AC1 Fundamentals Unit 8 – Time Constants
Exercise 2 – RC and RL Wave Shapes
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to demonstrate the effects of time
constants on RC and RL circuits by using square waves as the applied ac waveforms. You will
verify your results with a universal time constant chart.
EXERCISE DISCUSSION
- A square wave input signal is used to constantly charge and discharge the capacitor in a
simple RC circuit. - The frequency of the square wave has been selected to provide a long enough period for the
capacitor to fully charge and discharge. - Current flow is maximum when the charge on the capacitor is zero. Current flow is minimum
when the capacitor is fully charged. The current flow is in the opposite direction when the
capacitor discharges. - A sawtooth waveform is produced when the circuit resistance is increased. The larger
resistance increases the time constant. The period of the input square wave is now too short
for the capacitor to fully charge or discharge. - Since the time constant of a simple RL circuit is much smaller than that of an RC circuit, a
square wave input at a higher frequency (shorter period) can be used. - Maximum voltage across an inductor and minimum current flow result when an inductor
begins to charge. Since inductors oppose changes in current flow, the same conditions are
present when an inductor begins to discharge. - Current flow decreases at a rate governed by the RL time constant.
- Decreasing inductance shortens the charge and discharge time in relation to the period of the
applied square wave. This condition causes positive voltage spikes on the rising edge and
negative voltage spikes on the falling edge of the applied signal.