ECEN 301 Discussion #15 – 1 st Order Transient Response 1 Good, Better, Best Elder Oaks (October 2007) : As we consider various choices, we should remember that it is not enough that something is good. Other choices are better, and still others are best. Even though a particular choice is more costly, its far greater value may make it the best choice of all. I have never known of a man who looked back on his working life and said, “I just didn't spend enough time with my job.”
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ECEN 301 Discussion #15 – 1st Order Transient Response 1
Good, Better, BestElder Oaks (October 2007): As we consider various choices, we should remember
that it is not enough that something is good. Other choices are better, and still others are best. Even though a particular choice is more costly, its far greater value may make it the best choice of all.
I have never known of a man who looked back on his working life and said, “I just didn't spend enough time with my job.”
ECEN 301 Discussion #15 – 1st Order Transient Response 2
Lecture 15 – Transient Response of 1st Order Circuits
DC Steady-StateTransient Response
ECEN 301 Discussion #15 – 1st Order Transient Response 3
1st Order CircuitsElectric circuit 1st order system: any circuit containing
a single energy storing element (either a capacitor or an inductor) and any number of sources and resistors
Rs
R1
vs+–
L2
R2
L1
1st order2nd order
R1
R2 Cvs+–
ECEN 301 Discussion #15 – 1st Order Transient Response 4
Capacitor/Inductor Voltages/CurrentsReview of capacitor/inductor currents and voltages
• Exponential growth/decay
0.0
1.0
2.0
3.0
4.0
5.0
0.00 2.00 4.00 6.000.0
0.1
0.2
0.3
0.4
0.5
0.00 2.00 4.00 6.00
Capacitor voltage vC(t)Inductor current iL(t)
NB: neither can change instantaneously
Capacitor current iC(t)Inductor voltage vL(t)
NB: both can change instantaneously
dttdvCti )()(
t
C diC
tv )(1)(
dttdiLtv )()(
t
L dvL
ti )(1)(
NB: note the duality between inductors and capacitors
ECEN 301 Discussion #15 – 1st Order Transient Response 5
Transient ResponseTransient response of a circuit consists of 3 parts:
1. Steady-state response prior to the switching on/off of a DC source
2. Transient response – the circuit adjusts to the DC source3. Steady-state response following the transient response
R1
R2 Cvs+–
t = 0
DC Source
Switch Energy element
ECEN 301 Discussion #15 – 1st Order Transient Response 6
1. DC Steady State
1st and 3rd Step in Transient Response
ECEN 301 Discussion #15 – 1st Order Transient Response 7
DC Steady-StateDC steady-state: the stable voltages and currents in a
circuit connected to a DC source
voltagestatesteady as0)(
oltageinudctor v)()(
ttvdttdiLtv
L
LL
current statesteady as0)(
currentcapacitor )()(
ttidttdvCti
C
CC Capacitors act like open
circuits at DC steady-state
Inductors act like short circuits at DC steady-state
ECEN 301 Discussion #15 – 1st Order Transient Response 8
DC Steady-StateInitial condition x(0): DC steady state before a switch is first
activated• x(0–): right before the switch is closed• x(0+): right after the switch is closed
Final condition x(∞): DC steady state a long time after a switch is activated
R1
R2
Cvs
+–
t = 0
R3
R1
R2
Cvs
+–
t → ∞
R3
Initial condition Final condition
ECEN 301 Discussion #15 – 1st Order Transient Response 9
DC Steady-State• Example 1: determine the final condition capacitor voltage
• vs = 12V, R1 = 100Ω, R2 = 75Ω, R3 = 250Ω, C = 1uF
R1
R2
Cvs
+–
t = 0
R3
ECEN 301 Discussion #15 – 1st Order Transient Response 10
DC Steady-State• Example 1: determine the final condition capacitor voltage
• vs = 12V, R1 = 100Ω, R2 = 75Ω, R3 = 250Ω, C = 1uF
Vvv CC
0)0()0(
R1
R2
Cvs
+–
t → 0+
R3
1. Close the switch and find initial conditions to the capacitor
NB: Initially (t = 0+) current across the capacitor changes instantly but voltage cannot change instantly, thus it acts as a short circuit
ECEN 301 Discussion #15 – 1st Order Transient Response 11
DC Steady-State• Example 1: determine the final condition capacitor voltage
• vs = 12V, R1 = 100Ω, R2 = 75Ω, R3 = 250Ω, C = 1uF
VvvC
57.8)()( 3
R1
R2
Cvs
+–
t → ∞
R3
2. Close the switch and apply finial conditions to the capacitor
NB: since we have an open circuit no current flows through R2
V
vRR
Rv s
57.8
)12(350250
)(
:divider Voltage
31
33
ECEN 301 Discussion #15 – 1st Order Transient Response 12
DC Steady-StateRemember – capacitor voltages and inductor currents
cannot change instantaneously• Capacitor voltages and inductor currents don’t change right
before closing and right after closing a switch
)0()0(
)0()0(
LL
CC
ii
vv
ECEN 301 Discussion #15 – 1st Order Transient Response 13
DC Steady-State• Example 2: find the initial and final current conditions
at the inductoris = 10mA
is
t = 0
RLiL
ECEN 301 Discussion #15 – 1st Order Transient Response 14
DC Steady-State• Example 2: find the initial and final current conditions
at the inductoris = 10mA
is
t = 0
RLiL
1. Initial conditions – assume the current across the inductor is in steady-state.
isiL
NB: in DC steady state inductors act like short circuits, thus no current flows through R
ECEN 301 Discussion #15 – 1st Order Transient Response 15
DC Steady-State• Example 2: find the initial and final current conditions
at the inductoris = 10mA 1. Initial conditions – assume the current
across the inductor is in steady-state.
