Electrical Circuits
RC Time Constant
The RC time constant is the product of resistance and capacitance and describes how quickly a capacitor charges or discharges through a resistor. During charging from zero, the capacitor reaches about 63.2% of the supply voltage after one time constant, 86.5% after two and about 99.3% after five. The time constant is widely used to set delays, filter response and debounce timing.
RC Time Constant
τ = R × C
Variables and units
| Symbol | Quantity | Unit |
|---|---|---|
| τ | Time constant | s |
| R | Resistance | Ω |
| C | Capacitance | F |
Conditions of application
First-order RC circuit with a constant resistance and capacitance; ideal capacitor with negligible leakage. After one time constant the voltage reaches about 63.2% of its final change; after five time constants it is within about 1%.
Worked example
Solved
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Time constant and charge/discharge milestones for an RC circuit.