The RLC natural response falls into three categories: overdamped, critically damped, and underdamped.Written by Willy McAllister.
Introduction
The natural response of a resistor-inductor-capacitor circuit
In two prior articles, we covered an intuitive description of how the
What we're building to
The
We will solve for the roots of the characteristic equation using the quadratic formula:
By substituting variables
where,
Depending on the relative size of
- overdamped,
, leads to the sum of two decaying exponentials - critically damped,
, gives times a decaying exponential - underdamped,
, leads to a decaying sine
Modeling and solving the circuit - review
In a previous article we created and solved a
We proposed a solution with an exponential form (which worked out really nicely for us), and came up with what is called the characteristic equation with this form:
We solved for
By substituting variables
where
We revised our proposed solution to have this form,
We now take a close look at the expression for
Exact solution
If we want an exact answer for particular values of
Overdamped, critically damped, underdamped
We can get an impression of the full richness of the natural response by looking three possible outcomes in a qualitative sense.
The solution for
How the roots turn out:
relation | sign of | nickname | |
---|---|---|---|
overdamped | 2 real roots | ||
critically damped | 2 repeated roots | ||
underdamped | 2 complex roots |
How the response turns out,
relation | sign of | nickname | |
---|---|---|---|
overdamped | 2 decaying exponentials | ||
critically damped | |||
underdamped | decaying sine |
If your engineering studies take you into the area of Control Theory, these terms are used to describe how dynamic systems act. For example, the motion of a robot's arm can be described by a second-order differential equation. If your ask your robot to quickly reach for an object, you can describe how its hand moves using these words.
Let's take a look at the three possible outcomes in a bit more detail.
overdamped
Under this condition, the
(Convince yourself that
The current will be the superposition of two real exponentials that both decay to zero.
The circuit is said to be overdamped because two superimposed exponentials are both driving the the current to zero. A circuit will be overdamped if the resistance is high relative to the resonant frequency.
critically damped
The boundary between underdamped and overdamped is when
Solving a
This response is said to be critically damped.
underdamped
When
The current looks like a sine wave that diminishes over time. Think of the sound a bell makes when you strike it. The bell's note rings out and fades over time. That is an underdamped second-order mechanical system. For second-order electrical circuits, we borrow the term and say the underdamped system "rings" at a frequency of approximately
If we let the resistance get really small and eventually go to
The first example circuit we worked through earlier in this article had
We're not going to repeat the solution, but here are a few observations using the
The damping factor
The resonant frequency,
Looking at the terms under the square root:
Summary
The
The resulting characteristic equation is:
We solved for the roots of the characteristic equation using the quadratic formula:
By substituting variables
where
Depending on the relative size of
- overdamped,
, leads to the sum of two decaying exponentials - critically damped,
, leads to times decaying exponential - underdamped,
, leads to a decaying sine
Log in 315137771xy 8 years agoPosted 8 years ago. Direct link to 315137771xy's post “how to get the equation f...” how to get the equation for i= • (4 votes) Willy McAllister 6 years agoPosted 6 years ago. Direct link to Willy McAllister's post “Jonathan is correct. The ...” From the author:Jonathan is correct. The expression for current should be i = V0/L t e^(-at). This has been corrected in the article. The article still just states the result, without derivation. Inspired by Jonathan's solution in this clarification, I wrote up this more complete derivation of the Critically Damped case: (2 votes) seanwallawallaofficial 7 years agoPosted 7 years ago. Direct link to seanwallawallaofficial's post “why do you get barely any...” why do you get barely any questions, tips, or thanks on your electrical engineering section? • (2 votes) Willy McAllister 7 years agoPosted 7 years ago. Direct link to Willy McAllister's post “There are lots of questio...” There are lots of questions scattered throughout the EE subject area. You can see them if you add a /d at the end of the url, like this: https://www.khanacademy.org/science/electrical-engineering/d (3 votes) 浩然 费 a year agoPosted a year ago. Direct link to 浩然 费's post “If I add a SPDT switch in...” If I add a SPDT switch in the circuit, I can charge the capacitor. But if there is one way to charge without the switch. (For exmaple, I add a battery and it will work and form same-frequncy wave until battery does not have any energy) • (1 vote) Willy McAllister a year agoPosted a year ago. Direct link to Willy McAllister's post “Providing an initial char...” Providing an initial charge to a capacitor or initial current to an inductor is a tricky bit of work in a circuit simulator. It is difficult to implement a mechanical switch. They are usually not part of the simulator's primitive elements. It's possible to mimic a mechanical switch using very large transistors. I show an example of this on my web site, spinningnumbers.org. This is a follow-on to my work