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Lecture (03)First order Circuit (continue,..)
By:Dr. Ahmed ElShafee
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١
Step Response of an RC Circuit• When the dc source of an RC circuit is suddenly applied, the
voltage or current source can be modeled as a step function, and the response is known as a step response.
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢
• We assume an initial voltage V0 on the capacitor .
• Applying KCL,
• For t>0
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣
• Rearranging terms gives
• From integration table
• Integrating both sides and introducing the initial conditions
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٤
• Taking the exponential of both sides
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٥
• capacitor is uncharged initially,
• Calculating current
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٦
• v(t) has two components:– “natural response – Forced response’’
• forced response because it is produced by the circuit when an external “force’’ (a voltage source in this case) is applied.
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٧
• Forced response break to – “transient response – steady‐state response.’’
• Dies: – natural response eventually – transient component of the forced response, l
• Leaving only – the steady state component of the forced response.
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٨
• Another way to presents complete response
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٩
•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٠
• Note that if the switch changes position at time t0 instead of t=0
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١١
Example 01•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٢
• @ t<0, switch @ A• voltage division
• capacitor voltage cannot change instantaneously
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٣ Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٤
• t>0, switch @ B• Use thevenin theorem to find
Req connected to Cap
• Time constant
• Cap act ac open circuit @ steady state,
• as
•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٥
Example 02•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٦
• For t<0, switch is open, cap is open
• For t>0• Find Req
• Find time constant
• Find v(∞), the current flow through cap=0 (open) then
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٧
1
•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٨
Example 03•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II١٩
• For t<0
• For t>0• Find v(∞) Use V.D.
• The Thevenin resistance at the capacitor terminals
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٠
• Time constant
• To find current, that i is the sum of the currents through the resistor and the capacitor;
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢١
Example 04•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٢
• For t<0, switch is open,Cap is open
• For t>0, switch is on, voltage source =0
• Find v(∞), CD
5•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٣
u(-t)
• Time constant
• current
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٤
Step Response of an RL Circuit•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٥
• At t=0• we obtain A as• Substituting for A
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٦
• This is the complete response of the RL circuit
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٧
• If I0=0
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٨
• The voltage across the inductor is
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٢٩
Example 05•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٠
• t<0, switch on, 3 ohm is neglected, coil is short
• t>0, switch is off• Calculate current using ohm law, coil
is short
• Calculate Rth
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣١
• Calculate time constant
• Complete response
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٢
Example 06•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٣
• At t<0, the switch is closed so that the 5 ohm resistor is short circuited
• At t>0, switch is on, • Find current using CD:
• Find Req
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٤
•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٥
Example 07•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٦
• t<0
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٧
• 0<t<4
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٨
• Use KCL @ p
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٣٩
•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٤٠
Example 08•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٤١
• At t<0, S1, S1 are open
• At 0<t<2, S1 close, S2 open• Calculate current using CD
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٤٢
• At t>2,
• Calculate I(∞), using CD
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٤٣
•
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٤٤
Thanks,..See you next week (ISA),…
Dr. Ahmed ElShafee, ACU : Spring 2016, Electric Circuits II٤٥