Lab 13: MOSFET

Introduction:
The purpose of this experiment is to use the MOSFET to control the voltage across the DC motor. The speed of the motor will be proportional to the voltage across its terminals. Moreover, we will learn how to control the power supplied to a motor with a MOSFET, and how to regulate the behavior of the motor with a controller.

Procedure:
Part 1: Connect the motor to the MOSFET and the potentiometer as the diagram shown below:

Obtain the parts and build the circuit:

As we observed, the motor turn-on at 3.9 V.

When we turning the pot, we changing the voltage across the motor and the MOSFET. The pot  is the part which control the voltage across the MOSFET and the motor. The MOSFET is the part which control the current flow through the source and how much voltage can be drained out based on the source.

Slowly increase V_GS and try to make the motor shaft at approximately once per second; we can see that it is extremely hard  to do. Furthermore, the speed of the motor relate linearly to the V_GS. The speed of the motor realate linearly to the V_GS because the motor is directly connected to the drain. When the V_GS increases to about 5-6V, the resistance between the source and the drain  is about 1/2 ohms. Thus, the current and the voltage delivered to the motor increase.

Part 2: PWM Chopper MOSFET motor control.

In this part, we are controlling the motor using a technique called Pulse- Width Modulation (PWM).

Replace the pot in the previous part with square-wave generator set at 10KHz. Use the oscilloscope to displace the wareform of the moter voltage. 

Reduce the  square-wave frequency gradually to a few Hert, the motor turns on and off oscillating depend on the amplitude of the square-wave.