PC/CP320 Physical Computing

Controlling Power

Objective

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.)

Equipment

The items to be controlled were covered in the output transducer lab and may include:

The items to be used for power control may include:

Procedure

Relay

image To test the power control device do not use a resistance substitution box as the load resistor. Use a high current resistor or a rheostat rated for sufficient current.

Remember to put diodes across the coils of inductive devices to avoid back EMFs. (And think about which way the diode needs to go.)

  1. Start by investigating the devices you will be using to do the power control. Do only the devices assigned for this lab. Each of the following device-specific links points to a series of questions and tasks that you would normally want to do in order to understand the device.

  2. Look at your results from the previous lab where you used the relay. What voltage was required to switch it?
    Test this by using the variable supply and using it to switch the relay.
    Remember to put a Zener diode across the coil to avoid induced EMFs. (Think about which way the diode needs to go.)

  3. Wire up an op amp as a buffer. Feed the voltage above into the buffer and use the output of the op amp to try and control the relay. 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 switching the relay directly?

  4. Design and implement a circuit to turn the relay on and off using the bjt.
    Note: the order of the two elements in the voltage divider will make a difference in how easy it is to control power. Which way will allow a lower voltage for the control signal?

  5. Now use your op amp buffer to show that the relay can be switched with this new circuit.

    How does this new circuit get around the op amp limitation you mentioned earlier?
    Demonstrate and explain your findings to the lab supervisor.


DC motor

  1. 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.
    Test this by using the variable supply and using it to switch the relay.
    Remember to put a Zener diode across the motor to avoid induced EMFs. (Think about which way the diode needs to go.)

  2. Use the op amp buffer from above. Feed the voltage above into the buffer and use the output of the op amp to try and control 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?

  3. Design and implement a circuit to run the DC motor using the IRF510 power MOSFET.
    Note: the order of the two elements in the voltage divider will make a difference in how easy it is to control power. Which way will allow a lower voltage for the control signal?

  4. Now use your op amp buffer to show that the motor can be controlled with this new circuit.

    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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