Dual Power Supply Final
-
Upload
mahek19579328 -
Category
Documents
-
view
230 -
download
0
Transcript of Dual Power Supply Final
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 1/161
UNIT: 1
D.C.POWER SUPPLY
Introduction:
Most of the electronic devices and circuits require D.C. sources for their operation. Dry
cell and batteries are on form of the source but their voltages are low and they are expensive as
compared to conventional D.C. power supplies.
The regulated power supply is to provide the necessary dc voltage and current, with low
levels of ac ripple and with stability and regulation.
There are two types of power supplies
There are two types of power supplies:
(i) Unregulated power supply and
(ii) Regulated power supply.
i) Unregulated power supply:
In unregulated power supply as the A.C. input varies, the output D.C. voltage also varies.
As load varies the D.C. voltages also vary. The output D.C. voltage does not remain constant.
Such supply is called as unregulated supply.
The unregulated supply consists of three blocks
• Transformer
• Rectifier
• Filter
(Unregulated)
ii) Regulated power supply:
A regulated power supply will provide constant D.C. output voltage irrespective of line
voltage or load resistance variation. Such supply is called regulated power supply. An
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 2/161
unregulated power supply can be converted into a regulated power supply. The regulated power
supply can be made by adding a voltage regulator circuit.
A typical D.C. regulated power supply at least consists of the following Regulated
1. Transformer:
The job of the transformer is to step up or step down the A.C. voltage to suit the
requirements of the solid state device and circuit fed by the D.C. power supply. It also provides
isolation from the supply line which is an important safety consideration.
2. Rectifier:
It is a circuit which employs one or more diodes to convert A.C. voltage in to pulsating
D.C. voltage.
3. Filter:
The function of filter is to remove the fluctuation or pulsation (called Ripple) present in
the D.C. output voltage. Of course no filter can give an output voltage of D.C. battery in practice
but it approaches it so closely that the power supply performs well.
Voltage Regulator:
The function of voltage regulator is to keep yhe output voltage of D.C. supply constant
even when
• A.C. input to the transformer varies
• The transformer output vary upto certain value
• The load varies.
If at the output of D.C. regulated power supply different voltages are required a voltage
divider electronic circuits are used. It consists of a number of resistors connected in series across
the output terminal of the voltage regulator. Sometimes a potentiometer is included across the
output terminal so as to vary the output D.C. voltage.
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 3/161
UNIT: 2
TRANSFORMERS
These are components which only work with alternating currents and are used to
transform or change an alternating voltage up or down.
Working Principle of a Transformer
A transformer is a device that
(i) Transfer electric power from one circuit to another
(ii) It does so without a change of frequency
(iii) It accomplishes this by electromagnetic induction and(iv) Is when the two electric circuits are in mutual, inductive of each other.
Half-wave Rectifier with Transformer Coupled Input Voltage
RL
Vin VPri VSec
Fig: (2.1) Half-wave Rectifier with Transformer Coupled Input Voltage
A transformer is often used to couple the ac input voltages from the source to the
rectifier circuits. Transformer coupling provides two advantages. First, it allows the source
voltage to be stepped up or stepped down as needed. Second, the ac power source is electrically
isolated from the rectifier circuit, thus reducing the shock hazard.
Basic ac circuit recall that the secondary voltage of a transformer equals the turns ratio
i
Sec
N N
Pr times the primary voltage.
If NSec > NPri, the secondary voltage is greater than the primary voltage.
ii
Sec V N
N V Pr
Pr sec )(=
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 4/161
If NSec < NPri, the secondary voltage is less than the primary voltage. If NSec = NPri, then
VSec = VPri.
UNIT: 3
RECTIFIER CIRCUITS
Rectifier circuits are found in all dc power supplies that operate from an ac voltage
source. They convert the ac input voltage to a pulsating dc voltage. The most basic type of
rectifier circuit is the half-wave rectifier. Although half-wave rectifiers have some applications,
the full-wave rectifiers are the most commonly used type in dc power supplies. These are two
types of full-wave rectifiers:
(1) full-wave center-tapped rectifier
(2) full-wave bridge rectifier
Full-wave Center-tapped Rectifier
The full-wave center-tapped rectifier uses two diodes connected to the secondary of a
center-tapped transformer, as shown in Fig: (3.1). The input voltage is coupled through the
transformer to the center-tapped secondary. Half of the total secondary voltage appears between
the center tap and each end of the secondary winding.
