Capacitor Charging Time Calculator (RC Time Constant)
Calculate capacitor charging or discharging time using the RC time constant formula. Find voltage at any time during the exponential curve.
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RC Time Constant Guide
What is the RC Time Constant?
When a capacitor charges through a resistor (or discharges), the voltage doesn't change linearly — it follows an exponential curve. The time constant τ (tau) = R × C. Where: R = resistance in ohms. C = capacitance in farads. τ = time in seconds. Example: 10kΩ resistor + 100µF capacitor. τ = 10,000 × 0.0001 = 1 second. Charging from 0V toward V_supply: V(t) = V_supply × (1 - e^(-t/τ)). Discharging from V_initial toward 0: V(t) = V_initial × e^(-t/τ). Key benchmarks: 1τ: 63.2% charged. 2τ: 86.5%.
What's the key thing to understand about Practical Time Constants?
Component combinations and their τ: 1kΩ + 1nF: τ = 1µs. Radio frequency, signal filtering. 10kΩ + 1µF: τ = 10ms. Audio coupling, simple debouncing. 100kΩ + 10µF: τ = 1s. LED fade circuits, simple timers. 1MΩ + 100µF: τ = 100s. Long-period timers, slow envelope generators. 10MΩ + 1F: τ = 10,000,000s = ~115 days. Super-capacitor backup applications. Time to 'fully charge' (5τ): nanoseconds for RF circuits. Tens of microseconds for switching power supplies. Tens of milliseconds for audio circuits.
What's the key thing to understand about Filter Behaviour?
RC circuits naturally filter signals. Low-pass filter: cutoff frequency f_c = 1/(2π·R·C). Frequencies below f_c pass through. Frequencies above are attenuated. High-pass filter: same formula. Frequencies above f_c pass. Below are attenuated. Audio applications: high-pass to remove DC offset, low-frequency rumble. Low-pass to smooth digital PWM into analog signals. Power supply applications: large bulk capacitors (10,000µF+) smooth rectified mains. Time constant in seconds — slow ripple voltage.
What should I know about Practical Applications?
Switch debouncing: mechanical switches bounce 1-10ms when pressed. RC circuit (10kΩ + 0.1µF, τ=1ms) plus Schmitt trigger eliminates bouncing. Camera flash: large electrolytic capacitor (200-500µF) charges through low resistance, then discharges through flash tube in <1ms. Coupling: AC signals pass through, DC blocked. Used in audio amplifiers between stages. Timers: 555 timer chip uses RC for time delays — calculator gives the time constant. 1MΩ + 1µF = 1 second delay typical. Camera shutter timing circuits historically used RC delays in a similar way, before digital timing circuits became standard.