AC Components and Circuits The L/R Time Constant
When switch S is open, there can be no current flowing through the circuit. The entire source voltage, VT, is across the open switch contactsthere is no voltage across R and L. The instant the switch is closes—the very instantthere is still virtually no current flow through the circuit, and the full source voltage shifts from the switch to the inductor. There is, at that instant, no voltage across R. As time passes, the energy from the source begins to shift from the inductor and to the resistor. As a result, the voltage across the inductor drops and the resistor increases. As long as the switch remains closed, V_{R} + V_{L} = V_{T}. Also, the current through the circuit is increasing. After a certain length of time, there is no longer any voltage across the inductor, and the full amount of source voltage can be found across the resistor.
Energy converted to heat in the resistor is P = I^{2}R
Vs / R = Imax
Examples
Charge curve for an R/L circuit v_{C} = V_{T}(1 – e^{–tR/L})
Discharge curve for an R/L circuit v_{C} = V_{T} e^{–tR/L}


David L. Heiserman, Editor  Copyright © SweetHaven
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Revised: June 06, 2015