On the PC mainboard, there are many mosfet lights used to control the source of the CPU, chipset, and ram source. When the mosfet is dead, it will lose the voltage of the circuit, even worse, making the mainboard not power on, or trigger the fan to rotate a few times and then turn off.
Brief introduction about mosfet
Mosfet is a field transistor that works on magnetic field effect to generate electric current. They are made from N-P-N semiconductors for reverse mosfets and P-N-P for forward mosfets.
Mosfet forward : When we put the black rod at D, the red rod at S, the analog clock X1 hugs most of the needle.
Reverse mosfet is used on PC mainboard: When placing red rod in D, black rod in S, the clock will go up.
Mosfet is composed of 3 poles:
Gate pole (Gate): pole G.
Base pole (Drain): pole D.
Power pole (Source): pole S.
Characteristics: Poles G and D are always insulated, poles G and S are always insulated. The measurement case between these poles is that the mosfet is shorted or detected. To determine the legs of the mosfet, you rotate the 2 mosfet legs towards your body. On the left is pin G, on the right is pin S and above is pin D.
How to measure mosfet includes 2 ways: short-circuit measurement and check the quality of mosfet mainboard lights
1. Short-circuit measurement: When measuring short-circuit, we measure it directly on the mainboard by adjusting the analog clock to the X1 ohm scale (or the diode scale for digital clocks).
The red rod is placed at D, the black rod is placed at S.
Invert the measuring rod, i.e. red rod at S, black rod at D.
+ If both directions are measured: One clockwise is up a bit, and the other is up to the end of the scale => The light is not shorted on D – S.
+ On the contrary, when measuring in both directions, up to 0 ohms, the D-S lamp is short.
2. Check the quality of the Mosfet lamp: To measure the quality of the mosfet, you must completely remove the mosfet and put it on a piece of paper (insulation) to measure, or remove the G and S pins from the mainboard.
You measure 6-way measuring stick, only 1 clockwise up is good mosfet lamp.
The black rod is placed at D, the red rod is placed at S.
Black rod in D, red rod in G.
Reverse the measuring rod for red rod at D, black rod at S => The meter only goes up when placed here.
Red stick in D, black stick in G.
Red stick at G, black stick at S.
Red stick in S, black stick in G.
The following cases are damaged Mosfet lamps.
When measuring between G and S, there is a low impedance close to 0 ohms => The lamp is detected or shorted G – S.
Measured between G and D, there is a low impedance => It is a detected light or a short G – D.
Positive charge for G to turn on the light. Charge the positive by leaving the red rod at S, the black rod at G. Then keep the red rod at S, put the black rod on the D leg. If the clock needle goes to 0 ohms, the light is good. If the light does not turn on, the D – S leg is broken.
Give G a tone to turn off the light. Load the negative by leaving the black rod at D, the red rod at G. Then keep the black rod at D, put the red rod on the S pin. If the clock hand is still up to 0 ohms, it is a short D – S, otherwise the clock will not go up. good.
In the case of short-circuited measurement, or detected, you can replace the equivalent mosfet. For example, mosfet in the CPU source circuit, then replace the mosfet by getting from another main CPU source to replace. There are also some mosfets like: LM1117, APL1085, FAN1084, L1117… is a voltage stabilizer ic that generates a voltage of 1.5v, 3v3 if not clear, you can check the pinout with the datasheet
That’s it, you work hard to follow for a while and then remember. Because my post is too detailed, it looks a bit confusing. But actually the measurement is very fast, it takes about 15 seconds to finish ah. Wishing you success.
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