Second Order Circuits: Difference between revisions

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-\alpha B_1 + \omega_d B_2 =  \frac{1}{C} \left( \frac{-V_0}{R} - I_0 \right)
-\alpha B_1 + \omega_d B_2 =  \frac{1}{C} \left( \frac{-V_0}{R} - I_0 \right)
</math>
</math>
==== Characteristics ====
Voltage alternates between positive and negative values due to the two
types of energy-storage elements. It's like a mass suspended on a
spring.
The oscillation rate is fixed by <math>\omega_d</math>, which is why it
is called the '''damped radian frequency'''.
The oscillation amplitude decreases exponentially at a rate determined
by <math>\alpha</math>, so it is called the '''damping factor'''.
Notice that when <math>R \neq 0</math>, there is <math>\alpha > 0</math>
and the circuit is damped.


[[Category:Electrical Engineering]]
[[Category:Electrical Engineering]]

Revision as of 08:41, 8 March 2024

Second order circuits are circuits that have two energy storage elements, resulting in second-order differential equations.

One application of second order circuits is in timing computers. As we will see, an RLC circuit can generate a sinusoidal wave.

There are primarily two types of second order circuits:

  • Parallel RLC circuits
  • Series RLC circuits

This page will analyze them and derive some useful equations.

Series RLC Circuits

Natural Response

An unforced series RLC circuit

Consider an un-forced RLC circuit. We want to find .

First, we can use KVL and KCL

Next, we can use and substitution to get

Changing the order and moving the constants,

Moving constants away from the first term to get a second-order differential equation,

Parallel RLC Circuits

Natural Response

A parallel unforced RLC circuit

By KCL,

By differentiating once with respect to and rearranging some constants,

we get a homogeneous second-order differential equation, which has a standard solution that I will not go into detail. Briefly, it is solved by assuming since derivatives of must take the same form to cancel out to zero.

By applying the standard solution, we have

Characteristic Equation

The above simplifies to

This is the characteristic equation of the differential equation, as the root of the quadratic determines properties of

where

and

It can be pretty easily proven that the sum of the two roots is also a solution

Forms

Depending on the root, there are three forms:

  • Overdamped, where there are real, distinct solutions
  • Underdamped, where there are complex solutions
  • Critically damped, where the solutions are not distinct.


Overdamped

For an overdamped response, we have

The A's can be solved by substituting in and

Underdamped

For an underdamped response, we have

where there is damped radian frequency

From Euler's identity, the natural response comes to

The rest is identical to that of overdamped:

Characteristics

Voltage alternates between positive and negative values due to the two types of energy-storage elements. It's like a mass suspended on a spring.

The oscillation rate is fixed by , which is why it is called the damped radian frequency.

The oscillation amplitude decreases exponentially at a rate determined by , so it is called the damping factor.

Notice that when , there is and the circuit is damped.