The purpose of this lab is to investigate circuitry which is useful for constructing solid-state controllers for permanent-magnet DC motors.

Flyback Voltage.

Some interesting things happen when an inductive load such as a motor is switched on and off suddenly. To see some of these effects, we will use the single-sweep and storage capabilities of our oscilloscopes to capture the transients associated with turning a small motor on and off. Construct the circuit shown below and connect it to the oscilloscope as indicated.

Put the oscilloscope into storage mode and set it for single sweep. Set the vertical scale to 10 V/div and the time scale to 0.1 mS/div. Use DC-coupled external triggering and play with the trigger level until throwing the switch reliably starts the motor and triggers the 'scope. You will see a series of fast on-off cycles associated with closing the switch: these are caused by contact bounce in the mechanical switch. What happens to the voltage across the motor as a result of this contact bounce? Can you see similar transients associated with turning the motor off? Why or why not? Print one of the more spectacular transients for your notebook. Describe at least one way that such transients could damage the switch, especially if it were a solid-state switch.

To combat the negative transients, put a 1N4007 rectifier diode in parallel with the motor (it should be reverse biased when the power is on). Repeat the observations made above to see the effect of the diode.

A MOSFET On/Off Switch.

Use an IRF711 N-channel MOSFET to build the following circuit. "Float" the power supply so that the oscilloscope ground may connected to any arbitrary point within the circuit.

At what gate voltage will the switch turn-on? Make a plot of the drain-source voltage as a function of the gate-source voltage during steady-state operation. What would the drain-source voltage be ideally? What range of gate-source voltages will produce "reasonable" values?

Replace the potentiometer with a TTL-level ( 0-5 volts) square-wave generator set at 10 kHz. Use the oscilloscope to display the waveform of the motor voltage. Reduce the square-wave frequency gradually to a few hertz and observe the motor response. Explain what happens.

Pulse-Width Modulation (PWM) Controller.

We can vary the average voltage seen by the motor by controlling the duty cycle of the square-wave applied to the MOSFET gate. An effective PWM circuit can be made from a 555 timer chip as shown below. This circuit is a monostable which produces an output pulse whenever the trigger input falls below 1.6 V. The duration of the output pulse is T = 1.1RC. We will play with the 555 timer in more detail in a later lab.

Use the 10 kHz TTL square-wave as your trigger voltage. To achieve duty cycles ranging up to 90%, we will need output pulse durations of up to 90 microseconds. Choose C so that a 10 kilohm pot can be used to vary the duty cycle within this limit.

Construct the circuit. Adjust the pot until you get a 50% duty cycle, then measure the resistance of the pot. Does the value match your theoretical expectations? Use the circuit to drive the gate of the MOSFET switch. Measure the motor voltage with a multimeter. You should be able to smoothly vary the motor voltage by adjusting the duty cycle via the pot.