# 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 and a natural response such that:

The forced response is due to the switch being closed, which is the voltage V for . The natural response depends on the circuit values and is given below:

Define the pole frequency and the dampening factor as:

Depending on the values of and the system can be characterized as:

1. If the system is said to be **overdamped**. The solution for the system has the form:

2. If the system is said to be **critically damped** The solution for the system has the form:

3. If the system is said to be **underdamped** The solution for the system has the form:

How do you calculate these equations?

## Example:[edit]

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 and :

From these values note that . The system is therefore **underdamped**. The equation for the voltage across the capacitor is then:

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:

Therefore . 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 into the equation for the inductor and solving at the instant the switch was closed (t=0) gives:

Therefore . Once is known, the voltage across the inductor and resistor () is given by:

You have missed a lot of steps, where are they?

Image:Example1 underdamped.png