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Electronics 1
Lecture 6
Diode Circuit Analysis 2
Ahsan Khawaja
[email protected]
Lecturer
Room 102
Department of Electrical Engineering
AC Voltage
• The output of an AC power source is
sinusoidal and varies with time according to
the following equation:
– Δv = ΔVmax sin ωt
• Δv is the instantaneous voltage
• ΔVmax is the maximum output voltage of the source
• ω is the angular frequency of the AC voltage
AC Voltage, cont.
• The angular frequency is
2π
ω  2π ƒ 
T
– ƒ is the frequency of the source
– T is the period of the source
• The voltage is positive during one half
of the cycle and negative during the
other half.
•
Commercial electric power plants in the Pakistan use a frequency of 50 Hz.
– This corresponds with an angular frequency of _____ rad/s ???
Resistors in an AC Circuit
• Consider a circuit
consisting of an AC source
and a resistor
• The AC source is
symbolized by
• Δv = ΔvR = Δvmaxsin wt
• ΔvR is the instantaneous
voltage across the resistor
Resistors in an AC Circuit
• The instantaneous current in the resistor is
vR Vmax
iR 

sin ωt  I max sin ωt
R
R
• The instantaneous voltage across the
resistor is also given as
ΔvR = Imax R sin ωt
Resistors in an AC Circuit
• The graph shows the
current through and the
voltage across the
resistor.
• The current and the
voltage reach their
maximum values at the
same time.
• The current and the
voltage are said to be in
phase.
Resistors in an AC Circuit
• For a sinusoidal applied voltage, the current
in a resistor is always in phase with the
voltage across the resistor.
• The direction of the current has no effect on
the behavior of the resistor. Resistors
behave essentially the same way in both DC
and AC circuits.
Getting DC back out of AC, Why?
•
•
•
AC provides a means for us to distribute electrical power, but most devices
actually want DC
– bulbs, toasters, heaters, fans don’t care: plug straight in
– sophisticated devices care because they have diodes and transistors that
require a certain polarity rather than oscillating polarity derived from AC
this is why battery orientation matters in most electronics
Use diodes to “rectify” AC signal
Simplest (half-wave) rectifier uses one diode:
input voltage
AC source
load
diode only conducts
when input voltage is positive
voltage seen by load
Rectifier Circuit
• The arrow on the diode (
) indicates the
direction of the current in the diode.
• Because of the diode, the alternating current in the
load resistor is reduced to the positive portion of the
cycle.
A Half Wave Rectifier
Since the diode only allows current in
one direction, only the positive half of
the voltage is preserved.
Half-Wave Rectifier Circuit with
Resistive Load
For positive half-cycle of input, source forces positive current through
diode, diode is on, vo = vs.
During negative half cycle, negative current can’t exist in diode, diode is
off, current in resistor is zero and vo =0 .
12
Half-Wave Rectifier Circuit with
Resistive Load (contd.)
Using constant voltage model (second approx),
during on state of diode vo =(VP sinwt)- Vd,on.
Output voltage is zero when diode is off.
Often a step-down transformer is used to convert
240 V- 50 Hz voltage available from power line
to desired ac voltage level as shown.
Half-Wave Rectifier Circuit with
Resistive Load
vo  0,
vs  VDo
R
R
vo 
vs 
VDo ,
R  rD
R  rD
vs  VDo
Peak inverse voltage (PIV)
DC component and PIV
PIV is the maximum (peak) voltage that appears across the diode
when reverse biased.
Efficiency = 40.5 (Very low)
Doing Better: Full-Wave Rectification
The rectification process can be improved by
using a full-wave rectifier circuit.
Full-wave rectification produces a greater
DC output:
•
•
Half-wave: Vdc = 0.318Vm
Full-wave: Vdc = 0.636Vm
16
Full-Wave Rectifiers
A full-wave rectifier allows current to flow during both the positive
and negative half cycles or the full 360º.
Note that the output frequency is twice the input frequency.
Most power supplies use full-wave rectifiers. Half-wave rectifiers
see lesser applications like lo-cost power supplies.
The average VDC or VAVG = 2Vp/.
A Full Wave Rectifier
The rectifier we have just seen is called a halfwave rectifier since it only uses half of the
sinusoidal voltage. A full wave rectifier uses both
the negative and positive voltages.
Full-Wave Rectifier
Center-Tapped
This method of rectification employs two diodes connected to a centertapped transformer.
The peak output is only half of the transformer’s peak
secondary voltage.
Center-tap
Full-Wave Center Tapped
Note the current flow
direction during both
alternations.
The Full-Wave Bridge Rectifier
It employs four
diodes arranged such
that current flows in
the same direction
through the load
during each half of
the cycle.
A Full Wave Rectifier
1.4V (2 diodes)
10V
5V
0V
-5V
-10V
110.0ms
V(D5:2)
110.5ms
111.0ms
111.5ms
112.0ms
112.5ms
113.0ms
V(R4:2,D7:1)
Time
Note: Since a small voltage drop (around 0.7V)
now occurs over two diodes in each direction, the
voltage drop from a full wave rectifier is 1.4V.