RC Time Constant Calculator
Calculate charge/discharge timing for resistor-capacitor circuits.
RC Time Constant Formulas
Understanding RC Time Constants
The time constant τ (tau) defines how quickly a capacitor charges or discharges through a resistor. After one time constant, the capacitor reaches 63.2% of its final voltage. After 5τ, it's considered fully charged at 99.3%.
| Time | % Charged | % Discharged | Typical Use |
|---|---|---|---|
| 1τ | 63.2% | 36.8% | Timing threshold |
| 2τ | 86.5% | 13.5% | Fast response |
| 3τ | 95.0% | 5.0% | Settling time |
| 4τ | 98.2% | 1.8% | High precision |
| 5τ | 99.3% | 0.7% | Fully settled |
Common Applications
Low-Pass Filters
RC circuits create first-order low-pass filters. Cutoff frequency = 1/(2πRC). Signals above this frequency are attenuated.
Timer Circuits
555 timers and monostable circuits use RC timing for pulse generation. Adjust R and C to set delay duration.
Power Supply Filtering
Smooth ripple in DC power supplies. Larger τ provides better filtering but slower transient response.
Debouncing
Clean up noisy mechanical switch signals. Typical τ of 10-50ms absorbs bounce without affecting response.
Signal Coupling
AC coupling removes DC offset while passing signals. Choose τ much larger than the signal period.
Reset Circuits
Power-on reset delay ensures stable supply before MCU starts. 100ms is typical for most microcontrollers.
Design Tips
Component Selection
- • Use ceramic capacitors for high-frequency filtering
- • Electrolytic caps for large τ values (watch polarity)
- • Metal film resistors for precision timing
- • Account for ±20% tolerance in electrolytics
Common Pitfalls
- • Loading effects reduce effective resistance
- • Capacitor ESR affects high-frequency behavior
- • Temperature affects both R and C values
- • Leakage current matters for very long τ