RC Circuit
Part
04
Special Topics
Course instructor
Dr. Maryam Liaqat
Group Members
Group #3
1.Naeem
2.Shahzaib
3.H.M.Ayyan
4.Shahid Ali
5.Ayesha Qudrat
6.Noor ul Ain
7.Javeed Iqbal
Objectives
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Explain true, reactive, and apparent power.
Describe power triangle.
Define Power Factor
Significance of Apparent power
Rc Circuit as a Filter
Discuss About Ac Coupling.
Power in RC Circuits
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When there is both resistance and
capacitance, some of the energy is alternately
stored and returned by the capacitance and
some is dissipated by the resistance.
The amount of energy converted to heat is
determined by the relative values of the
resistance and the capacitive reactance.
The Power in the resistor is true power:
Ptrue = I2R
Power in RC Circuits
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Unit of true power is Watt
The Power in the capacitor is reactive
power:
Pr = I2XC
Unit of reactive power is VAR(Volt-ampere
reactive).
Power Triangle for RC Circuits
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The apparent power (Pa) is the resultant of
the average power (Ptrue) and the reactive
power (PR). Pa = I2totZ
Ptrue = PacosΘ
Power Factor
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The term cos , in the previous slide, is called
the power factor:
PF = cos 
The power factor can vary from 0 for a purely
reactive circuit to 1 for a purely resistive
circuit.
In an RC circuit, the power factor is referred
to as a leading power factor because the
current leads the voltage.
Significance of Apparent
Power
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Apparent power is the power that appears to
be transferred between the source and the
load.
Apparent power consists of two components;
a true power component, that does the work,
and a reactive power component, that is
simply power shuttled back and forth
between source and load.
Apparent power is expressed in Volt-amperes (VA).
Basic Applications
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RC series circuit
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Used for filtering
Low Pass Filter
 High Pass Filter
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Low-Pass RC Filter
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A capacitor acts as an open to dc.
As the frequency is increased, the capacitive
reactance decreases.
As capacitive reactance decreases, output
voltage across the capacitor decreases.
A series RC circuit, where output is taken
across the capacitor, finds application as a
low-pass filter.
Low Pass Filter (frequency cutoff)
1
1
fc 

2 2RC
 = Time Constant = RC
fc = frequency cutoff
 Vout
Attenuation Gain  20 log
 Vin

 (dB)

High-Pass RC Filter
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For the case when output voltage is
measured across the resistor.
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At dc, the capacitor acts an open, so the
output voltage is zero.
As frequency increases, the capacitive
reactance decreases, resulting in more
voltage being dropped across the resistor.
The result is a high-pass filter.
High Pass Filter (Frequency cutoff)
1
1
fc 

2 2RC
 = Time Constant = RC
fc = frequency cutoff
 Vout
Attenuation Gain  20 log
 Vin

 (dB)

AC Coupling
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An RC network can be used to create a dc voltage
level with an ac signal superimposed on it.
This is often found in amplifiers, where the dc
voltage is required to bias the amplifier.
Summary
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In an RC circuit, part of the power is resistive
and part is reactive.
The phasor combination of resistive power
and reactive power is called apparent power.
Apparent power is expressed in volt-amperes
(VA).
The power factor indicates how much of the
apparent power is true power.
Summary
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A power factor of 1 indicates a purely
resistive circuit, and a power factor of 0
indicates a purely reactive circuit.
A filter passes certain frequencies and rejects
others.
An RC network can be used to create a dc
voltage level with an ac signal superimposed
on it.
Trouble shooting
Effect of an Open Capacitor
When the capacitor is open.
• There is no current.
• Thus, the resistor or voltage remains at zero.
• The total source voltage is across the open
capacitor.
Effect of shorted capacitor
Capacitors rarely short; but when a capacitor does short
out, the voltage across it is zero, the current equals ViR, and
the total voltage appears across the resistor, as shown in Figure