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First Order And Second Order Response Of RL And RC Circuit Topic 5 First-Order and Second-Order Response of RL and RC Circuit • • • • • • • Natural response of RL and RC Circuit Step Response of RL and RC Circuit General solutions for natural and step response Sequential switching Introduction to the natural and step response of RLC circuit Natural response of series and parallel RLC circuit Step response of series and parallel RLC circuit Natural response of RL and RC Circuit RL- resistor-inductor • RC-resistor-capacitor • First-order circuit: RL or RC circuit because their voltages and currents are described by first-order differential equation. • • Natural response: refers to the behavior (in terms of voltages and currents) of the circuit, with no external sources of excitation. Natural response of RC circuit Consider the conditions below: 1. At t < 0, switch is in a closed position for along time. 2. At t=0, the instant when the switch is opened 3. At t > 0, switch is not close for along time • For t ≤ 0, v(t) = V0. For t ≥ 0: ic iR 0 dv(t ) v(t ) C 0 dt R dv(t ) v(t ) 0 dt RC dv(t ) v(t ) dt RC dv(t ) 1 dt v(t ) RC du 1 dv u RC v (t ) 1 1 V0 u du RC t 0 dv 1 ln v(t ) ln V0 (t 0) RC v(t ) t voltan ln RC V0 v(t ) V0 e t RC • Thus for t > 0, v(t ) V0 e t RC v(t ) V0 ic (t ) e R R t RC 1 1 2 2 W (t ) C v(t ) C V0 e 2 2 2t RC The graph of the natural response of RC circuit v(t ) V0 V0 e t0 t RC t0 • The time constant, τ = RC and thus, v(t ) V0 e t • The time constant, τ determine how fast the voltage reach the steady state: Natural response of RL circuit Consider the conditions below: 1. At t < 0, switch is in a closed position for along time. 2. At t=0, the instant when the switch is opened 3. At t > 0, switch is not close for along time For t ≤ 0, i(t) = I0 For t > 0, • v(t ) R i (t ) 0 1 R t du dv i (0) u 0 di (t ) L L R i (t ) 0 dt R ln i (t ) ln i (0) (t 0) di (t ) L R i (t ) L dt i (t ) R Current di (t ) R t ln dt i (t ) L L i (0) du R dv u L i (t ) i (t ) i (0) e t R L • Thus for t > 0, i(t ) I 0 e t R L v(t ) i (t ) R RI 0 e t R L 1 2 w(t ) Li (t ) 2 1 2 2t R L LI 0 e 2 Ex. The switch in the circuit has been closed for along time before is opened at t=0. Find a) IL (t) for t ≥ 0 b) I0 (t) for t ≥ 0+ c) V0 (t) for t ≥ 0+ d) The percentage of the total energy stored in the 2H inductor that is dissipated in the 10Ω resistor. Ans. a) The switch has been closed for along time prior to t=0, so voltage across the inductor must be zero at t = 0-. Therefore the initial current in the inductor is 20A at t = 0-. Hence iL (0+) also is 20A, because an instantaneous change in the current cannot occur in an inductor. • The equivalent resistance and time constant: Req 2 40 10 10 L 2 0.2 saat Req 10 • The expression of inductor current, iL(t) as, i L (t ) i(0 ) e 20 e 5 t t A t0 b) The current in the 40Ω resistor can be determine using current division, 10 i0 i L 10 40 Note that this expression is valid for t ≥ 0+ because i0 = 0 at t = 0-. • The inductor behaves as a short circuit prior to the switch being opened, producing an instantaneous change in the current i0. Then, • i0 (t ) 4e 5t A t0 c) The voltage V0 directly obtain using Ohm’s law V0 (t ) 40i0 5t 160e V t0 d) The power dissipated in the 10Ω resistor is 2 V0 p10 (t ) 10 10t 2560 e W t0 • The total energy dissipated in the 10Ω resistor is W10 (t ) 2560e 0 256 J 10t dt • The initial energy stored in the 2H inductor is 1 2 W (0) L i (0) 2 1 2 400 400 J 2 • Therefore the percentage of energy dissipated in the 10Ω resistor is, 256 100 64% 400 First-Order and Second-Order Response of RL and RC Circuit • • • • • • • Natural response of RL and RC Circuit Step Response of RL and RC Circuit General solutions for natural and step response Sequential switching Introduction to the natural and step response of RLC circuit Natural response of series and parallel RLC circuit Step response of series and parallel RLC circuit Step response of RC circuit • The step response of a circuit is its behavior when the excitation is the step function, which maybe a voltage or a current source. Consider the conditions below: 1. At t < 0, switch is in a closed