Electronics Handbook/Circuits/RLC Series Analysis

Consider an RLC series circuit

1. If L = 0 then the cicuit is reduced to RC series
2. If C = 0 then the cicuit is reduced to RL series
3. If R = 0 then the cicuit is reduced to LC series
4. If R, L , C are not zero

RC Series

• Differential Equation

$C{\frac {dV}{dt}}+{\frac {V}{R}}=0$

${\frac {dV}{dt}}=-{\frac {1}{RC}}V$

${\frac {1}{V}}dV=-{\frac {1}{RC}}dt$

$\int {\frac {1}{V}}dV=-\int {\frac {1}{RC}}dt$

ln V = $-{\frac {1}{RC}}+C$

$V=e^{-}({\frac {1}{RC}})t+C$

$V=Ae^{-}({\frac {1}{RC}})t$ với $A=e^{C}$

• Time Constant

t	V(t)	% Vo
0	A = eC = Vo	100%
${\frac {1}{RC}}$	.63 Vo	60% Vo
${\frac {2}{RC}}$	Vo
${\frac {3}{RC}}$	Vo
${\frac {4}{RC}}$	Vo
${\frac {5}{RC}}$	.01 Vo	10% Vo

• Circuit Impedance

Z/_θ

$Z=Z_{R}+Z_{C}$

Z = R /_0 + ( 1 / ωC ) /_ - 90

Z = = |Z|/_θ = ${\sqrt {R^{2}+({\frac {1}{\omega C}})^{2}}}$ /_ Tan^-1 ${\frac {1}{\omega RC}}$

Z(jω)

$Z=Z_{R}+Z_{C}$

Z = $R+{\frac {1}{j\omega C}}={\frac {1+j\omega RC}{j\omega C}}$

$Z={\frac {1}{X_{C}}}(1+j\omega T$)

• Angle Difference Between Voltage and Current

There is a difference in angle Between Voltage and Current . Current leads Voltage by an angle θ

$Tan\theta ={\frac {1}{\omega RC}}={\frac {1}{f}}{\frac {1}{2\pi RC}}=t{\frac {1}{2\pi RC}}$

The difference in angle between Voltage and Current relates to the value of R , C and the Angular of Frequency ω which also relates to f and t . Therefore when change the value of R or C , the angle difference will be changed and so are ω , f , t

$\omega ={\frac {1}{RC}}{\frac {1}{Tan\theta }}$

$f={\frac {1}{2\pi }}{\frac {1}{RC}}{\frac {1}{Tan\theta }}$

$t=2\pi RCTan\theta$

• First Order Equation of Circuit

$L{\frac {dI}{dt}}+IR=0$

${\frac {dI}{dt}}=-I{\frac {R}{L}}$

$\int {\frac {1}{I}}dI=-\int {\frac {L}{R}}dt$

ln I = $(-{\frac {L}{R}}+c)$

I = $e^{(}-{\frac {L}{R}}+c)t$

I = $e^{c}e^{(}-{\frac {L}{R}}t)$

I = $Ae^{(}-{\frac {L}{R}}t)$

• Time Constant RL

τ = ${\frac {L}{R}}$

I = A $e^{-}({\frac {L}{R}})t$

t	I(t)	% Io
0	A = eC = Io	100%
${\frac {1}{RC}}$	.63 Io	63% Io
${\frac {2}{RC}}$	Io
${\frac {3}{RC}}$	Io
${\frac {4}{RC}}$	Io
${\frac {5}{RC}}$	.01 Io	10% Io

• Circuit Impedance

$Z=Z_{R}+Z_{L}$ = R/_0 + ω L/_90

Z = |Z|/_θ = ${\sqrt {R^{2}+(\omega L)^{2}}}$/_Tan^-1$\omega {\frac {L}{R}}$

Z(jω)

$Z=Z_{R}+Z_{L}=R+j\omega L$

$Z=R+j\omega L=R(1+j\omega {\frac {L}{R}})$

• Angle of Difference Between Voltage and Current

In RL series circuit, only L is the component that depends on frequency . There is no difference between voltage and current on R . There is an angle difference between voltage and current by 90 degree . When connect R and L in series , there is a difference in angle between voltage and current from 0 to 90 degree which can be expressed as a mathematic formula below

$Tan\theta =\omega {\frac {L}{R}}=2\pi f{\frac {L}{R}}=2\pi {\frac {1}{t}}{\frac {L}{R}}$

$\omega =Tan\theta {\frac {R}{L}}$

$f={\frac {1}{2\pi }}Tan\theta {\frac {R}{L}}$

$t=2\pi {\frac {1}{Tan\theta }}{\frac {L}{R}}$

• In Summary

RL series circuit has a first order differential equation of current

Which has one real root of the form

$I(t)=Ae^{\frac {t}{T}}$

$A=e^{c}$

${\frac {d}{dt}}f(t)+{\frac {1}{T}}=0$

Which has solution in the form

$f(t)=Ae^{\frac {-t}{T}}$