Enter the resistance and capacitance to find the time constant for a Resistor-Capacitor (RC) circuit.
Enter the resistance and inductance to find the time constant for a Resistor-Inductor (RL) circuit.
Results
Time Constant (τ)
100.00 ms
Full Charge/Discharge (5τ)
500.00 ms
Charging & Discharging Curve
This chart visualizes how the voltage (for RC circuits) or current (for RL circuits) changes over time. The horizontal axis represents time in multiples of the calculated time constant (τ). Notice how the curve reaches approximately 63.2% of its final value at 1τ.
Understanding the Time Constant
What is a Time Constant?
The time constant, denoted by the Greek letter tau (τ), is a measure of how quickly a circuit responds to a change in voltage or current. In simple terms, it represents the time required for the voltage or current in a charging or discharging circuit to reach approximately 63.2% of the difference between its initial and final values.
The Formulas
The time constant is calculated differently for RC and RL circuits:
- For a Resistor-Capacitor (RC) circuit:
τ = R × C - For a Resistor-Inductor (RL) circuit:
τ = L / R
Where R is resistance in Ohms (Ω), C is capacitance in Farads (F), and L is inductance in Henrys (H).
Why 5τ for Full Charge/Discharge?
While theoretically the circuit never reaches 100% of its final value, it gets very close. After five time constants (5τ), the circuit has reached over 99.3% of its final value. This is considered fully charged or discharged for most practical engineering purposes. Here's a quick breakdown:
- After 1τ: ~63.2% complete
- After 2τ: ~86.5% complete
- After 3τ: ~95.0% complete
- After 4τ: ~98.2% complete
- After 5τ: ~99.3% complete
Frequently Asked Questions
What does a small time constant mean?
A small time constant (τ) indicates a fast-responding circuit. This means the capacitor or inductor will charge and discharge very quickly. This is desirable in high-frequency applications.
What does a large time constant mean?
A large time constant (τ) indicates a slow-responding circuit. The capacitor or inductor takes a longer time to charge or discharge. This is useful for creating timing delays or in filtering out high-frequency noise.
Where are time constants used in real life?
Time constants are a fundamental concept in electronics. They are used in timing circuits (like in a blinking LED circuit), filters in audio equipment to separate different frequencies, and in power supplies to smooth out voltage ripples.
