In the actual use of MOS tubes, MOS tubes can be used to amplify current and also as electronic switches.

So how does the MOS tube control the direction of current?

As a voltage-controlled device, the MOS tube controls the current at the output drain by applying a voltage to the input gate, that is, the current is controlled by applying a voltage to the G pole.

Taking the use of a voltage comparator (such as LM358) to control a MOS tube as an example, a control voltage (connected to the comparator's non-inverting input) and a reference voltage (connected to the comparator's inverting input) are simultaneously sent to the voltage comparator (the comparator power supply is connected to positive 12V and ground), and the output of the comparator is pulled up by a 5.1K resistor and connected to the G pole.

 In the initial stage, if the control voltage is higher than the reference voltage, the G pole can basically be added to 12V, which can make the MOS tube turn on quickly and output current. Since the current is very small at the beginning, the control voltage is much higher than the reference voltage.

As the current increases and gradually reaches a certain value, the reference voltage will rise rapidly. When it approaches and exceeds the control voltage, the comparator outputs a low level (close to 0V), which cuts off the MOS tube and reduces the current. After the current decreases, the reference voltage drops again, the tube turns on again, and the current increases again. This cycle repeats to achieve dynamic control of the current and control the current direction from drain to source or from source to drain as specified by the MOS tube type (N channel or P channel).