CT
D1
D2
RL
2
secV
2
secV
Vin
-
-
+
+
F
Fig: (3.1) A Full-wave Center-tapped Rectifier
For a positive half-cycle of the input voltage, the polarities on the secondary are as shown
in Fig: (3.2.a). This condition forward-biases the diode D1 and reverse-biases the diode D2 .The
current path is through D1 and the load resistor R L, as indicated.
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 5/161
D1
D2
RL
Vin
-
+
+
F
-
+
- +
+ -
0
0
Vout
Fig: (3.2.a) During positive half-cycles, D1 is forward-biased and D2 is reverse-biased.
For a negative half-cycle of the input voltage, the voltage polarities on the secondary are
as shown in Fig: (3.2.b). This condition reverse-biases D1 and forward-biases D2 .The current
path is through D2 and R L, as indicated. Because the output current during both the positive and
negative portions of the input cycle is in the same direction through the load, the output voltage
developed across the load resistor is a full-wave rectified dc voltage.
D1
D2
RL
Vin
F
-
+
- +
+ -
0
0
Vout
Fig: (3.2.b) During negative half-cycles, D2 is forward-biased and D1 is reverse-biased.
• Peak value of output voltage for the full-wave center-tapped rectifier:
V sceVp
out Vp 7.02
)()( −=
• Average value of output for the full-wave center-tapped rectifier:
π
)(2 out Vp AGV V =
• Diode peak inverse voltage for the full-wave center-tapped rectifier:
V V PIV out P 7.02 )( +=
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 6/161
Full-wave Bridge Rectifier
The full –wave bridge rectifier uses four diodes, as shown in Fig: (3.3.a). When the input
cycle is positive, diodes D1 and D2 are forward-biased and conduct current through R L. During
this time, diodes D3 and D4 are reverse-biased.
RL
Vin
F
-
+ 0
Vout
D1
D4D2
D3
Fig: (3.3.a) During positive half-cycles of the input, D1 and D2 are forward-biased and conduct current, D3 and D4
are reverse-biased.
When the input cycle is negative as shown in Fig: (3.3.b), diodes D3 and D4 are forward-
biased and conduct current in the same direction through R L as during the positive half-cycle.
During the negative half-cycle, D1 and D2 are reverse-biased. A full-wave rectifier output voltage
appears across R L as a result of this action.
RL
Vin
F
-
+ 0
Vout
-
+
-
+
D1
D4
D2
D3
Fig: (3.3.b) During negative half-cycles of the input, D3 and D4 are forward-biased and conduct current, D1 andD2 are reverse-biased.
• Peak value of output voltage for the full-wave bridge rectifier:
V Vpout Vp 4.1(sec))( −=
• Average value of output voltage for the full-wave bridge rectifier:
π
)(2 out VpV AVG =
• Diode peak inverse voltage for the full-wave bridge rectifier:
PIV = VP (out) + 0.7
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 7/161
UNIT: 4
POWER SUPPLY FILTER
A power supply filter ideally eliminates the fluctuations in the output voltage of a half –
wave rectifier and produces a constant-level dc voltage. The 60Hz pulsating dc output of a half-
wave rectifier or the 120Hz pulsating output of a full-wave rectifier must be filtered to reduce the
large voltage variations. Fig: (4.1) illustrates the filtering concepts showing a nearly smooth dc
output voltage from the filter. The small amount of fluctuation in the filter output voltage is
called ripple.
Vin
0 VFull-wave
Rectifier Filter
0 VVout
0
Fig: (4.1) Power supply filteringCapacitor Filter
A half wave rectifier with a capacitor filter is shown in Fig: 4.2. During the positive first
quarter-circle of the input, the diode is forward bias and presents a low resistance path, allowing
the capacitor to charge to within 0.7V of the input peak. When the input begins to decrease
below its peak, the capacitor retains its charge and the diode becomes reversed biased since the
cathode is more positive than the anode. During the remaining part of the cycle, the capacitor can
discharge only through the load resistor at a rate determines by the R LC time constant.
