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Chapter 2Alternating Current (AC)
Transformer
Example 2.1A step-up transformer has 25 turns on the primary coil and 500 turns on the secondary
coil. If this transformer is to produce an output of 4800 V with a 12-mA current, what input current and voltage are needed?
A 2.025
5001012 3
A
N
NII
p
ssp
V 200500
25V 4800
s
psp N
NVV
Impedance Matching
pp
sp V
N
N
RI
2
1
RN
NR
s
peq
2
Transmission of power
Alternating Current (AC) Circuits
tVV sinmax
tItR
V
R
VI sinsin max
max
Root Mean Square (rms) Values
tII 22max
2 sin
2max2
12 II av
max212 III avrms
max21 VVrms
Power in an AC circuit
tRIRIP 22max
2 sin
RIRIP rmsav22
max21
R
V
R
VP rmsav
22max
2
1
AC circuits and Impedance
1 / C L
= 2 f
Capacitors in AC circuits
C
rmsrms X
VI
L
rmsrms X
VI
Inductors in AC circuits
The resistor voltage is in phase with the current
The capacitor voltage 90o lags the current
The inductor voltage 90o leading the current
RC Circuits in AC RL Circuits in AC
2222 LRXRZ L 2
222 1
C
RXRZ C
Resonance in RLC circuits
LCCL
1or
1
LCfo 2
1
Selectivity and Q of a Circuit
Metal Detectors in AirportsThe metal detector is essentially a resonant circuit. The portal you step through is an inductor (a large loop of conducting wire) that is part of the circuit. The frequency of the circuit is tuned to the resonant frequency of the circuit when there is no metal in the inductor. When you walk through with metal in your pocket, you change the effective inductance of the resonance circuit, resulting in a change in the current in the circuit. This change in current is detected, and an electronic circuit causes a sound to be emitted as an alarm.