场效应管好坏判断及替换原则（The principle of good and bad judgment and replacement of field effect transistor）.doc

场效应管好坏判断及替换原则（The principle of good and bad judgment and replacement of field effect transistor） Judge the quality of field effect transistor and triode with Multimeter First, the quality of MOS type FET is qualitatively judged First use the multimeter R * 10K block (built-in 9V or 15V battery), the negative form of the grid (black) (G), is a pen (red) ground source (S). Charging between the grid and the source is a slight deflection of the pointer at this time. Then use multimeter R * 1 Omega block, the negative probe connected to the drain (D), is the pen connected source (S), multimeter indication for a few ohms, the FET is good. Two. Qualitatively determine the electrode of junction type FET The multimeter dial to R * 100 files, the red pen arbitrarily pick one pin, the black pen is connected to another pin, the third is suspended. If the hands have a slight swing, prove the third gate. To observe the effect is more obvious, also can use the body near or with fingers dangling feet, just to see the hands to make substantial deflection, that is left foot gate, the remaining two feet are the source and drain. Judge reason: JFET input resistance is greater than 100M, and transconductance is very high, when the grid open, the space electromagnetic field is easy to sense voltage signal on the gate, so that the pipe tends to cut-off, or tends to conduction. If the induction voltage is directly added to the grid, the phenomenon will be more obvious due to the strong input interference signal. As to the left hand greatly deflection, it means that the pipe tends to stop, increase the drain source resistance between RDS and drain source current decreases IDS. On the other hand, to the right hand large deflection, that pipe tends to conduction, RDS down, IDS up. But the hand toward what direction should be considered, the polarity of the induced voltage (voltage or reverse voltage) and the working point of the tube. Matters needing attention： (1) test shows that, when the h