Circuits & Networks II: Transients Lecture 1

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Questions and Answers

What is the name given to a sudden change of state in a circuit?

Transient

Resistors are time-invariant elements.

True (A)

Inductors can consume power.

False (B)

Capacitors can store energy.

<p>True (A)</p> Signup and view all the answers

When a DC supply is connected to an inductor through a switch, what happens to the inductor at the instant the switch is closed?

<p>The inductor acts as an open circuit. (A)</p> Signup and view all the answers

What happens to an inductor as time approaches infinity after a DC supply is connected?

<p>The inductor acts as a short circuit. (D)</p> Signup and view all the answers

When a DC supply is connected to a capacitor through a switch, what happens to the capacitor at the instant the switch is closed?

<p>The capacitor acts as a short circuit. (A)</p> Signup and view all the answers

What happens to a capacitor as time approaches infinity after a DC supply is connected?

<p>The capacitor acts as an open circuit. (A)</p> Signup and view all the answers

What is the term used to describe the state of an inductor immediately after a switch is closed?

<p>Transient state</p> Signup and view all the answers

What is the term used to describe the state of an inductor after a sufficiently long time has passed?

<p>Steady state</p> Signup and view all the answers

What is the time constant of an RL circuit?

<p>L/R</p> Signup and view all the answers

What is the complete solution for the current in an RL circuit with DC excitation?

<p>The complete solution is the sum of the transient and steady-state responses. (A)</p> Signup and view all the answers

Which of the following components is responsible for providing the steady-state response in an RL circuit?

<p>Resistor (B)</p> Signup and view all the answers

Which of the following components is responsible for providing the transient response in an RL circuit?

<p>Inductor (A)</p> Signup and view all the answers

What is the order of the differential equation for an RLC circuit with DC excitation?

<p>Second Order (B)</p> Signup and view all the answers

What are the three types of damping that can occur in an RLC circuit?

<p>Overdamped, critically damped, and underdamped (C)</p> Signup and view all the answers

Flashcards

Transient

A sudden change in the state of a circuit, often caused by the opening or closing of a switch, causing the response of the circuit to be highly dependent on time.

Resistor

The time-invariant element in a circuit that does not store energy but dissipates it as heat.

Inductor

The time-variant element in a circuit that stores energy in a magnetic field when current flows through it.

Capacitor

The time-variant element in a circuit that stores energy in an electric field when a voltage is applied across it.

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Transient State of Inductor

The state of an inductor immediately after a switch is closed (at t=0+), where it behaves like an open circuit. This happens because the inductor resists instantaneous changes in current.

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Steady State of Inductor

The state of an inductor after sufficient time has passed (at t=∞), where it behaves like a short circuit. This is because the current through the inductor stabilizes.

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Transient State of Capacitor

The state of a capacitor immediately after a switch is closed (at t=0+), where it behaves like a short circuit. This happens because the capacitor resists instantaneous changes in voltage.

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Steady State of Capacitor

The state of a capacitor after sufficient time has passed (at t=∞), where it behaves like an open circuit. This is because the voltage across the capacitor stabilizes.

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Initial Current of an Inductor

The value of current flowing through an inductor before a switch is operated (at t=0-) which reflects the energy stored in the inductor.

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Initial Voltage of a Capacitor

The value of voltage across a capacitor before a switch is operated (at t=0-) which reflects the energy stored in the capacitor

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Time Constant of RL Circuit

The time constant of an RL circuit, representing the time it takes for the current to reach approximately 63.2% of its final value. It is calculated as τ = L/R.

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Time Constant of RC Circuit

The time constant of an RC circuit, representing the time it takes for the voltage across the capacitor to reach approximately 63.2% of its final value. It is calculated as τ = RC.

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Complete Solution for Current

The complete solution for the current in a circuit is the sum of the transient response (ICF) and the steady-state response (IPI).

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Complementary Function (ICF)

The part of the current response in a circuit that represents the temporary, time-dependent behavior, often exponentially decaying. It is calculated from the complementary equation of the circuit.

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Particular Integral (IPI)

The part of the current response in a circuit that represents the stable, constant behavior after sufficient time has passed. It is calculated based on the steady-state conditions of the circuit.

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RLC Series Circuit

A type of circuit with a resistor, inductor, and capacitor connected in series. It exhibits a second-order response due to the interaction of these elements.

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Discriminant (∆)

The discriminant of the characteristic equation of an RLC series circuit, calculated as ∆ = (R/2L)2 - 1/(LC)

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Overdamped Response

A type of response in an RLC circuit where the roots of the characteristic equation are real and distinct (∆ > 0). The circuit returns to its steady state slowly, without any oscillations.

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Critically Damped Response

A type of response in an RLC circuit where the roots of the characteristic equation are real and equal (∆ = 0). The circuit returns to its steady state the fastest possible way without any oscillations.

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Underdamped Response

A type of response in an RLC circuit where the roots of the characteristic equation are complex conjugates (∆ < 0). The circuit returns to its steady state with oscillations.

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Laplace Transform

The technique of representing circuit variables like currents and voltages as functions of Laplace variable 's' to solve differential equations.

