Definition
The time, in seconds, required for the voltage across a charging capacitor in a resistor-capacitor (RC) circuit to reach approximately 63.2% of the applied voltage, or for a discharging capacitor to fall to approximately 36.8% of its starting voltage. It equals the resistance in ohms multiplied by the capacitance in farads (T = R × C).
Plain English
A number that tells you how quickly a capacitor charges up or drains down through a given resistance. After one time constant, the capacitor is about two-thirds of the way to fully charged.
Context Anchor
Seen in aircraft electrical and electronics training when discussing timing circuits, filters, and how quickly a stored electrical charge changes.
Derivation
Capacitive comes from the Latin capacitas, meaning capacity to hold — a capacitor holds an electric charge. Time constant means a fixed time value that characterises how the circuit behaves. Together: the fixed time value that describes how a charge-holding component fills or empties.
Why Pilots Care
Capacitors appear in radios, instruments, and ignition timing circuits. When something charges or discharges too slowly or too quickly, it often points back to the time constant of the circuit, which helps technicians diagnose faults.
Analogy
Think of filling a bucket through a narrow hose. A bigger bucket (more capacitance) or a narrower hose (more resistance) makes it take longer to fill. The time constant is a single number that captures both effects.
Grounding Statement
Picture a small reservoir filling through a narrow pipe: the time to reach most of its level depends on pipe size and reservoir size.
Intuition Check
Do not read “time constant” as the total time required for the capacitor to become fully charged or fully discharged. One time constant means it has made most of the change, not all of it.
Example Sentence 1
The technician calculated the capacitive time constant to confirm the timing circuit was discharging at the correct rate.
Example Sentence 2
A higher capacitive time constant slows how quickly the circuit reacts to voltage changes.