Definition
The time, in seconds, required for the voltage across a capacitor in a resistor-capacitor (RC) circuit to rise to approximately 63.2% of its final value when charging, or to fall to 36.8% of its starting value when discharging. It is calculated by multiplying the resistance (R, in ohms) by the capacitance (C, in farads), and is represented by the Greek letter tau (τ).
Plain English
A number that tells you how quickly a capacitor in a circuit will charge up or discharge through a resistor. The bigger the resistor or the bigger the capacitor, the longer it takes.
Context Anchor
Seen in aircraft electrical and avionics discussions, especially when describing filters, timing circuits, and circuits that smooth or delay changing signals.
Derivation
RC simply stands for the two components involved: the Resistor and the Capacitor. 'Time constant' means a fixed value, specific to that circuit, that describes how its behavior changes over time. The combination tells you it is a timing value built from those two parts.
Why Pilots Care
Technicians use it to diagnose charging rates, filter response, and timing behavior in aircraft power and avionics circuits.
Analogy
Think of filling a bucket through a narrow hose. A bigger bucket (more capacitance) or a narrower hose (more resistance) means it takes longer to fill. The RC time constant is the number that captures both of those factors in one value.
Grounding Statement
After one RC time constant, the circuit has made most of its voltage change, but it still needs more time to settle fully.
Intuition Check
Do not read constant as meaning the voltage stays the same. Here, constant means a fixed timing value that controls how fast the circuit changes.
Example Sentence 1
After replacing the capacitor in the timing circuit, the technician calculated the new RC time constant to confirm the delay would still meet the manufacturer's specification.
Example Sentence 2
A longer RC time constant in the filter slowed the voltage response during the preflight electrical check.