1. Pin identification of junction field effect transistors

The gate of the FET corresponds to the base of the transistor, and the source and drain correspond to the emitter and collector of the transistor, respectively. Put the multimeter in the R×1k gear, and use two test pens to measure the forward and reverse resistances between each two pins respectively. When the forward and reverse resistances between two pins are equal, and both are several KΩ, the two pins are the drain D and the source S (interchangeable), and the remaining pin is the gate G . For junction field effect transistors with 4 pins, the other pole is the shielding pole (grounded in use).

2. Judgment grid

Touch one electrode of the tube with the black test lead of the multimeter, and touch the other two electrodes with the red test lead respectively. If the resistance values measured twice are very small, it means that it is a forward resistance, the tube is an N-channel field effect transistor, and the black test lead is also connected to the grid.

The manufacturing process determines that the source and drain of the FET are symmetrical and can be used interchangeably without affecting the normal operation of the circuit, so there is no need to distinguish them. The resistance between source and drain is about several thousand ohms.

Note that this method cannot be used to determine the gate of the IGBT. Because the input resistance of this kind of tube is extremely high, and the inter-electrode capacitance between the gate and source is very small, as long as there is a small amount of charge during measurement, a high voltage can be formed on the inter-electrode capacitance, which is easy to damage the tube.

3. Estimate the amplification capability of the FET

Turn the multimeter to R×100, the red test lead is connected to the source S, and the black test lead is connected to the drain D, which is equivalent to adding a 1.5V power supply voltage to the FET. At this time, the pointer indicates the resistance value between DS electrodes. Then pinch the grid G with your fingers, and add the induced voltage of the human body to the grid as an input signal. Due to the magnification of the tube, both UDS and ID will change, which is also equivalent to the change of the resistance between the DS electrodes. It can be observed that the needle has a relatively large swing. If the needle swings very little when the grid is pinched by hand, it means that the amplifying ability of the tube is weak; if the needle does not move, it means that the tube has been damaged.

Since the 50Hz AC voltage induced by the human body is relatively high, and different FETs may have different working points when measuring with the resistance gear, the needle may swing to the right or left when the grid is pinched by hand. The RDS of a few tubes is reduced, causing the hands to swing to the right, and the RDS of most tubes is increased, causing the hands to swing to the left. Regardless of the swing direction of the hands, as long as there is a significant swing, the tube has the ability to magnify.

This method is also suitable for measuring MOS tubes. In order to protect the MOS field effect tube, it is necessary to hold the insulating handle of the screwdriver by hand, and touch the grid with a metal rod, so as to prevent the human body induced charge from being directly added to the grid and damaging the tube.

After each measurement of the MOS tube, the GS junction capacitor will be charged with a small amount of charge, and the voltage UGS will be established, and then the time measurement needle may not move. At this time, short-circuit the GS poles.

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