# Review of AC-circuit

Electronics - Devices

Oct 5, 2013 (4 years and 7 months ago)

170 views

Review of AC
-
circuit

Keys to deal with R, L, and C in an AC
-
circuit

Across R, voltage and current are in
-
phase

Across L, the current lags behind voltage by 90
o

Across C, the current leads voltage by 90
o

When two elements are in series

the same current
passing through them

When two elements are in parallel

the same voltage
across them

Also,

Basics

I and V are in
-
phase

I lags V by 90 degree

I leads V by 90 degree

R

L

C

RLC in series

Key: I
R
, I
L
, and I
C

are all in phase

D
V
L

~ X
L

D
V
C

~ X
C

D
V
R

~ R

D
V
max

~ Z

What if RLC are not in series?

Which of the following is true

(a)
D
V
c

=
D
V
L
,
I
C

= I
L

(b)
D
V
c

=
D
V
L
,
I
R

= I
L

(c)
D
V
c

=
D
V
R
,
I
R

= I
L

(d)
D
V
L

=
D
V
R
,
I
R

= I
L

How do you draw the phasor diagram?

D
V
L

D
V
R

I
R

= I
L

I
C

(e)
D
V
c

=
D
V
L
+
D
V
R
,
I
C

= I
L

(f)
D
V
c

=
D
V
L
+
D
V
R
,
I
R

= I
L

I
max

D
V
c

=
D
V
R
+
D
V
L

Power dissipation

Only R dissipates power

P =
I

*
D
V, always

For L and C,
I

and

D
V are 90
o

out of phase

P
L
= 0, P
C

= 0

For R,
I

and

D
V are in
-
phase

P
R
=
I
D
V =
I
rms
2
R

Resonance in RLC circuit in series

For a fixed AC voltage source,
maximum power dissipation occurs
when X
L

= X
C
, ie. When Z = R

P
R
=
I
D
V =
I
rms
2
R

Where
I
rms

= V
rms
/Z

i.e.