E E 2415
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Transcript of E E 2415
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E E 2415
Lecture 7Natural and Step Responses of RL and RC Circuits
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Conservation of Charge (1/4)
• Energy transferred if v10 v20
• Total system charge is conserved
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Conservation of Charge (2/4)
Initial stored energy:
At equilibrium:
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Conservation of Charge (3/4)
Initial Charge:Final Charge:
Since
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Conservation of Charge (4/4)
Final stored energy:
Energy consumed in R:
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Conservation of Flux Linkage (1/3)
• Energy transferred if i10 i20
• Total system flux linkage is conserved.
Initial stored energy:
At equilibrium:
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Conservation of Flux Linkage (2/3)
Initial flux linkage:
Final flux linkage:
Since
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Final stored energy:
Energy consumed in R:
Conservation of Flux Linkage (3/3)
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Natural RL Response (1/2)
• Inductor has initial current, io.• Switch opens at t = 0• Inductor current can’t change
instantaneously
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Natural RL Response (2/2)
KVL:
Separatethe variables:
Integrate:
Exponential of both sides:
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Natural RC Response (1/2)• Capacitor has initial voltage, vo.
• Switch closes at t = 0.• Capacitor voltage can’t change
instantaneously
KCL:
Separate the variables:
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Natural RC Response (2/2)
Integrate:
Exponential of both sides:
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RL Step Response (1/4)
• Make-before-break switch changes from position a to b at t = 0.
• For t < 0, Io circulates unchanged through inductor.
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RL Step Response (2/4)
• For t > 0, circuit is as below.• Initial value of inductor current, i, is Io.• The KVL equation provides the
differential equation.
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RL Step Response (3/4)
Solution has two parts:
Steady State Response
Transient Response
Determine k by initial conditions:
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RL Step Response (4/4)
• Inductor behaves as a short circuit to DC in steady state mode
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RC Step Response (1/3)• Switch closes at t = 0.
• Capacitor has initial voltage, Vo.
v-i relationship:
By KVL & Ohm’s Law:
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RC Step Response (2/3)
• Response has two parts– steady state– transient
• Use initial voltage to determine transient
Steady State Response Transient Response
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RC Step Response (3/3)
• Capacitor becomes an open circuit to DC after the transient response has decayed.
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Unbounded Response (1/5)
• Need Thévenin equivalent circuit from terminal pair connected to inductor
• Let initial current = 0A in this example.
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Unbounded Response (2/5)
Voltage divider to get vx:
Then Thévenin voltage
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Unbounded Response (3/5)
Therefore:
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Unbounded Response (4/5)
Steady state:
Transient:
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Unbounded Response (5/5)
Use initial conditions to determine k.
Complete response is unbounded: