PC/CP320 Physical Computing

Controlling Power (DC Motor)


We have various output transducers whose power can be controlled using various techniques. The pairing of an output transducer and a control circuit is not limited to the ones you will be investigating; although some pairings make more sense than others. The specific objectives for this year are:

Remember that any control element is used in a voltage divider with the load. This is true whether you are testing the device or using it normally. For testing purposes, you can simply replace the normal load with a fixed resistor of similar size and current rating.

control setup
Remember that you need to put diodes across the coils of inductive devices to avoid back EMFs. (Think about which way the diode needs to go.)



  1. Power MOSFET investigation

    Determine the MOSFET characteristics as follows:
    Power MOSFET (N channel) : IRF510

  2. DC motor investigation

    Look at your results from the previous lab where you used the DC motor. What voltage was required to make it run?
    Test this by using the variable supply and using it to run the motor.

    Remember to put a Zener diode across the motor to avoid induced EMFs. (Think about which way the diode needs to go.)

  3. Driving a DC motor using an op amp

    Wire up an op amp as a buffer. Feed the voltage from the variable supply above into the buffer and use the output of the op amp to try and run the motor. Does it work?
    If it doesn't work, observe the voltages at the input and output of the buffer. How should they be related? Is that what actually happens?

    What is the limitation of the op amp that prevents it from running the motor directly?

  4. Driving a DC motor using an op amp and MOSFET

    Now instead of using the op amp to run the motor directly, feed it into the gate of the MOSFET in the left configuration from above to run the DC motor.
    mosfet control element

    Note: the order of the two elements in the voltage divider will make a difference in how easy it is to control power. Why does the left configuration allow a lower voltage for the control signal?

    How does this new circuit get around the op amp limitation you mentioned earlier?

    Demonstrate and explain your findings to the lab supervisor.



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