​Pin identification of JFET transistor

Pin identification of JFET transistor

The gate of the Field-effect transistor is equivalent to the base of the transistor, and the source and drain correspond to the emitter and collector of the transistor respectively. Place the multimeter in R × Measure the forward and reverse resistance between each two pins using two probes in 1k gear. When the forward and reverse resistances between two pins are equal and both are K Ω, then these two pins are drain D and source S (interchangeable), and the remaining pin is gate G. For a JFET with four pins, the other electrode is a shielding electrode (grounded in use).

2. Judgment gate

Touch one electrode of the diode with the black pen of the multimeter, and touch the other two electrodes with the red pen. If the resistance values measured twice are very small, it means they are positive resistance. The diode belongs to N-channel field-effect transistor, and the black probe is also connected to the grid.

The manufacturing process determines that the source and drain of Field-effect transistor are symmetrical and can be interchanged, which does not affect the normal operation of the circuit, so it is unnecessary to distinguish them. The resistance between the source and drain is approximately several thousand ohms.

Note that this method cannot be used to determine the grid of insulated grid Field-effect transistor. Because the input resistance of this type of diode is extremely high and the inter pole capacitance between the gate sources is very small, a small amount of charge can form a high voltage on the inter pole capacitance during measurement, which can easily damage the diode.

3. Estimating the amplification ability of Field-effect transistor

Set the multimeter to R × For gear 100, the red probe is connected to the source electrode S, and the black probe is connected to the drain electrode D, which is equivalent to adding 1.5V power supply voltage to the Field-effect transistor. At this point, the needle indicates the resistance value between the D-S poles. Then pinch the gate G with your fingers and apply the induced voltage from the human body as an input signal to the gate. Due to the amplification effect of the diode, both UDS and ID will change, which is equivalent to a change in the resistance between the D-S electrodes. A significant oscillation of the watch needle can be observed. If the swing of the watch needle is very small when pinching the gate by hand, it indicates that the amplification ability of the diode is weak; If the watch needle does not move, it indicates that the pipe has been damaged.

Since the 50Hz AC voltage sensed by the human body is high, and the working points of different FETs may be different when measuring with resistance gear, the meter needle may swing to the right or left when the grid is pinched by hand. A few pipes have reduced RDS, causing the watch needle to swing to the right, while most pipes have increased RDS, causing the watch needle to swing to the left. Regardless of the swing direction of the watch needle, as long as there is a clear swing, it indicates that the diode has amplification ability.

This method is also applicable to measuring MOS transistor. In order to protect MOS Field-effect transistor, it is necessary to hold the insulating handle of the Screwdriver with your hand and touch the grid with a metal rod to prevent the induced charge of the human body from directly adding to the grid and damaging the diode.

After each measurement of the MOS transistor, a small amount of charge will be charged to the G-S junction capacitor, establishing a voltage UGS. Then, the meter needle may not move during the measurement. At this point, a short circuit between the G-S electrodes is sufficient.