EE3300/EE5300 Electronics Applications
Week 4 Practical

Written by Luke McCarthy and A/Prof Bronson Philippa
College of Science and Engineering, James Cook University
Last updated 24 March 2025

Equipment

  • 2x LM358 dual op-amps.
  • 1x Motorised potentiometer (either pre-built 50 kΩ, or custom built 10 kΩ).
  • 1x 10 kΩ potentiometer.
  • 2x Breadboards (Likely you will need the extra space as today we build one large circuit).
  • 1x 2N3904 NPN BJT.
  • 1x 2N3906 PNP BJT.
  • 1x MJE2955 PNP Power BJT.
  • 1x MJE3055 NPN Power BJT.
  • Various capacitors.
  • Various resistors.

Instructions to students

  • Work individually on these activities.
  • Focus on neat circuit breadboarding.
  • Include power supply decoupling capacitors.

Exercise 1: Constructing a Motor Driver Circuit

Motivation

This week we explored different classes of amplifier and discussed some of their advantages and disadvantages. In Exercise 1 you will use a Class B amplifier (push-pull stage) to drive a motorised potentiometer.

Your task

Your task is to characterise a motorised potentiometer load, then construct and interpret a typical motor-driver circuit arrangement.

Procedure

  1. Familiarise yourself with your motorised potentiometer. Figure 1 illustrates the pinout configurations of two motorised potentiometer designs. (You will be given one of these).

Figure 1
Figure 1:

Pinout diagram for two motorised potentiometer designs.

Zoom:

  1. Measure the resistance across the outer terminals of the potentiometer section and determine which pin will correspond to the wiper.

    Use your multi-meter to measure the overall resistance and from the wiper to one end of the potentiometer, as you rotate the motor arm manually, to verify the range of the outer legs over which the potentiometer can sweep.

  2. If not already present, attach a small piece of tape to the arm of the motorised potentiometer. This will act as a flag and make it easier for you to see the motor rotating and in what direction.

  3. Next, rotate the arm of the motorised potentiometer to the approximate middle of its range of motion. You can do this by rotating all the way clockwise/anticlockwise and then moving back the other way slightly.

    Begin with a small DC voltage across the motor pins. Slowly increase the voltage magnitude until the motor starts turning. What is the minimum applied voltage needed for the motor to turn and move the potentiometer wiper? What happens when you reverse the input polarity (swap the DC connections around)?

  4. Construct the circuit shown in Figure 2. Note: The motorised potentiometer draws the most current of any load we’ve covered thus far in the unit. This means you may need to increase the current limit dial on your DC power supply to allow more current to flow from the supply. We will discuss this current in more detail in the next section, but for now you should choose a value that will protect your equipment and allow your motor to turn.

Figure 2
Figure 2:

Motor driver circuit.

If you power on the circuit without Vin connected, the output signal may swing to one of the rail voltages (or slightly less e.g. +/- 13V). Why might the output swing high (or low) with the non-inverting input floated?

Zoom:

  1. Connect a 1 Vpp, 0.5 Hz square wave to the circuit (non-inverting input) and observe the motorised potentiometer. What happens to the arm of the motorised potentiometer? What happens as you increase the signal amplitude? What about if you increase the frequency?

Interpretation

To be marked off, you will need to be able to discuss these questions with your tutor:

  1. Based on your findings, choose a magnitude of input signal that you believe is suitable for driving the motorised potentiometer.

  2. Why do we use power transistors for our push-pull stage, rather than the 2N2904/6 models that we’ve used in previous practicals?

  3. What is the role of the 10 Ω + 100 nF branch in parallel with the circuit output?

Exercise 2: Designing a Current Limiting Protection Arrangement

Motivation

Another big part of robust circuit design is using hardware designs to enforce circuit protection. In this activity, we will design circuitry to limit the possible output current, for example, to protect the output transistors in the event of a short circuit at the output.

Your task

Modify your circuit design to implement current limiting protection.

Procedure

  1. Consider the current limiter circuit design in Figure 3.

Figure 3
Figure 3:

Current limiter circuit.

Zoom:

  1. Answer the following questions:

    1. What are the roles of the 2N3904/6 transistors?

    2. The 2N3904/6 are small signal transistors. Why is it appropriate to use small signal transistors for this circuit, rather than the power transistors of the push-pull stage?

    3. Determine a current limit you would like to set for your protection circuit. This must permit enough current for your motor to turn (test this) but remain below the rated limits for your circuit components.

