or power field effect tube, and its full name is V-groove MOS field effect tube. It is a new high-efficiency, power switching device developed after MOSFET. It not only inherits the high input impedance (≥108W) and small driving current (about 0.1μA) of MOS field effect tube, but also has excellent characteristics such as high withstand voltage (up to 1200V), large working current (1.5A~100A), high output power (1~250W), good linearity of transconductance, and fast switching speed. It is precisely because it combines the advantages of electron tubes and power transistors that it is widely used in voltage amplifiers (voltage amplification can reach thousands of times), power amplifiers, switching power supplies and inverters. VMOS field effect power tube has the advantages of extremely high input impedance and large linear amplification area, especially it has a negative current temperature coefficient, that is, when the gate-source voltage remains unchanged, the on-current will decrease with the increase of tube temperature, so there is no tube damage caused by the "secondary breakdown" phenomenon. Therefore, the parallel connection of VMOS tubes is widely used. As we all know, the gate, source and drain of traditional MOS field effect tubes are roughly on the same horizontal plane of the chip, and its working current basically flows in the horizontal direction. VMOS tubes are different. From Figure 1, we can see its two major structural features: first, the metal gate adopts a V-groove structure; second, it has vertical conductivity. Since the drain is led out from the back of the chip, ID does not flow horizontally along the chip, but starts from the heavily doped N+ region (source S), flows through the P channel into the lightly doped N-drift region, and finally vertically downward to the drain D. The direction of the current is shown by the arrow in the figure. Because the cross-sectional area of ​​the flow increases, a large current can pass. Since there is a silicon dioxide insulating layer between the gate and the chip, it still belongs to an insulated gate MOS field effect tube. Typical domestically produced VMOS field effect tubes include VN401, VN672, VMPT2, etc.