An online calculator to calculate the current through and voltages across a resistor, a capacitor and an inductor in series when the input a step voltage of the form \( V_0 u(t) \) where \( u(t) \) is the unit step function.
We first give the formulas used in the series RLC calculator.
The formulas developed in the series RLC circuit response to a step voltage are presented here as they are used in the calculator.
In the formulas below, \( \alpha = \dfrac{R}{2 L} \)
When a voltage step function of the form \( V_0 u(t) \) we have three possible cases to consider:
Case 1: The circuit is underdamped when \( \dfrac{1}{L C} \gt \left(\dfrac{R}{2 L}\right)^2 \)
Let \( \omega = \sqrt {\dfrac{1}{L C} - \left(\dfrac{R}{2 L}\right)^2} \)
The current and voltages are given by
\( i(t) = \dfrac{V_0}{\omega L} \; \sin (\omega t) \; e^{-\alpha t} \)
\( v_R(t) = R \; i(t) = V_0 \dfrac{2 \alpha}{\omega } \sin (\omega t) e^{-\alpha t} \)
\( v_L(t) = L \; \dfrac{d i}{dt} = V_0 ( \cos (\omega t)- \dfrac{\alpha}{\omega} \sin (\omega t) ) e^{-\alpha t} \)
\( v_C(t) = V_0 - V_0 \left\{ \cos (\omega t) + \dfrac{\alpha}{\omega} \sin (\omega t) \right\} e^{-\alpha t} \)
Case 2: The circuit is overdamped when \( \dfrac{1}{L C} \lt \left(\dfrac{R}{2 L}\right)^2 \)
Let \( \beta = \sqrt { \left(\dfrac{R}{2 L}\right)^2 - \dfrac{1}{L C} } \) and rewrite \( I(s) \) as
The current and voltages are given by
\( i(t) = \dfrac{V_0}{\beta L} \; \sinh (\beta t) \; e^{-\alpha t} \)
\( \quad \quad = \dfrac{V_0}{2\beta L} \; \left\{ e^{ (\beta - \alpha) t} - e^{ ( - \beta - \alpha) t} \right\} \)
\( v_R(t) = V_0 \dfrac{\alpha}{\beta } \; \left\{ e^{ (\beta - \alpha) t} - e^{ ( - \beta - \alpha) t} \right\} \)
\( v_L(t) = L \; \dfrac{d i}{dt} = \dfrac{V_0}{2\beta} \left\{ (\beta - \alpha) e^{ (\beta - \alpha) t} + (\beta + \alpha) e^{ ( - \beta - \alpha) t} \right\} \)
\( v_C(t) = V_0 - V_0 \left\{ \dfrac{\beta + \alpha}{2 \beta} e^{(\beta - \alpha) t} + \dfrac{\beta - \alpha}{2 \beta} e^{(-\beta - \alpha) t} \right\} \)
Case 3: The circuit is critically damped \( \dfrac{1}{L C} = \left(\dfrac{R}{2 L}\right)^2 \)
The current and voltages are given by
\( i(t) = \dfrac{V_0}{ L} \; t \; e^{-\alpha t} \)
\( v_R(t) = 2 V_0 \alpha \; t \; e^{-\alpha t} \)
\( v_L(t) = V_0 e^{- \alpha t} \left( 1 - \alpha t \right) \)
\( v_C(t) = V_0 - V_0(1+\alpha t)e^{-\alpha t} \)