Qualitatively judge the quality of field-effect transistors and transistors
Sep 24,2024
Qualitatively judge the quality of field-effect transistors and transistors
1、 Qualitative assessment of the quality of field-effect transistors
First, use the multimeter R × 10k Ω (with a built-in 15V battery), connect the negative probe (black) to the gate (G), and the positive probe (red) to the source (S). Charging between the gate and source causes a slight deviation in the multimeter pointer. Switch to the R × 1 Ω range of the multimeter again, connect the negative probe to the drain (D) and the positive probe to the source (S). If the multimeter reading is a few ohms, it indicates that the field-effect transistor is good.
2、 Determine the electrodes of junction field-effect transistors
Set the multimeter to the R × 100 position, connect the red probe to any pin, and connect the black probe to the other pin, leaving the third pin hanging. If there is a slight oscillation of the pointer, it proves that the third pin is the grid. To achieve a more obvious observation effect, one can also use the human body to approach or touch the suspended foot with fingers. As long as the pointer is significantly deflected, it indicates that the suspended foot is the gate, and the other two feet are the source and drain, respectively.
Reason for judgment: The input resistance of JFET is greater than 100M Ω, and the transconductance is very high. When the gate is open, the spatial electromagnetic field can easily induce a voltage signal on the gate, causing the tube to tend to turn off or turn on. If the human induced voltage is directly applied to the gate, the above phenomenon will be more pronounced due to strong input interference signals. If the pointer deviates significantly to the left, it means that the tube tends to cut off, the resistance RDS between the drain and source increases, and the current IDS between the drain and source decreases. On the contrary, if the needle deviates significantly to the right, it indicates that the tube is trending towards conduction, RDS ↓, IDS ↑. But the direction in which the pointer is deflected depends on the polarity of the induced voltage (forward or reverse voltage) and the operating point of the tube.
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