Some methods for detecting FETs with a pointer-type multimeter - News - Global IC Trade Starts Here Free Products

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1. Using the resistance measurement method to determine the electrode of the junction field effect transistor According to the phenomenon that the positive and negative resistance values ​​of the PN junction of the field effect transistor are different, the three electrodes of the junction field effect transistor can be discriminated. Specific method: Put the multimeter on the R×1k file, and select two electrodes to measure the positive and negative resistance values. When the positive and negative resistance values ​​of a certain two electrodes are equal and several thousand ohms, the two electrodes are the drain D and the source S, respectively. Because the junction and source are interchangeable for the junction field effect transistor, the remaining electrode must be the gate G. It is also possible to arbitrarily contact the black meter (red pen) of the multimeter to one electrode, and the other pen to contact the remaining two electrodes in turn to measure the resistance value. When the two measured resistance values ​​are approximately equal, the electrode touched by the black test lead is the gate, and the remaining two electrodes are the drain and the source, respectively. If the measured resistance values ​​are both large, it means that the reverse of the PN junction, that is, the reverse resistance, can be judged to be an N-channel FET, and the black meter is connected to the gate; if it is measured twice The resistance value is small, indicating that it is a forward PN junction, that is, a forward resistance, which is determined to be a P-channel field effect transistor, and a black meter pen is also a gate. If the above situation does not occur, the black and red test leads can be exchanged as described above until the gate is discriminated.
2, using the resistance method to determine the good effect of the FET is measured by a multimeter to measure the source and drain of the FET, the gate and source, the gate and drain, the gate G1 and the gate G2 The resistance value between the two is the same as the resistance value indicated in the FET manual to determine whether the tube is good or bad. Specific method: first place the multimeter in R × 10 or R × 100, measure the resistance between the source S and the drain D, usually in the range of tens of ohms to several thousand ohms (in the manual, various models The resistance of the tube is different. If the measured resistance is greater than the normal value, it may be due to poor internal contact; if the measured resistance is infinite, it may be an internal fault. Then, the multimeter is placed in the R×10k file, and the resistance between the gates G1 and G2, between the gate and the source, and between the gate and the drain is measured. When the resistance values ​​are infinite, It is indicated that the tube is normal; if the above resistance values ​​are too small or are passages, the tube is bad. It should be noted that if the two gates are broken in the tube, the component substitution method can be used for detection.
3. Using the resistance-resistance method to identify the FET without the mark First, use the method of measuring the resistance to find two pins with resistance values, that is, the source S and the drain D, and the remaining two legs are the first gate G1. And a second gate G2. Write down the resistance value between the source S and the drain D measured by the two meter pens, measure the meter once again, record the measured resistance value, and measure the resistance value twice, the black meter pen The connected electrode is the drain D; the red test lead is connected to the source S. The S and D poles discriminated by this method can also be verified by the method of estimating the amplification capability of the tube, that is, the black meter pen with large amplification capability is connected to the D pole; the ground of the red test pen is 8 pole, two The test results should be the same. After determining the positions of the drain D and the source S, the circuit is mounted according to the corresponding positions of D and S. Generally, G1 and G2 are also aligned in sequence, which determines the positions of the two gates G1 and G2. Thus, the order of the D, S, G1, and G2 pins is determined.
4. Judging the size of the transconductance by measuring the change of the reverse resistance value. When measuring the transconductance performance of the VMOSN channel enhancement type FET, the red stylus can be used to connect the source S and the black stylus to the drain D. This is equivalent to A reverse voltage is applied between the source and the drain. At this time, the gate is open, and the reverse resistance of the tube is very unstable. The ohmic file of the multimeter is selected in the high resistance of R×10kΩ, and the voltage in the meter is high. When the gate G is touched by hand, the reverse resistance value of the tube is obviously changed. The larger the change, the higher the transconductance value of the tube. If the transconductance of the tube is small, use this method to measure When the reverse resistance does not change much.
5. Estimate the amplification capability of the FET by the inductive signal input method. Use the R×100 file of the multimeter resistance, the red pen to the source S, the black pen to the drain D, and the field effect tube plus 1.5V. The power supply voltage, at this time, the resistance value between the drain and source indicated by the hands. Then, the gate G of the junction field effect transistor is pinched by hand, and the induced voltage signal of the human body is applied to the gate. Thus, due to the amplification of the tube, the drain-source voltage VDS and the drain current Ib both change, that is, the resistance between the drain and the source changes, and thus the needle has a large amplitude swing. If the hand-pinched gate needle swings less, the tube's amplification ability is poor; the needle swing is large, indicating that the tube has a large amplification capability; if the needle does not move, the tube is bad.
According to the above method, we use the R×100 file of the multimeter to measure the junction field effect transistor 3DJ2F. First open the G pole of the tube, and measure the drain-source resistance RDS to 600Ω. After pinching the G-pole, the hands will swing to the left, and the indicated resistance RDS is 12kΩ. The amplitude of the needle swing is large, indicating that the tube is good. And has a larger magnification.
There are several points to be made when using this method: First, when the test field effect tube is pinched by the gate, the multimeter hand may swing to the right (the resistance value decreases) or may swing to the left (the resistance value increases). This is because the AC voltage induced by the human body is higher, and the operating points of different FETs measured by the resistance file may be different (or work in the saturation zone or in the unsaturated zone). Tests show that the RDS of most tubes increases. That is, the hands are swung to the left; the RDS of a few tubes is reduced, causing the hands to swing to the right. However, no matter how the direction of the needle swings, as long as the swing of the hands is large, the tube has a large amplification capability.
Second, this method is also applicable to MOS FETs. However, it should be noted that the input resistance of the MOS FET is high, and the induced voltage allowed by the gate G should not be too high, so do not directly pinch the gate by hand. It must be used to hold the insulated handle of the screwdriver and touch it with a metal rod. The gate is touched to prevent the body's induced charge from being directly applied to the gate, causing gate breakdown. Third, each time the measurement is completed, the GS should be short-circuited. This is because the GS junction capacitance will be charged with a small amount of charge, and the VGS voltage is established, so that the hands may not move when the measurement is performed again, and only the GS interelectrode charge short circuit is released.