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What Is the Time Constant for a Circuit?

An electric circuit is a device made of interconnected electrical elements such as wires, resistors, capacitors and coils. Each of these components alters the flow of electricity in the circuit in one way or another. The time constant is a means of measuring how the flow of electricity is changed by certain components. Capacitors have one time constant and coils have a different one.

RC Time Constant
- A capacitor is made up out of two metal plates separated by either air or some other non-conductive substance. When connected to a battery, the battery charges one of the plates in a positive manner and the other plate in a negative manner. The resistive-capacitive, or RC, time constant is a measure of how long it takes to either charge or discharge a capacitor by 63.2 percent. This length of time depends on both the capacitance of the capacitor and the resistance of the circuit. The RC time constant is equal to the resistance multiplied by the capacitance.
RL Time Constant
- A coil, also known as an inductor, is a spiral of wire. It is usually wound around a length of metal such as iron. When a current passes through the coil, it creates a magnetic field around the coil. If the current changes, this magnetic field also changes. The resistive-inductive, or RL, time constant is a measure of how rapidly current in the coil builds up or dies away by 63.2 percent. This length of time depends not only on the inductance of the coil but also on the resistance of the circuit. The RL time constant is equal to the inductance divided by the resistance.
In Filters
- Both resistive-capacitive and resistive-inductive circuits are often used to filter out noise from electric signals such as radios or electrocardiograms. The time constants are used to determine what values of resistance, capacitance and/or inductance are needed for effective filtering. If the time constant is made too short or long relative to the length of the signal, then the filter will not work. It may even distort the shape of signal and add more noise.
General Rule
- The time constant of an RC or RL circuit can be used to predict how long it will take the current and voltage in a circuit to attain their maximum values after the circuit has been switched on. As a rule of thumb, it takes a length of time equal to five times the value of the time constant. This rule applies only if the power supplied to the circuit is steady.

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