and opened position for along time. 2. At t=0, the instant when the switch is opened and closed 3. At t > 0, switch is not close and opened for along time • For t ≤ 0, v(t)=V0 For t > 0, Vs v(t ) Ri (t ) 1 du dv RC u Vs dv(t ) t ln v(t ) V ln V V s 0 s Vs v(t ) RC RC dt volt v(t ) Vs t 1 dv(t ) ln dt RC V V 0 s RC Vs v(t ) t RC v(t ) Vs V0 Vs e 1 dv(t ) dt t RC v(t ) Vs Vs V0 Vs e • Thus for t >0 V Vs V0 Vs e V f Vn Where V f V s Vn V0 Vs e t t • Vf = force voltage or also known as steady state response • Vn = transient voltage is the circuit’s temporary response that will die out with time Step Response: RC circuit force total Natural • The current for step response of RC circuit dv i (t ) C dt 1 t C (V0 Vs )e 1 t V0 Vs e R Vs V0 t e R R i (t ) i(0 )e t Step response of RL circuit Consider the conditions below: 1. At t < 0, switch is in a opened position for along time. 2. At t=0, the instant when the switch is closed 3. At t > 0, switch is not open for along time i(t)=I0 for t ≤ 0. For t > 0, • Vs Ri (t ) v(t ) di (t ) Vs Ri (t ) L dt Vs L di (t ) i (t ) R R dt R di (t ) dt V s L R i (t ) R di dt L i Vs R R du dv L u Vs R i (t ) R t du dv I 0 u Vs L 0 R R t ln i (t ) Vs R ln I 0 Vs R L i (t ) Vs R R t ln Current Vs L I0 R i(t ) Vs R I 0 Vs R e t R L • Thus, i(t ) I 0 Vs R I 0 Vs R e di (t ) v(t ) L dt t R L Vs R I 0 e t 0 t R L t 0 t0 t0 Ex. The switch in the circuit has been open for along time. The initial charge on the capacitor is zero. At t = 0, the switch is closed. Find the expression for a) i(t) for t ≥ 0 b) v(t) when t ≥ 0+ Answer (a) • Initial voltage on the capacitor is zero. The current in the 30kΩ resistor is (7.5)( 20) i (0 ) 3mA 50 The final value of the capacitor current will be zero because the capacitor eventually will appear as an open circuit in terms of dc current. Thus if = 0. • The time constant, τ is • (20 30)10 (0.1) 10 5ms 3 6 • Thus, the expression of the current i(t) for t ≥ 0 is i ( t ) i (0 )e 3e 3e t t 5103 200t mA t0 Answer (b) • The initial value of voltage is zero and the final value is Vf (7.5)(20) 150V • The capacitor vC(t) is v C ( t ) Vf V0 Vf e 150 (0 150)e 150 150e • 200t t 200t V t0 Thus, the expression of v(t) is v( t ) 150 150e (150 60e 200t 200t (30)(3)e )V t 0 200t First-Order and Second-Order Response of RL and RC Circuit • • • • • • • Natural response of RL and RC Circuit Step Response of RL and RC Circuit General solutions for natural and step response Sequential switching Introduction to the natural and step response of RLC circuit Natural response of series and parallel RLC circuit Step response of series and parallel RLC circuit General solutions for natural and step response • There is common pattern for voltages, currents and energies: v(t ) V f V0 V f e i(t ) I f I 0 I f e W (t ) W f W0 W f e t t 2t The general solution can be computed as: x(t ) x f x0 x f e t Write out in words: the unknown the final the initial the final t var iable as a value of the value of the value of the e time cons tan t var iable function of time var iable var iable When computing the step and natural responses of circuits, it may help to follow these steps: 1. Identify the variable of interest for the circuit. For RC circuits, it is most convenient to choose the capacitive voltage, for RL circuits, it is best to choose the inductive current. 2. Determine the initial value of the variable, which is its value at t0. 3. Calculate the final value of the variable, which is its value as t→∞. 4. Calculate the time constant of the circuit, τ. First-Order and Second-Order Response of RL and RC Circuit • • • • • • • Natural response of RL and RC Circuit Step Response of RL and RC Circuit General solutions for natural and step response Sequential switching Introduction to the natural and step response of RLC circuit Natural response of series and parallel RLC circuit Step response of series and parallel RLC circuit Sequential switching • • Sequential switching is whenever switching occurs more than once in a circuit. The time reference for all switching cannot be t = 0. Example… The first switch moves from a to b at t=0. The second switch closed at t=1ms. Find the current, i for t ≥ 0. • • Step 1: the current at