How to judge a good or a bad diode

How to judge a good or a bad diode

First, to qualitatively judge the quality of MOS-type FET, first use the multimeter R× 10kΩ block (built-in 9V or 15V battery), connect the negative pen (black) to the gate (G), and connect the positive pen (red) to the source (S). Charging between the gate and the source, the multimeter pointer is slightly deflected. Then use the multimeter R×1Ω block, connect the negative pen to the drain (D), the positive pen to the source (S), if the multimeter indicator value is a few ohms, it means that the FET is good.

    Second, qualitatively judge the electrode of the junction FET to dial the multimeter to R×100 gear, the red meter pen is arbitrarily connected to one leg, and the black pen is connected to another leg diode, so that the third foot is suspended. If a slight swing of the hand is observed, the third foot is a gate. To obtain a more obvious observation effect, you can also use the human body to approach or touch the hanging foot with your fingers, as long as you see the hand deflected significantly, it means that the hanging foot is the gate, and the other two feet are the source and drain.

    Reason for judgment: The input resistance of the JFET is greater than 100MΩ, and the transconductance is very high, and when the gate is open, the spatial electromagnetic field is easy to induce a voltage signal on the gate, so that the diode tends to be cut-off, or tends to be on. If the human induced voltage is applied directly to the gate, the above phenomenon will be more obvious due to the strong input interference signal. If the needle is greatly deflected to the left, it means that the diode tends to be cut-off, the drain-source resistance RDS increases, and the drain-source current decreases IDS. Conversely, the needle is greatly deflected to the right, indicating that the diode tends to be conductive, RDS↓, IDS↑. However, the direction in which the needle is deflected depends on the polarity of the induced voltage (forward or reverse voltage) and the operating point of the diode.