at KA where all the articles are updated. If you navigate to the Natural and Forced Response topic, find the article on RLC Variations. The simulation models have giant MOSFETS acting like mechanical switches to initialize the voltage. Remove the spaces and try this URL, https : //spinningnumbers . org/a/rlc-natural-response-variations.html#under-damped (Scroll down to the end of the Under Damped chapter to find the simulation model.) (2 votes) Deepak Gautam 6 years agoPosted 6 years ago. Direct link to Deepak Gautam's post “The article begins by tel...” The article begins by telling: But at a later point, the article also tells: If ( R/2L > ωo ) and ( R > ωo ), both implies an overdamped circuit, this means both are equivalent statements. How can ( R/2L > ωo ) means the same thing as ( R > ωo )? • (1 vote) Willy McAllister 6 years agoPosted 6 years ago. Direct link to Willy McAllister's post “Equation (1) is correct. ...” Equation (1) is correct. (2 votes) ronen peri 3 years agoPosted 3 years ago. Direct link to ronen peri's post “In the hidden section abo...” In the hidden section about the underdamped natural frequency approximation, it states that we can only approximate the frequency as w0 because the damping factor under the radical shifts the frequency over time (the wavelength changes). I think this is misleading and not true. The natural frequency does not change; only the envelope changes. The natural frequency is determined by the roots of the differential equation, which in turn are characterized by the "damping factor" alpha and the "frequency" omega. But these names are only naming convention after the case that the damping factor equals 0 (the only case we can state that w is the natural frequency, as it is the only factor which determines the roots). Otherwise, the frequency is a "combination" of alpha and w. And it stays constant! The name "damping factor" arise from the fact that when it is not zero, the sinusoidal decays. The only way to change the system's FIXED natural frequency is by changing the boundary conditions. • (1 vote) Erkan Giray Arat 4 years agoPosted 4 years ago. Direct link to Erkan Giray Arat's post “I used a program to graph...” I used a program to graph the overdamped current solution, and I am pretty sure it never goes up to two amperes as it is shown. The equation 1.25*(exp(-x)-exp(-9x)) has a zero derivative at (-ln(1/9))/8=0.2747 which makes the function value 0.8443, never rises above this. • (1 vote) Willy McAllister 4 years agoPosted 4 years ago. Direct link to Willy McAllister's post “https://spinningnumbers.o...” https://spinningnumbers.org/a/rlc-natural-response-variations.html#over-damped image: https://spinningnumbers.org/i/rlc_overdamped_current.svg (1 vote) muhammad atif 6 years agoPosted 6 years ago. Direct link to muhammad atif's post “Suppose a parallel RLC ci...” Suppose a parallel RLC circuit is critically damped at the natural frequency of 100 Hz. I have values for R and L. How do I calculate a value of C such that it is critically damped. I know it can be calculated using 1/2RC = 1/sqrt(LC) but how do I incorporate an element of 100 Hz from this formula? This formula doesn't give me the critical damped circuit with the natural frequency of 100 Hz. Please tell me. • (1 vote) wmburtis 5 years agoPosted 5 years ago. Direct link to wmburtis's post “I don't really have a goo...” I don't really have a good feel for what the i-t curves would look like for these three variations of the RLC natural response. Why don't you give specific examples with their graphs? A more general criticism of the entire EE course -- there are not nearly enough problems to try to solve to test the reader's understanding. Please provide problems for each module!! • (1 vote) Willy McAllister 5 years agoPosted 5 years ago. Direct link to Willy McAllister's post “The currents are fully wo...” The currents are fully worked out and plotted. Search in this article for "Example of over damping" and "Example of critical damping". These are hints that open up to reveal the plots. If you follow this link to the derivation of the under damped RLC you will find a plot of the current for that variation, https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/ee-natural-and-forced-response/a/w/ee/a/ee-rlc-natural-response-derivation (1 vote)Want to join the conversation?
V0/W0*e−αt*t in crirically damped?
http://spinningnumbers.org/a/rlc-natural-response-variations.html#critically-damped
This website is a continuation of the EE contributions I made at KA.
on other sections, i see over 100 questions but on this section, i hardly ever see 10
( α > ωo ) implies overdamped circuit. Or,
( R/2L > ωo ) implies overdamped circuit..................... (1)
"A circuit will be overdamped if the resistance is high relative to the resonant frequency."
This means,
( R > ωo ) implies overdamped circuit..................... (2)
For equation (2) you are taking a very qualitative statement and trying to turn it into an equation, but this doesn't work. The units are all wrong... ohms > radians/sec. I'm just trying to say you end up with an over damped circuit if the resistance is high. In technical terms, a circuit is over damped when equation (1) is true.
lets begin with the end solution for the differential eqn:
i = <some function of t> * <a sinusoidal>. in our RLC case:
i = 5e^-t * sin2t.
We can clearly see that the argument of the sine function depends linearly on t. Nothing else. The multiplier (or Eigenvalue) acts only as an envelope function which does nothing in term of frequency.