RLVin
+
-
+
-
+
-
+ -
0
VP(in)
t0
0t0
VP(in) –0.7VVc
Fig: (4.2) (a) Initial charging of capacitor (diode is forward-biased) happens only once when power is turn on.
RLVin
+
-
+
-
+ -
0
VP(in)
t0 0t0
Vc
t1
t1
Fig: (4.2) (b) Discharging through R L after peak of positive alternation (diode is reverse biased)
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 8/161
RLVin
+
-
+
-
+ -
0
VP(in)
t0 0t0
Vct1
t1
t2t2
Fig: (4.2) (c) Charging back to peak of input (diode is forward-biased)
Fig: (4.2) Operation of a half-wave rectifier with a capacitor filter
Ripple Voltage
The capacitor quickly at the beginning of a cycle and slowing discharges after the
positive peak. The variation in the output voltage due to charging and discharging is called the
ripple voltage.
0
Ripple
(a) Half-wave
0
Ripple
(b) Full-wave
Same Slope
Fig: (4.3) Comparison of ripple voltage for half-wave and full-wave signals with the same filter
capacitor and load and derived from same sine wave input
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 9/161
UNIT: 5
THE VOLTAGE REGULATORS
There are many type of circuit to regulate a certain dc voltage. Discrete circuits can be
constructed using feedback transistors to get a voltage regulator. There also exits many IC types
of voltage regulators. The well-known types of voltage regulator ICs are;
(1) The 78XX series - for positive regulators
(2) The 79XX series - for negative regulators
(3) The LM 317 - for adjustable positive regulators
(4) The LM 337 - for adjustable negative regulators
Fixed Positive Linear Voltage Regulators
The 78XX series of IC regulators is representative of three terminal devices that providea fixed positive output voltage. The three terminals are input, output and ground as indicated in
the standard fixed voltage configuration in Fig: (5.1.a).The last two digits in the part number
designate the output voltage. For example, the ‘7805’ is a +5V regulator. Other available output
voltages are given in Table: 5.1.
Capacitors although not always necessary are sometime used on the input and output as
indicated in Fig: (5.1.b). The output capacitor acts basically as a line filter to improve transient
response. The input capacitor is use to prevent unwanted oscillations when the regulator is some
distance from the power supply filter such that the line has a significant inductance.
The 78XX can produce output current in excess of 1A when used with an adequate heat
sink. The 78LXX series can provide up to 100mA, the 78MXX series can provide up to 500mA,
and the 78TXX series can provide in excess of 3A.
The input voltage must be at least 2V above the output voltage in order to maintain
regulation. The circuits have internal thermal overload protection and short-circuit current-
limiting features. Thermal overload occurs when the internal power dissipation becomes
excessive and the temperature of the device exceeds a certain value.
78 XX+ INPUT + OUTPUT
IN OUT
GNDC1 C2
78 XX
1 2 3
PIN 1 - INPUT
2 - GROUND
3 - OUTPUT
Fig: (5.1.a) Pin Layout Fig: (5.1.b) Standard configuration
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 10/161
TYPE NUMBER OUTPUT VOLTAGE
7805 +5V
7806 +6V
7808 +8V
7809 +9V
7812 +12V
7815 +15V
7818 +18V
7824 +24V
Table (5.1) 78XX series
Fixed Negative Linear Voltage Regulators
The 79XX series is typical of three-terminals IC regulators that provide a fixed negative
output voltage. This series is the negative counterpart of the 78XX series and shares most of the
same features and characteristics. Fig: (5.2.a & b) and Table (5.2) indicate the pin layout; the
standard configuration and part numbers with corresponding output voltage that are available.
79 XX
1 2 3
79 XX- INPUT - OUTPUT
IN OUT
GND
PIN 1 - GROUND
2 - INPUT
3 - OUTPUT
C1 C2
Fig: (5.2.a) Pin Layout Fig: (5.2.b) Standard configuration
Table (5.2) 79XXseries
5.3 Adjustable
Positive Linear
Voltage Regulators
Fig: (5.3.a)
Pin Layout
Fig: (5.3.b) Standard configuration
The LM317 is an excellent example of the three- terminal positive regulator with an
adjustable output voltage. Notice that there is an input, an output and an adjustable terminal.
