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Chapter 5 First-Order Circuits 5.1 5.2 5.3 5.4 5.5 1 The Source-Free RC Circuit The Source-Free RL Circuit Unit-step Function Step Response of an RC Circuit Step Response of an RL Circuit 5.1 The Source-Free RC Circuit (1) A first-order circuit is characterized by a first-order differential equation. By KCL iR iC 0 Ohms law v dv C 0 R dt Capacitor law • Apply Kirchhoff’s laws to purely resistive circuit results in algebraic equations. • Apply the laws to RC and RL circuits produces differential equations. 2 5.1 The Source-Free RC Circuit (2) The natural response of a circuit refers to the behavior (in terms of voltages and currents) of the circuit itself, with no external sources of excitation. Decays more slowly Time constant RC Decays faster • The time constant, of a circuit is the time required for the response to decay by a factor of 1/e or 36.8% of its initial value. • v decays faster for small t and slower for large t. 3 5.1 The Source-Free RC Circuit (3) The key to working with a source-free RC circuit is finding: v(t ) V0 e t / where RC 1. The initial voltage v(0) =V0 across the capacitor. 2. The time constant = RC. 4 5.1 The Source-Free RC Circuit (4) Example 1 Refer to the circuit below, determine vC, vx and io for t ≥ 0. Assume that vC(0) = 30 V. 5 Solution e1 6 5.1 The Source-Free RC Circuit (5) Example 2 The switch in circuit below is opened at t = 0, find v(t) for t ≥ 0 and Wc(0). • Please refer to lecture or textbook for more detail elaboration. Answer: V(t) = 8e–2t V, 5.333J 7 Solution e2 8 9 5.2 The Source-Free RL Circuit (1) A first-order RL circuit consists of a inductor L (or its equivalent) and a resistor, R (or its equivalent) By KVL vL vR 0 di L iR 0 dt Inductors law 10 di R dt i L Ohms law i (t ) I 0 e Rt / L 5.2 The Source-Free RL Circuit (2) A general form representing a RL i (t ) I 0 e where • 11 • • t / L R The time constant, of a circuit is the time required for the response to decay by a factor of 1/e or 36.8% of its initial value. i(t) decays faster for small t and slower for large t. The general form is very similar to a RC source-free circuit. 5.2 The Source-Free RL Circuit (3) Comparison between a RL and RC circuit A RC source-free circuit A RL source-free circuit i (t ) I 0 e t / 12 where L R v(t ) V0 e t / where RC 5.2 The Source-Free RL Circuit (4) The key to working with a source-free RL circuit is finding: i (t ) I 0 e t / where L R 1. The initial voltage, i(0) = I0 through the inductor. 2. The time constant = L/R. 13 5.2 The Source-Free RL Circuit (5) Example 3 Find i and vx in the circuit. given that i(0) = 5 A. 14 Solution E3 1 6 1 6 KVL @ loop i2. Use KVL @ loop i1 di1 3i1 i1 i2 2(3i1 ) 0 dt di1 10i1 i2 0 (1) dt 2(3i1 ) i2 i1 5i2 0 7i1 6i2 0 7 i2 i1 6 (2) di1 53dt ln i1 Insert (2) in (1), i1 i (t ) i (0) i (t ) 53t ln 53t i (0) t 0 i (t ) i (0)e 53t 5e 53t A i1 15 v x 3i1 3(5e 53t ) 15e 53tV Solution E3 16 17 5.2 The Source-Free RL Circuit (6) Example 4 For the circuit, find i(t) for t > 0. 1 8 Solution E4 19 Exercise Determine i, io, vo for all t . [Assume the switch was closed for a long time] 20 @t<0 @t>0 21 5.3 Unit-Step Function (1) The unit step function u(t) is 0 for negative values of t and 1 for positive values of t. 22 0, u (t ) 1, t0 t0 0, u (t to ) 1, t to 0, u (t to ) 1, t to t to t to 5.3 Unit-Step Function (2) Represent an abrupt change for: voltage source. current source: 23 5.4 The Step-Response of a RC Circuit (1) The step response of a circuit is its behavior when the excitation is the step function, which may be a voltage or a current source. • Initial condition: v(0-) = v(0+) =V0 • Applying KCL, dv v Vs u (t ) c 0 dt R or v Vs dv u (t ) dt RC 24 •Where u(t) is the unit-step function 5.4 The Step-Response of a RC Circuit (2) Integrating both sides and considering the initial conditions, the solution of the equation is: t0 V0 v(t ) t / V ( V V ) e 0 s s Final value at t -> ∞ 25 Complete Response = Natural response (stored energy) = V0e–t/τ + + Initial value at t = 0 t 0 Source-free Response Forced Response (independent source) Vs(1–e–t/τ) 5.4 The Step-Response of a RC Circuit (3) Three steps to find out the step response of an RC circuit: i. The initial capacitor voltage, v(0). ii. The final capacitor voltage, v() — DC voltage across C. iii. The time constant, . v (t ) v () [v (0) v ()] e 26 t / Note: The above method is a short-cut method. You may also determine the solution by setting up the circuit formula directly using KCL, KVL , ohms law, capacitor and inductor VI laws. 5.4 The Step-Response of a RC Circuit (4) Example 5 Find v(t) for t > 0 in the circuit in below. Assume the switch has been open for a long time and is closed at t = 0. Calculate v(t) at t = 0.5. • Please refer to lecture or textbook for more detail elaboration. Answer: v(t ) 15e 2t 5 and v(0.5) = 0.5182V 27 Solution E5 28 5.5 The Step-response of a RL Circuit (1) The step response of a circuit is its behavior when the excitation is the step function, which may be a voltage or a current source. • Initial current i(0-) = i(0+) = Io • Final inductor current i(∞) =Vs/R • Time constant t = L/R t Vs Vs i (t ) ( I o )e u (t ) R R 29 5.5 The Step-Response of a RL Circuit (2) Three steps to find out the step response of an RL circuit: i. The initial inductor current, i(0) at t = 0+. ii. The final inductor current i(). iii. The time constant . i (t ) i () [i (0) i ()] e 30 t / Note: The above method is a short-cut method. You may also determine the solution by setting up the circuit formula directly using KCL, KVL , ohms law, capacitor and inductor VI laws. 5.5 The Step-Response of a RL Circuit (4) Example 6 The switch in the circuit shown below has been closed for a long time. It opens at t = 0. Find i(t) for t > 0. 31 Solution E6 32 5.5 The Step-Response of a RL Circuit (5) Example 7 Switch S1 is closed at t = 0 and switch S2 is closed at t = 2s, Calculate i(t) for all t. Find i(1) and i(3) 33 Solution E7 34 35