AC1 Fundamentals Unit 8 – Time Constants
Exercise 1 – RC Time Constants
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine the time constant of an
RC circuit by using calculated and measured values. You will verify your results with an
oscilloscope.
EXERCISE DISCUSSION
- The time constant (τ) of a circuit is the amount of time required for current (inductive circuit)
or voltage (capacitive circuit) to reach 63% of its maximum value. - An RC circuit time constant (in seconds) is the product of the circuit resistance (in ohms) and
capacitance (in farads). - An RL circuit time constant is the division of the circuit resistance divided into the
inductance (in henries). - Five time constants are required for the component to reach 99% maximum.
- There are no time constants for a purely resistive circuit because the circuit reacts to changes
in voltage or current instantaneously. - Universal time constant charts are used to determine the amount of voltage across a capacitor
or current through an inductor when the time constant is known. - Charging and discharging curves are equal and opposite.