mAii sL
10)0(is
iL
ECEN 301 Discussion #15 – 1st Order Transient Response 16
DC Steady-State• Example 2: find the initial and final current conditions
at the inductoris = 10mA
is
t = 0
RLiL
1. Initial conditions – assume the current across the inductor is in steady-state.
2. Throw the switch
NB: inductor current cannot change instantaneously
RLiL
ECEN 301 Discussion #15 – 1st Order Transient Response 17
DC Steady-State• Example 2: find the initial and final current conditions
at the inductoris = 10mA
is
t = 0
RLiL
1. Initial conditions – assume the current across the inductor is in steady-state.
2. Throw the switch3. Find initial conditions again (non-steady state)
NB: inductor current cannot change instantaneously
–R+
LiL
NB: polarity of R
mAii LL
10)0()0(
ECEN 301 Discussion #15 – 1st Order Transient Response 18
DC Steady-State• Example 2: find the initial and final current
conditions at the inductoris = 10mA
is
t = 0
RLiL
1. Initial conditions – assume the current across the inductor is in steady-state.
2. Throw the switch3. Find initial conditions again (non-steady state)4. Final conditions (steady-state)
NB: since there is no source attached to the inductor, its current is drained by the resistor R
AiL 0)(
ECEN 301 Discussion #15 – 1st Order Transient Response 19
2. Adjusting to Switch
2nd Step in Transient Response
ECEN 301 Discussion #15 – 1st Order Transient Response 20
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
dttdv
Rti
RCdttdi S
CC )(1)(1)(
+ R –
+C–
iCvs+–
)(1)(1)( tvRC
tvRCdt
tdvSC
C NB: Review lecture 11 for derivation of these equations
ECEN 301 Discussion #15 – 1st Order Transient Response 21
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
dttdv
Rti
RCdttdi S
CC )(1)(1)(
)(1)(1)( tvRC
tvRCdt
tdvSC
C
ECEN 301 Discussion #15 – 1st Order Transient Response 22
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
dttdv
Rti
RCdttdi S
CC )(1)(1)(
)(1)(1)( tvRC
tvRCdt
tdvSC
C
NB: Constants
ECEN 301 Discussion #15 – 1st Order Transient Response 23
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
dttdv
Rti
RCdttdi S
CC )(1)(1)(
)(1)(1)( tvRC
tvRCdt
tdvSC
C
NB: similarities
ECEN 301 Discussion #15 – 1st Order Transient Response 24
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
)()()(:generalIn
001 tybtxadttdxa
ECEN 301 Discussion #15 – 1st Order Transient Response 25
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
)()()(:generalIn
001 tybtxadttdxa
Capacitor/inductor voltage/current
ECEN 301 Discussion #15 – 1st Order Transient Response 26
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
)()()(:generalIn
001 tybtxadttdxa
Forcing function(F – for DC source)
ECEN 301 Discussion #15 – 1st Order Transient Response 27
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
)()()(:generalIn
001 tybtxadttdxa
Combinations of circuit element parameters