t=0- is determined by assuming that the first switch at point a and the second switch opened for along time. Therefore, the current, i(0-)=10A. At t=0, the time constant of RL circuit is obtained as L 1ms . R • Thus the current for 0 ≤ t ≤ 1ms is, i 10e 1000t A • At t=t1=1ms, i (t1 ) 10e 1 3.68 A When switch is closed at t=1ms, the equivalent resistance is 1Ω. Then, L 2 1 2ms R 1 • Thus i for t ≥ 1ms is i i (t1 ) e 3.68 e ( t t1 ) 1 ( t t1 ) 1 A The graph of current for t ≥ 0 First-Order and Second-Order Response of RL and RC Circuit • • • • • • • Natural response of RL and RC Circuit Step Response of RL and RC Circuit General solutions for natural and step response Sequential switching Introduction to the natural and step response of RLC circuit Natural response of series and parallel RLC circuit Step response of series and parallel RLC circuit Second order response for RLC RLC circuit: consists of resistor, inductor and capacitor • Second order response : response from RLC circuit • Type of RLC circuit: • 1. 2. Series RLC Parallel RLC Natural response of parallel RLC • Summing all the currents away from node, V 1 t dv vd I 0 C 0 0 R L dt • Differentiating once with respect to t, 2 1 dv v d v C 2 0 R dt L dt 2 d v 1 dv v 0 2 dt RC dt LC • Assume that v Ae st As st A st As e e e 0 RC LC 2 st s 1 2 Ae s 0 RC LC st characteristic equation • Characteristic equation is zero: s 1 2 s 0 RC LC • The two roots: 2 1 1 1 s1 2 RC 2 RC LC 2 1 1 1 s2 2 RC 2 RC LC • The natural response of series RLC: v A1 e A2 e s1t s2t • The two roots: s1 0 2 s2 0 2 2 2 • where: 1 2 RC 0 1 LC • Summary Parameter Terminology s1, s2 Characteristic equation α 0 Neper frequency Resonant radian frequency Value in natural response s1 2 0 2 s2 2 0 1 2 RC 0 1 LC 2 • • 1. 2. 3. The two roots, s1 and s2 depend on the value of α and ωo. 3 possible conditions are: If ωo < α2 , the voltage response is overdamped If ωo > α2 , the voltage response is underdamped If ωo = α2 , the voltage response is critically damped Overdamped voltage response • Overdamped voltage expression: v A1 e A2 e s1t s2t • The constants of A1 and A2 can be obtained from these two equations: v(0 ) A1 A2 (1) dv(0 ) s1 A1 s2 A2 (2) dt • Whereby, vo+ is obtained from the circuit at the instant when the switching occurred. • The value of v(0+) = V0 and the initial value of dv/dt is dv(0 ) iC (0 ) dt C The process for finding the overdamped response, v(t) : 1. Find the roots of the characteristic equation, s1 dan s2, using the value of R, L and C. 2. Find v(0+) and dv(0+)/dt using circuit analysis. 3. The values of A1 and A2 is obtained by solving the equation below: v(0 ) A1 A2 dv(0 ) iC (0 ) s1 A1 s2 A2 dt C 4. Substitute the value for s1, s2, A1 dan A2 to determine the expression for v(t) for t ≥ 0. • The response of overdamped voltage for v(0) = 1V and i(0) = 0 Underdamped voltage response ωo2 > α2, the roots of the characteristic equation are complex and the response is underdamped. • When • The roots s1 and s2 as, s1 (0 ) 2 j 0 2 jd s2 jd • ωd : damped radian frequency 2 2 • The underdamped voltage response of a parallel RLC circuit is v( t ) B1 e t B2 e cos d t t sin d t • The constants B1 dan B2 are real number. The two simultaneous equation that determine B1 and B2 are: v(0 ) V0 B1 dv(0 ) iC (0 ) 1B1 d B2 dt C Response of underdamped voltage for v(0) = 1V and i(0) = 0 Critically Damped voltage response • A circuit is critically damped when ωo2 = α2 ( ωo = α). The two roots of the characteristic equation are real and equal: 1 s1 s2 2 RC • The solution for the voltage is v(t ) D1t e t D2 e t •The two simultaneous equation that determine D1 and D2 are, v(0 ) V0 D2 dv(0 ) iC (0 ) D1 D2 dt C Response of the critically damped voltage for v(0) = 1V and i(0) = 0 The step response of a parallel RLC circuit • From the KCL, iL iR iC I dv v I iL C dt R • Because • We get di vL dt 2 dv d iL L 2 dt dt • Thus, 2 L diL d iL iL LC 2 I R dt dt 2 d iL 1 diL iL I 2 dt RC dt LC LC • There are two approaches to solve the equation: – direct approach – indirect approach. Indirect Approach • From the KCL: 1 t v dv vd C I 0 L R dt • Differentiate once with respect to t: 2 v 1 dv d v C 2 0 L R dt dt 2 d v 1 dv v 0 2 dt RC dt