  2. Use your knowledge of the base-emitter voltage of the small-signal transistors and your chosen current limit to choose suitable resistor values.

  3. Build the circuit onto the output stage of your motor driver circuit but do not turn it on yet.

  4. BE VERY CAREFUL WITH THIS STEP AS WE WILL BE CREATING A FAULT CONDITION. As a secondary line of protection, set a current limit on your bench power supply (slightly higher than your own circuit’s limit). Test your current limiting circuit by short-circuiting the terminals of your motor together. DO THIS WHILE THE CIRCUIT IS TURNED OFF BEFORE TURNING THE POWER ON.

  5. Read the current measurement off the power supply screen. Does this match your designed current limit? Note: You must ensure that your designed current limit is below the current limit of the bench supply (or the supply will block current flow before your current limiter activates).

Exercise 3: Constructing a Proportional Position Controller

Motivation

Now that we have a robust, protected motor driver output stage, we will build a simple position controller circuit. Based on your past practicals and Exercise 1, you should have noticed that an applied voltage across the DC motor turns the arm with a speed proportional to the applied voltage magnitude. In this Exercise, we will use a control arrangement to synchronise a position input with the position of the motorised potentiometer output.

If you’ve had some exposure to control systems, you may be familiar with the concept of a PID (Proportional, Integral, Derivative) controller. We will build just the proportional section today.

Your task

Your task is to construct a proportional motor controller with feedback from the motorised potentiometer.

Part 1: Understanding the Proportional Controller

  1. Construct the circuit in Figure 4. When choosing your breadboard layout, consider that you will soon need to connect the output of your controller stage to the input of your previously built motor driver circuit. Note: Because we are using the LM358 dual op-amps, you should build this circuit using only one IC.

Figure 4
Figure 4:

Proportional controller circuit.

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  1. Based on your understanding of the circuit, and your test results when applying common mode and differential mode inputs (as shown in Figure 4), answer the following questions:

    1. What is the role of the left-side op-amp with the 100 kΩ resistor configuration?

    2. What is the role of the right-side op-amp with the 220 kΩ feedback?

Part 2: Combining with the Motor Driver

  1. Connect the controller output to the input of your motor driver circuit and setup the resistor configurations at the input and output as shown in Figure 5. Notice that we will use a 10k potentiometer at our input to set the desired position of our motorised potentiometer output.

    Note: You will need to vary the values of the output series resistors (that go from the motorised potentiometer to the rails) based on the resistance of your motorised potentiometer. As depicted in Figure 5, a 50k potentiometer should use ~22k resistors, whereas a 10k potentiometer should use 4.7k resistors (about 5 times less).

Figure 5
Figure 5:

Proportional controller connected to motor driver.

Zoom:

  1. In the configuration of Figure 5, with the inverting terminal 100k resistor connected to ground, vary the position of the non-inverting input with the 10k input potentiometer. Use two channels of your oscilloscope to observe the input/output behaviours as you move the input potentiometer. You may gain further insight from measuring the output of the motorised potentiometer wiper and then the voltage across the motor.

  2. Answer the following questions.

    1. What do you notice about the input and output behaviours?

    2. Is your circuit effectively behaving as a position controller?

  3. Now, change the feedback resistor in the gain stage from 220k to the much smaller 22k resistor, as depicted in the red box in Figure 6. Disconnect the inverting input 100k resistor from ground and instead connect to a feedback path from the wiper of the motorised potentiometer, again depicted in Figure 6.

Figure 6
Figure 6:

Proportional controller with feedback from motorised potentiometer.

Zoom:

  1. Vary the input potentiometer and observe the new behaviour of your motorised potentiometer.

Interpretation

  1. What do you notice about the movement of the motorised potentiometer arm before and after connecting your feedback path?
  2. Why is it important that we reduce the effective gain of the proportional controller before connecting our feedback path?

Conclusion

Your tutor will mark you off for completing this activity and being able to discuss the results. If you do not finish on time, you have one week to complete it. Bring your completed circuit or evidence of your work to a subsequent lab session for marking.

When you leave, make sure that the lab is just as neat or even neater than when you arrived.

Acknowledgements

This activity was adapted from Lab 10 in Learning the Art of Electronics by Hayes and Horowitz, with current limiting design based on Chapter 9 of The Art of Electronics by Horowitz and Hill.