RC time constant
Time constant of an RC circuit / From Wikipedia, the free encyclopedia
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The RC time constant, denoted τ (lowercase tau), the time constant (in seconds) of a resistor–capacitor circuit (RC circuit), is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads), i.e.:
- [seconds]
It is the time required to charge the capacitor, through the resistor, from an initial charge voltage of zero to approximately 63.2% of the value of an applied DC voltage, or to discharge the capacitor through the same resistor to approximately 36.8% of its final charge voltage. These values are derived from the mathematical constant e, where and . The following formulae use it, assuming a constant voltage applied across the capacitor and resistor in series, to determine the voltage across the capacitor against time:
- Charging toward applied voltage (initially zero voltage across capacitor, constant V0 across resistor and capacitor together) [1]
- Discharging toward zero from initial voltage (initially V0 across capacitor, constant zero voltage across resistor and capacitor together)