LC • The solution for v depends on the roots of the characteristic equation: v A1 e A2 e s1t v B1 e t B2 e v D1t e s2t cos d t t t sin d t D2 e t • Substitute into KCL equation : iL I A1 e A2 e s1t s2t t iL I B1 e cos d t t B2 e sin d t t t iL I D1 t e D2 e Direct Approach • It is much easier to find the primed constants directly in terms of the initial values of the response function. A1 , A2 , B1 , B2 , D1 , D2 • The primed constants could be determined from: iL (0) and diL (0) dt • The solution for a second-order differential equation consists of the forced response and the natural response. • If If and Vf is the final value of the response function, the solution for the step function can be written as: Function of the same form i If as the natural response function of the same form v Vf as the natural response Natural response of a series RLC • The procedures for finding the natural response of a series RLC circuit is similar as the natural response of a parallel RLC circuit as both circuits are described by the similar form of differential equations. Series RLC circuit • Summing the voltage around the loop, di 1 t Ri L i d V0 0 dt C 0 • Differentiate once with respect to t, 2 di d i i R L 2 0 dt dt C 2 d i R di i 0 2 dt L dt LC • The characteristic equation for the series RLC circuit is, R 1 s s 0 L LC 2 • The roots of the characteristic equation are, 2 s1, 2 R 1 R 2L 2 L LC @ s1, 2 0 2 2 • Neper RLC, frequency (α) for series R rad / s 2L And the resonant radian frequency, 0 1 rad / s LC The current response will be overdamped, underdamped or critically damped according to, 0 2 0 2 2 0 2 2 2 • Thus the three possible solutions for the currents are, i(t ) A1 e A2 e s1t i(t ) B1e t B2e i(t ) D1t e cos d t t t s2t sin d t D2 e t Step response of series RLC • The procedures are the same as the parallel circuit. Series RLC circuit • Use KVL, di v Ri L vC dt • The current, i is related to the capacitor voltage (vC ) by expression, dv C iC dt • Differentiate once i with respect to t 2 di d vC C 2 dt dt • Substitute 2 into KVL equation, d vC R dvC vC V 2 dt L dt LC LC • Three possible solutions for vC are, s1t t vC V f A1 e A2 e vC V f B1 e B2 e t s2t cos d t sin d t t t vC V f D1 t e D2 e Example 1 (Step response of parallel RLC) The initial energy stored in the circuit is zero. At t = 0, a DC current source of 24mA is applied to the circuit. The value of the resistor is 400Ω. 1. What is the initial value of iL? 2. What is the initial value of diL/dt? 3. What is the roots of the characteristic equation? 4. What is the numerical expression for iL(t) when t ≥ 0? Solution 1. No energy is stored in the circuit prior to the application of the DC source, so the initial current in the inductor is zero. The inductor prohibits an instantaneous change in inductor current, therefore iL(0)=0 immediately after the switch has been opened. 2. The initial voltage on the capacitor is zero before the switch has been opened, therefore it will be zero immediately after. Because di L vL dt thus di L (0 ) 0 dt 3. From the circuit elements, 0 12 2 1 10 8 16 10 LC (25)(25) 9 1 10 2 RC (2)( 400)( 25) 5 10 rad / s 4 25 10 2 8 • Thus the roots of the characteristic equation are real, s1 5 10 3 10 4 4 20 000 rad / s s 2 5 10 3 10 4 80 000 rad / s 4 4. The inductor current response will be overdamped. i L I f A1 e A2 e s1t s2t • Two simultaneous equation: i L (0) I f A1 A2 0 di L (0) s1 A1 s 2 A2 0 dt A1 32mA A2 8mA • Numerical solution: 24 32e iL (t ) 80000t 8e untuk t0 20000t mA Example 2 (step response of series RLC) • No energy is stored in the 100mH inductor or 0.4µF capacitor when switch in the circuit is closed. Find vC(t) for t ≥ 0. Solution • The roots of the characteristic equation: 2 280 10 280 s1 0.10.4 0 .2 0. 2 1400 j 4800 rad / s s 2 1400 j 4800 rad / s 6 • The roots are complex, so the voltage response is underdamped. Thus: 1400t vC 48 B1 e cos 4800t 1400t B2 e sin 4800t t0 • No energy is stored in the circuit initially, so both vC(0) and dvC(0+)/dt are zero. Then: vC (0) 0 48 B1 dvC (0 ) 0 4800 B2 1400 B1 dt • Solving for B1’and B2’yields, B1 48V B2 14V • Thus, the solution for vC(t), 48 48 e 1400t cos 4800 t V vC (t ) 14 e 1400t sin 4800 t for t0