(Constants)
ECEN 301 Discussion #15 – 1st Order Transient Response 28
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
Fabtx
dttdx
aa
0
0
0
1 )()(:written-re generalIn
)()()(:generalIn
001 tybtxadttdxa
Forcing Function F
ECEN 301 Discussion #15 – 1st Order Transient Response 29
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
Fabtx
dttdx
aa
0
0
0
1 )()(:written-re generalIn
FKtxdttdx
S )()(:written-re generalIn
DC gain
ECEN 301 Discussion #15 – 1st Order Transient Response 30
General Solution of 1st Order Circuits• Expressions for voltage and current of a 1st
order circuit are a 1st order differential equation
Fabtx
dttdx
aa
0
0
0
1 )()(:written-re generalIn
FKtxdttdx
S )()(:written-re generalIn
Time constantDC gain
ECEN 301 Discussion #15 – 1st Order Transient Response 31
General Solution of 1st Order CircuitsThe solution to this equation (the complete response)
consists of two parts: • Natural response (homogeneous solution)
• Forcing function equal to zero• Forced response (particular solution)
FKtxdttdx
S )()(
ECEN 301 Discussion #15 – 1st Order Transient Response 32
General Solution of 1st Order CircuitsNatural response (homogeneous or natural solution)
• Forcing function equal to zero
)()(
0)()(
txdttdx
txdttdx
NN
NN
/)( tN etx
Has known solution of the form:
ECEN 301 Discussion #15 – 1st Order Transient Response 33
General Solution of 1st Order Circuits
Forced response (particular or forced solution)
FKxtx SF )()(
FKtxdttdx
SFF )()(
F is constant for DC sources, thus derivative is zero
NB: This is the DC steady-state solution
FKtx SF )(
ECEN 301 Discussion #15 – 1st Order Transient Response 34
General Solution of 1st Order Circuits
Complete response (natural + forced)
)(
)()()(
/
/
xe
FKe
txtxtx
t
St
FN
)()0(
)()0(:for Solve
xxxx
Solve for α by solving x(t) at t = 0
)()]()0([)( / xexxtx t
Initial conditionFinal condition
Time constant
ECEN 301 Discussion #15 – 1st Order Transient Response 35
General Solution of 1st Order Circuits
Complete response (natural + forced)
)()]()0([)( / xexxtx t
Transient Response Steady-State Response
ECEN 301 Discussion #15 – 1st Order Transient Response 36
3. DC Steady-State + Transient Response
Full Transient Response
ECEN 301 Discussion #15 – 1st Order Transient Response 37
Transient ResponseTransient response of a circuit consists of 3 parts:
1. Steady-state response prior to the switching on/off of a DC source
2. Transient response – the circuit adjusts to the DC source3. Steady-state response following the transient response
R1
R2 Cvs+–
t = 0
DC Source
Switch Energy element
ECEN 301 Discussion #15 – 1st Order Transient Response 38
Transient ResponseSolving 1st order transient response:
1. Solve the DC steady-state circuit: • Initial condition x(0–): before switching (on/off)• Final condition x(∞): After any transients have died out (t → ∞)