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Partial Fractions Method

A mathematical method for decomposing a complex Laplace function into simpler fractions, aiding in the process of finding the inverse Laplace transform.

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Inverse Laplace Transform

The process of obtaining a time-domain solution from a Laplace function in the s-domain.

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Two Switching Operations

The process of solving a circuit with two switching operations, requiring the analysis of both initial states and steady states.

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Initial Conditions

The initial condition of a circuit, representing the state of an inductor or capacitor before a switching event. It is represented by IL(0) for inductor and VC(0) for capacitor.

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Steady State Before Switching

The current or voltage values calculated in the circuit before a switching operation, indicating the status of the inductor or capacitor before the change.

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Transient or Steady State After Switching

The values of current or voltage calculated in the circuit after a switching operation, indicating the transient or steady-state response of the circuit after the change.

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Initial Energy Stored in an Inductor

The energy stored in an inductor due to its initial current. It is calculated as ½ LI2.

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Initial Energy Stored in a Capacitor

The energy stored in a capacitor due to its initial voltage. It is calculated as ½ CV2.

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S-Domain Analysis

A technique for analyzing circuits in the s-domain, using Laplace transforms to represent circuit variables and solving equations in the s-domain.

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Initial Current of Inductor in S-Domain

The initial condition of an inductor in the s-domain represented as IL(0)/s.

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Initial Voltage of Capacitor in S-Domain

The initial condition of a capacitor in the s-domain represented as VC(0)/s.

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Study Notes

Circuits & Networks-II: Transients - Lecture 1

  • Unit-II covers transients in electrical circuits.
  • The lecture discusses RL, RC and RLC circuits with switching actions for DC and AC supplies in both time and Laplace domains.
  • A transient is a sudden change of state in a system, and the response during this transient period depends on time.
  • Key electrical components include resistors (R), inductors (L), and capacitors (C).
  • Resistors are time-invariant; they do not store energy.
  • Inductors and capacitors store energy. Inductors store energy in a magnetic field, and capacitors store energy in an electric field.
  • Inductors and capacitors exhibit specific transient behavior when a switch is closed or opened.

Circuit Transient Behavior

  • Resistor: Responds instantaneously to changes in voltage, i.e., no transient period needed. Current flows immediately. Ohm's Law applies (V = IR).
  • Inductor: Does not allow immediate changes in current. Initially acts as an open circuit (t = 0+), then becomes a short circuit (t → ∞).
  • Capacitor: Does not allow immediate changes in voltage. Initially acts as a short circuit (t = 0+), then becomes an open circuit (t → ∞).

Transient, Study State, and Initial Conditions

  • The table below summarizes the transient and steady-state behavior of inductors and capacitors:
Element t = 0+ (transient) t → ∞ (study state) t = 0⁻ (initial conditions) Energy Stored
Inductor, L OPEN SHORT Depends on the value of current at t = 0 1/2LI²
Capacitor, C SHORT OPEN Depends on the value of voltage at t = 0 1/2CV²

Switching Operation w.r.t. Time for L and C

  • t = 0+: Immediately after a switch is closed for an inductor or capacitor with a supply. Current/Voltage cannot change instantly.
  • t = ∞: Sufficient time has passed after the switch is closed, resulting in a steady-state response (for inductors, it is short circuit, and for capacitors, it is open circuit).
  • t = 0⁻: Time period before the switch is closed. An inductor/capacitor has an initial current/voltage which cannot change instantly.

Inductor and Capacitor with Initial Current/Voltage

  • Equations describing transient behavior are derived for inductors with an initial current and capacitors with an initial voltage.

  • To solve circuits with initial conditions and more than one switching operation, the complete response method can be utilized

Conclusion

  • Inductors and capacitors need time to react to changes in the circuit, showing transient responses before settling into the steady-state behavior. This transient behavior is characterized by time constants related to their values and circuit resistance.

Circuits & Networks-II: Transients - Lecture 2

  • The second lecture expands on the study of transients, covering RL circuits with DC excitation and two-branch switching.
  • The complete solution to an RL circuit with DC excitation is the sum of the complementary function (natural response) and the particular integral (forced response).
  • The method involved determining the complementary function roots, finding the particular integral, applying initial conditions to find arbitrary constants, and combining results for the complete solution.

Circuits & Networks-II: Transients - Lecture 3

  • The third lecture introduces RLC circuits with DC excitation to explore transient behavior in more complicated circuits
  • A second-order differential equation describes an RLC circuit.
  • The general solution is composed of a complementary function and a particular integral, and its form depends on the discriminant (Δ).
  • If the discriminant is positive, the roots are real and distinct; if zero, they are real and equal; if negative, they are complex conjugates. These cases correspond to overdamped, critically damped, and underdamped circuits respectively.

Summary:

  • The provided notes detail transient analysis for RL, RC, and RLC circuits with various sources.
  • Switching operations and initial conditions are considered crucial parameters in solving transient response problems.
  • Techniques for solving these problems include the use of differential equations, particular integral methods, time constants, and initial conditions.
  • Laplace transform is introduced as one tool for solving such problems in more advanced cases.

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