RC Discharge Calculator

When a charged capacitor is connected across a resistor, it discharges exponentially. The voltage drops according to V(t) = V₀ × e^(−t/τ), where τ = RC is the same time constant used for charging. The key difference is direction: charging approaches Vcc from below, while discharging approaches zero from above.

After one time constant (1τ), the capacitor retains 36.8% of its initial voltage — it has lost 63.2%. After 5τ, only 0.7% remains, which is effectively zero for most practical purposes. The discharge rate depends entirely on R and C: a larger resistor or capacitor means slower discharge.

Discharge timing is fundamental to many circuits. Monostable timers (like the 555) use RC discharge to set pulse width. Touch sensors detect discharge rate changes caused by finger capacitance. Sample-and-hold circuits rely on slow discharge through high-impedance buffers to maintain a voltage reading.

Key Points

• Discharge follows V(t) = V₀ × e^(−t/τ) — exponential decay
• After 1τ: 36.8% remains, after 5τ: 0.7% remains
• Same time constant τ = RC as charging, but curve is inverted
• Larger R or C → slower discharge

Applications

• Timer and delay circuits (555 monostable)
• Touch and proximity sensing
• Sample-and-hold circuits
• Power supply soft-start and sequencing
• Debouncing circuits