# Practical Electronics/Parallel RC

## Parallel RC

### Circuit Impedance

${\displaystyle {\frac {1}{Z}}={\frac {1}{Z_{R}}}+{\frac {1}{Z_{C}}}}$
${\displaystyle {\frac {1}{Z}}={\frac {1}{R}}+j\omega C={\frac {j\omega CR+1}{R}}}$
${\displaystyle Z=R{\frac {1}{j\omega CR+1}}}$

### Circuit Response

${\displaystyle I=I_{R}+I_{C}}$
${\displaystyle I={\frac {V}{R}}+C{\frac {dV}{dt}}}$
${\displaystyle V=(IR-RC{\frac {dV}{dt}})}$

## Parallel RL

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### Circuit Impedance

${\displaystyle {\frac {1}{Z}}={\frac {1}{Z_{R}}}+{\frac {1}{Z_{L}}}}$
${\displaystyle {\frac {1}{Z}}={\frac {1}{R}}+{\frac {1}{j\omega L}}={\frac {R+j\omega L}{j\omega RL}}}$
${\displaystyle Z={\frac {j\omega RL}{R+j\omega L}}=j\omega L{\frac {1}{1+j\omega {\frac {L}{R}}}}}$

### Circuit Response

${\displaystyle I=I_{R}+I_{L}}$
${\displaystyle I={\frac {V}{R}}+{\frac {1}{L}}\int Vdt}$
${\displaystyle V=IR-{\frac {R}{L}}\int Vdt}$

## Parallel LC

### Circuit Impedance

${\displaystyle {\frac {1}{Z}}={\frac {1}{Z_{L}}}+{\frac {1}{Z_{C}}}}$
${\displaystyle {\frac {1}{Z}}={\frac {1}{j\omega L}}+j\omega C={\frac {(j\omega )^{2}LC+1}{j\omega L}}}$
${\displaystyle Z={\frac {j\omega L}{(j\omega )^{2}LC+1}}}$

### Circuit response

${\displaystyle I=I_{L}+I_{C}}$
${\displaystyle I={\frac {1}{L}}\int Vdt+C{\frac {dV}{dt}}}$

## Parallel RLC

### Circuit Impedance

${\displaystyle {\frac {1}{Z}}={\frac {1}{Z_{R}}}+{\frac {1}{Z_{L}}}+{\frac {1}{Z_{C}}}}$
${\displaystyle {\frac {1}{Z}}={\frac {1}{R}}+{\frac {1}{j\omega L}}+j\omega C}$
${\displaystyle {\frac {1}{Z}}={\frac {(j\omega )^{2}LC+j\omega L+R}{j\omega RL}}}$
${\displaystyle {\frac {1}{Z}}={\frac {(j\omega )^{2}{\frac {LC}{R}}+j\omega {\frac {L}{R}}+1}{j\omega L}}}$

### Circuit response

${\displaystyle I=I_{R}+I_{L}+I_{C}}$
${\displaystyle I={\frac {V}{R}}+{\frac {1}{L}}\int Vdt+C{\frac {dV}{dt}}}$
${\displaystyle I={\frac {V}{R}}+{\frac {1}{L}}\int Vdt+C{\frac {dV}{dt}}}$
${\displaystyle V=IR-{\frac {R}{L}}\int Vdt-CR{\frac {dV}{dt}}}$

#### Natural Respond

${\displaystyle 0={\frac {V}{R}}+{\frac {1}{L}}\int Vdt+C{\frac {dV}{dt}}}$

#### Forced Respond

${\displaystyle I_{t}=IR+L{\frac {dI}{dt}}+{\frac {1}{C}}\int Idt}$

Second ordered equation that has two roots

ω = -α ± ${\displaystyle {\sqrt {\alpha ^{2}-\beta ^{2}}}}$

Where

${\displaystyle \alpha ={\frac {R}{2L}}}$
${\displaystyle \beta ={\frac {1}{\sqrt {LC}}}}$

The current of the network is given by

A eω1 t + B eω2 t

From above

When ${\displaystyle {\alpha ^{2}=\beta ^{2}}}$, there is only one real root
ω = -α
When ${\displaystyle {\alpha ^{2}>\beta ^{2}}}$, there are two real roots
ω = -α ± ${\displaystyle {\sqrt {\alpha ^{2}-\beta ^{2}}}}$
When ${\displaystyle {\alpha ^{2}<\beta ^{2}}}$, there are two complex roots
ω = -α ± j${\displaystyle {\sqrt {\beta ^{2}-\alpha ^{2}}}}$

