Electronics I lab
DC Power Supply
Objective
•
•
Design and construct circuits that transform sinusoidal (AC) voltages into constant
(DC) voltages.
Evaluate the performance of simple rectifier and regulator circuits.
Theory:
•
DC power supplies are the essential part of most of the electronic equipments
since most of the equipments need DC power for operation. The power which is
readily available everywhere is AC (in Gaza 220v/60Hz). DC power supplies
convert this AC power into DC that can be used to operate most of our
equipments. The main components of a DC power supply are rectifier diodes and
capacitors. As you have seen in the last lab that rectifier diodes under normal
condition can conduct only in the forward direction. This property of diodes is
used to remove (or invert) the negative cycle of the AC voltage so that we have
uni-directional voltage but not a constant one. Further, capacitors are used to
smooth out this varying unidirectional voltage and make it as smooth as possible,
since capacitor tries to resist the change in its induced voltage, a suitable value of
capacitor or a combination of capacitors are used to smooth out the change in the
uni-directional voltage and make it constant. The output of the power supply is a
fairly smooth DC voltage.
There are 4 main operations to mos t powe r supplies:
• Transfor mer (step do wn the voltage )
• Rectification (convert AC to pulsating DC)
• Filtering (Smooth pulsating DC to reduce peaks and raise valleys)
• Regulation (Control output voltage level to make it robust against input voltage
fluctuations and load changes)
Transformer :
•
•
•
The power transformer provides separate windings to isolate the primary and
secondary circuits. Isolation is necessary for safety considerations. Bizarre
accidents are possible with non-isolated systems.
A transformer also provides line voltage step down or up as needed by the DC
power supply. Using a transformer, voltages can be transformed from one level to
another with minimum power loss.
All transformers have a voltage rating, frequency rating, and volt-ampere rating ,
which is the maximum power (real or imaginary) that the secondary can deliver to
a load. This rating is a function of the physical size of the iron core. Ideally, the
load is resistive and the volt-ampere rating then represents the power in watts.
Rectification
The rectification process uses diode s to pe rmit current from the transformer in one
direction and block reverse current. The alternating current thus becomes
unidirectional current known as DC. At this po int the current is in pulses rather than
be ing continuo us and s moot h. But there is only one direction of the current thus it is
DC.There are three types of rectifier circuits; half-wave, full- wave center-tap, and
full-wave bridge.
Although we can use four individual power diodes to make a full wave bridge
rectifier, pre- made bridge rectifier components are available "off-the-shelf" in a range
of different voltage and current sizes that can be soldered directly into a PCB circuit
board or be connected by spade connectors. The image to the right shows a typical
single phase bridge rectifier with one corner cut off. This cut-off corner indicates that
the terminal nearest to the corner is the positive or +ve output terminal or lead with
the opposite (diagonal) lead being the negative or -ve output lead. The other two
connecting leads are for the input alternating voltage from a transformer secondary
winding.
The Smoothing Capacitor
Pulsating DC voltages are generally not useful for electronics. It is preferred to have a
relatively constant DC voltage. The filter is frequently just a large capacitance to
smooth
the raw voltage fluctuations from the rectifier. More advanced designs will include a
series inductor in addition to the capacitor for significantly improved filtering and also
for improved efficiency in the rectification process.
The filter capacitance is usually of the electrolytic type in order to keep the physical
size to a minimum. The smoothing capacitor converts the full- wave rippled output of
the rectifier into a smooth DC output voltage. Two important parameters to consider
when choos ing a suitable a capacitor are its Working Voltage, which must be higher
tha n the no- load output value of the rectifier and its Capacitance Value, which
determines the amount of ripple that will appear superimposed on top of the DC
voltage. Too low a value and the capacitor has little effect. As a general rule of
thumb, we are looking to have a ripple voltage of less than 100mV peak to peak.
Ripple factor :
The ipe factor is an indication of the effectiveness of a filter and is defined as
r=
V r ( pp )
V DC
where V r ( pp ) is the peak-to-peak ripple voltage and Vdc is the dc(average )value of
the filter's output voltage .the lower the ripple factor the better the filter .the ripple
factor can be lowered by increasing the value of the filter capacitor .
V r ( pp )
V DC
1
)V p ( rect )
≅(
fR l C
1
)V p ( rect )
≅ (1 −
2fR l C
Surge Curre nt in Capacitor Input Filter
When the power is first applied to a power supply, the filter capacitor is uncharged.
At the instant the switch is closed, voltage is connected to the rectifier and the
uncharged capacitor appears as a short , An initial surge of current is produced
through the forward-biased diodes. It is possible that the surge current could destroy
the diodes, for this reason a surge-limiting resistor R surge, is sometimes connected.
The value of this resistor must be small to avoid a significant voltage drop across it.
The diode must have a forward current rating that can ha ndle the momentary surge of
current. This rating Is specified in the datasheet of the diode as I FSM
R surge =
V p (sec) − 1.4
I FSM
Voltage Regulator
While filers can reduce the ripple from power supplies to a low value , the most
effective a approach is a combination of a capacitor –input filter used with a voltage
regulator .A voltage regulator is connected to the output of a filtered rectifier and
maintains a constant output voltage despite changes in the input ,the load current ,or
the temperature .The capacitor-input filter reduces the input ripple to the regulator to
an acceptable level .
Most regulators are integrated circuits and have three terminal –an input terminal ,an
output terminal ,and a reference terminal .The input to the regulator is first filtered
with a capacitor to reduce the ripple to <10%.the regulator reduces the ripple to a
negligible amount .In addition ,most regulators have an internal voltage reference
,short circuit protection . Some common voltages that these are available for include
+5, +12,+15,-5,-12, and-15.Three-terminal regulator s designed for fixed output
voltages require only external capacitors to complete the regulation portion of the
power supply .Filtering is accomplished by a large value capacitor between the input
voltage and ground .An output capacitor (typically .1 uF to 1uF)is connected from
output to ground to improve the transient response .
Worksheet
•
Study the effect of the capacitor
1-Change the value of C1 :1n ,100n,1u,50u,100u,470u.
For every value compute the ripple factor and plot the output voltage
D1
D2
D1N4007
V
D1N4007
V1
VOFF = 0
VAMPL = 17
FREQ = 60
V+
R1
C1
1k
100u
V-
D3
D1N4007
D4
D1N4007
0
•
Study the effect of regulator.
Plot the output
Note : regulator from OPAMP library
D1N4007
1
IN
D1N4007
C2
V1
VOFF = 0
VAMPL = 17
FREQ = 60
.33u
C1
100u
D3
D1N4007
D4
D1N4007
0
OUT
3
D2
D1
GND
U1
LM7805C
2
C3
.1u
R1
1k