MOSFET Lab

Purposes:

1. To investigate two circuits that can control the voltage across an electric motor.

2. To gain some insight into how DC brushed motors behave.

3. To control the power supplied to a motor with a MOSFET.

4. To regulate the behavior of a motor with a controller.

Materials:

N-type power MOSFET (IRF510 or NTE2382), 10kohm potentiometer, 2.2kohm resistor, 22-AWG wire, oscilloscope, 1N4007 rectifier 

Procedures:

Part 1: 

First we were required to construct the circuit shown in the schematic diagram shown below and to connect it to an oscillator. The second photo below shows the circuit itself. 

To begin, we were required to put the oscilloscope into storage mode and to set it for a single sweep. We made sure to set the vertical scale to 10 V/div as well as the time scale to 0.1mS/div. We adjusted the trigger level until we successfully started the motor. We indeed heard some off and on cycles. Next we attempted to neutralize the transients by putting a 1N4007 rectifier diode in parallel with the motor. We turned on the motor to investigate the effect of the diode onto the motor's cyclical sound. We did notice a longer cycle. 

Next we were required to construct the circuit in the diagram below which included the load resistor and the diode.  The photos below show both the schematic and actual circuits.

We noticed that the motor turned on at a gate voltage of 7.23 V. Next, we slowly increased Vgs from an initial value of zero. We noticed that with the increase in voltage, the motor begin spinning faster with the cycles being shorter. So I would definitely say that we were controlling the motor voltage well.  We measured the internal resistance of the multimeter and noticed that it had a value of 0.5 ohm. Then we were instructed to disconnect the motor from the circuit and to connect it onto the oscilloscope. We measured an induced back voltage of 8 V by hand.

Part 2: 

Now for the second part, we were to control the average voltage applied to the motor using pulse-width modulation.   

We were now required to replace the potentiometer with a 0-10 V square wave function generator and we set it to 10 kHz. We then used an oscilloscope to display the waveform of the voltage of the motor. Here is what we observed:

We now varied the duty cycle wave to 100% and noticed that the motor sped faster. The following photo shows proof that it sped faster. Notice how the cycles became shorter.

When then changed the DC source knob to 30% of maximum power and we made the following observation in the photo below:

Conclusion:

We considered this lab to be a success since we successfully completed all of our objectives. In all honesty, I thought that this was one of the most boring labs we've had all semester because it was based entirely on just observations without any data nor follow-up theoretical computations.