Pick your Circuit Type (RC or RL), enter your Resistance (R) alongside either Capacitance (C) or Inductance (L), and the Time Constant Calculator returns your Time Constant (τ) plus the Cutoff Frequency, 63.2% Charge Time, and 95% Charge Time — everything you need to understand how quickly your circuit responds.
Time Constant (τ)
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Cutoff Frequency
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63.2% Charge Time
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95% Charge Time
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The RC time constant (τ) is the product of resistance (R) and capacitance (C). It represents the time required for a capacitor to charge to 63.2% of its final voltage or discharge to 36.8% of its initial voltage.
For an RL circuit, the time constant τ = L/R, where L is inductance in henries and R is resistance in ohms. This represents the time for current to reach 63.2% of its final value.
The cutoff frequency fc = 1/(2πτ), where τ is the time constant. This frequency represents the -3dB point where the circuit's response is 70.7% of its maximum value.
Theoretically, a capacitor never fully charges, but it reaches 95% charge in 3τ, 99% in 5τ, and is considered fully charged after 5 time constants (99.3% charged).
No, the time constant τ is specifically the time to reach 63.2% charge. Full charging time is typically considered to be 5τ, when the capacitor reaches 99.3% of its final voltage.
For RC circuits: resistance in ohms (Ω) and capacitance in farads (F) gives τ in seconds. For RL circuits: inductance in henries (H) and resistance in ohms gives τ in seconds.
Yes, the time constant is the same for both filter types using the same R and C values. The difference is in the circuit configuration and which signal you measure, not the time constant itself.