#### Resonance Response

At resonance, the impedance of the frequency dependent components cancel out . Therefore the net voltage of the circui is zero

${\displaystyle Z_{L}-Z_{C}=0}$ and ${\displaystyle V_{L}+V_{C}=0}$

${\displaystyle \omega L={\frac {1}{\omega C}}}$
${\displaystyle \omega ={\sqrt {\frac {1}{LC}}}}$
${\displaystyle Z=Z_{R}+(Z_{L}-Z_{C})=Z_{R}=R}$
${\displaystyle I={\frac {V}{R}}}$

At Resonance Frequency

${\displaystyle \omega ={\sqrt {\frac {1}{LC}}}}$ .
${\displaystyle I={\frac {V}{R}}}$ . Current is at its maximum value

Further analyse the circuit

At ω = 0, Capacitor Opened circuit . Therefore, I = 0 .
At ω = 00, Inductor Opened circuit . Therefore, I = 0 .

With the values of Current at three ω = 0 , ${\displaystyle {\sqrt {\frac {1}{LC}}}}$ , 00 we have the plot of I versus ω . From the plot If current is reduced to halved of the value of peak current ${\displaystyle I={\frac {V}{2R}}}$ , this current value is stable over a Frequency Band ω1 - ω2 where ω1 = ωo - Δω, ω2 = ωo + Δω

• In RLC series, it is possible to have a band of frequencies where current is stable, ie. current does not change with frequency . For a wide band of frequencies respond, current must be reduced from it's peak value . The more current is reduced, the wider the bandwidth . Therefore, this network can be used as Tuned Selected Band Pass Filter . If tune either L or C to the resonance frequency ${\displaystyle \omega ={\sqrt {\frac {1}{LC}}}}$ . Current is at its maximum value ${\displaystyle I={\frac {V}{R}}}$ . Then, adjust the value of R to have a value less than the peak current ${\displaystyle I={\frac {V}{R}}}$ by increasing R to have a desired frequency band .

• If R is increased from R to 2R then the current now is ${\displaystyle I={\frac {V}{2R}}}$ which is stable over a band of frequency
ω1 - ω2 where
ω1 = ωo - Δω
ω2 = ωo + Δω

For value of I < ${\displaystyle I={\frac {V}{2R}}}$ . The circuit respond to Wide Band of frequencies . For value of ${\displaystyle I={\frac {V}{R}}}$ < I > ${\displaystyle I={\frac {V}{2R}}}$ . The circuit respond to Narrow Band of frequencies

## Summary

Circuit Symbol Series Parallel
RC
A parallel RC Circuit
Impedance Z ${\displaystyle Z_{t}=R+{\frac {1}{\omega C}}={\frac {\omega CR+1}{\omega C}}}$ ${\displaystyle {\frac {1}{Z_{t}}}={\frac {1}{Z_{R}}}+{\frac {1}{Z_{C}}}={\frac {1}{R}}+\omega C={\frac {R}{\omega CR+1}}}$
Frequency ${\displaystyle \omega _{o}=2f_{o}}$ ${\displaystyle Z_{R}=Z_{C}}$
${\displaystyle R={\frac {1}{\omega C}}}$
${\displaystyle \omega ={\frac {1}{CR}}}$
${\displaystyle {\frac {1}{R}}={\frac {1}{\omega C}}}$
${\displaystyle {\frac {1}{R}}=\omega C}$
${\displaystyle \omega ={\frac {1}{CR}}}$
Voltage V ${\displaystyle V=IR+{\frac {1}{C}}\int Idt}$ ${\displaystyle I={\frac {V}{R}}+C{\frac {dV}{dt}}}$
Current I ${\displaystyle \int Idt=C(V-IR)}$ ${\displaystyle {\frac {dV}{dt}}={\frac {1}{C}}(I-{\frac {V}{R}})}$
Phase Angle Tan θ = 1/2πf RC
f = 1/2π Tan CR
t = 2π Tan CR
Tan θ = 1/2πf RC
f = 1/2π Tan CR
t = 2π Tan CR