The interval between the instant in which the current begins to flows for in inductor in series with a resistor and the instant in which the current is 63,2% of the final value is the time constant of the RL series circuit. It is also the value between the instant in which the current is open and the instant in which the current decays to 37,2% of maximum.
Formula 1
T = L / R
Where:
T is the time constant in seconds (s)
L is the inductance in henrys (H)
R is the resistance in ohm (Ω)
Derivated Formulas:
Formula 2
L = T x R
Formula 3
R = L / T
Application Example:
What is the time constant of a circuit formed by a 30 kohm resistor in series with a 10 mH inductor?
Data:
R = 20 000 Ω
L = 10 mH = 0,01 H
T = ?
Using formula 1:
T = 0,01/20 000 = 0,0000005 = 0,5 us
