# Electronics/RCL time domain simple

When the switch is closed, a voltage step is applied to the RCL circuit. Take the time the switch was closed to be 0s such that the voltage before the switch was closed was 0 volts and the voltage after the switch was closed is a voltage V. The voltage across the capacitor consists of a forced response $v_{f}$ and a natural response $v_{n}$ such that:

$v_{c}=v_{f}+v_{n}$ The forced response is due to the switch being closed, which is the voltage V for $t\geq 0$ . The natural response depends on the circuit values and is given below:

Define the pole frequency $\omega _{n}$ and the dampening factor $\alpha$ as:

$\alpha ={\frac {R}{2L}}$ $\omega _{n}={\frac {1}{\sqrt {LC}}}$ Depending on the values of $\alpha$ and $\omega _{n}$ the system can be characterized as:

1. If $\alpha >\omega _{n}$ the system is said to be overdamped. The solution for the system has the form:

$v_{n}(t)=A_{1}e^{{\big (}-\alpha +{\sqrt {\alpha ^{2}-\omega _{n}^{2}}}{\big )}t}+A_{2}e^{{\big (}-\alpha -{\sqrt {\alpha ^{2}-\omega _{n}^{2}}}{\big )}t}$ 2. If $\alpha =\omega _{n}$ the system is said to be critically damped The solution for the system has the form:

$v_{n}(t)=Be^{-\alpha t}$ 3. If $\alpha <\omega _{n}$ the system is said to be underdamped The solution for the system has the form:

$v_{n}(t)=e^{-\alpha t}{\big [}B_{1}\cos({\sqrt {\omega _{n}^{2}-\alpha ^{2}}}t)+B_{2}\sin({\sqrt {\omega _{n}^{2}-\alpha ^{2}}}t){\big ]}$ ## Example:

Given the following values what is the response of the system when the switch is closed?

 R L C V 1kΩ 0.5H 100nF 1V

First calculate the values of $\alpha$  and $\omega _{n}$ :

$\alpha ={\frac {R}{2L}}=1000$

$\omega _{n}={\frac {1}{\sqrt {LC}}}\approx 4472$

From these values note that $\alpha <\omega _{n}$ . The system is therefore underdamped. The equation for the voltage across the capacitor is then:

$v_{c}(t)=1+e^{-1000t}{\big [}B_{1}\cos(4359t)+B_{2}\sin(4359t){\big ]}$

Before the switch was closed assume that the capacitor was fully discharged. This implies that v(t)=0 at the instant the switch was closed (t=0). Substituting t=0 into the previous equation gives:

$0=1+B_{1}$

Therefore $B_{1}=-1$ . Similarly at the instant the switch is closed, the current in the inductor must be zero as the current can not instantly change. Substituting the equation for $v_{c}(t)$  into the equation for the inductor and solving at the instant the switch was closed (t=0) gives:

$i(t)={\frac {dv_{c}(t)}{dt}}C$
$0=100\cdot 10^{-9}{\big [}4359B_{2}-1000B_{1}{\big ]}$

Therefore $B_{2}\approx -0.229$ . Once $v_{c}(t)$  is known, the voltage across the inductor and resistor ($V_{out}$ ) is given by:

$v_{out}=e^{-1000t}{\big [}\cos(4359t)+0.229\sin(4359t){\big ]}$

Figure 2: Underdamped Resonse

Image:Example1 underdamped.png