The external fixed resistor R 1 and the external variable resistor R 2 provide the output voltage
adjustment. Vout can be varied from 1.2V to 37V depending on the resistor values. The LM317
can provide over 1.5A of output current to a load.
Type Number Output Voltage
7905 -5V
7905.2 -5.2V
7906 -6V
7908 -8V
7912 -12V
7915 -15V
7918 -18V
7924 -24V
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 11/161
The LM317 is operated as a “floating” regulator because the adjustment terminal is not
connected to ground, but floats to whatever voltage is across R 2. This allows the output voltage
to be much higher than that of a fixed-voltage regulator.
IREF =11
REFR
V25.1R
V =
A constant 1.25V reference voltage (VREF), is maintained by the regulator between the
output terminal and the adjustment terminal. This constant reference voltage produces a constant
current (IREF) through R 1 regardless of the value of R 2. IREF also flows through R 2
2REF1REF2R 1R OUT R IR IVVV +=+=
)R R (I 21REF +=
)R R (R
V21
1
REF+=
)1( 2
R
RV V REF OUT +=
Adjustable Negative Linear Voltage Regulators
LM337
1 2 3
PIN 1 -ADJ2 - INPUT3 - OUTPUT
- OUTPUT- INPUT IN OUT
LM337
ADJ
R2
R1
Fig: (5.4.a) Pin Layout Fig: (5.4.b) Standard configuration
The LM 337 is the negative output counterpart of the LM 317 and is a good example of
the type of IC regulator. Like the LM 317, the LM 337 requires two external resistors for output
voltage adjustments as shown in Fig: (5.4.b). The output voltage can be adjusted from -1.2V to
-37V, depending on the external resistors values.
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 12/161
UNIT: 6
THE REGULATED DUAL VOLTAGE DC POWER SUPPLY
By combining the step down transformer, rectifier, filters and voltage regulators 7812 and
7912 together, we get a regulated dual voltage ( ±12V) dc power supply circuit as shown in
Figure: (6.1).
This is a simple circuit, which gives regulated -12 -0- +12V supply. ICs 7812 and 7912
are used here as positive and negative regulators respectively. The unregulated input voltage
must always be higher than the regulators output voltage by at least 3V in order for it to work. If
the input/output voltage difference is greater than 3V then the excess potential must be dissipated
as heat. The maximum power dissipated in this type of series regulator is the power dissipated in
the internal pass transistor, which is approx.(VS max - Vout) IL max. Hence, as the load current
increases, the power dissipated in the internal pass transistor increases. If ILoad exceeds 0.75 A,
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 13/161
the IC package should be secured to a heat sink. When this is done, I Load can increase to about 1.5
A.
The filter capacitors, C1 & C2, are chosen to maintain the voltage input range to the
regulator. Capacitors C5 & C6 improve the regulators ability to react to sudden changes in load
current and to prevent uncontrolled oscillations. C3 and C4 are small film capacitors to improve
transient response and filter high frequency line noise.
Fig: (6.1) Circuit diagram of the regulated dual voltage DC power supply
PCB fabrication:
The PCB is fabricated using positive laser print of the circuit layout. The steps followed are
discussed below. The PCB layout is shown in Fig. 6.2.
The printed circuit layout is printed on the photo paper using a laser printer.
On the clean copper clad of the size of circuit layout, the mirror image of the layout is
transferred using a household hot electric iron.
The photo paper is removed by dipping the copper clad in warm water.
For etching the copper clad is immersed in a tray containing ferric chloride aqueous
solution. The copper clad plate is moved up and down continuously. When etching is
complete the copper clad plate is removed and rinsed with water and then dried in open
air.
The laser toner ink is removed by using a smooth sand paper.
The drilling is made by using a hand drill of 0.8 mm diameter.
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 14/161
Again the circuit layout is scrubbed with a fine sand paper.
Thus the P.C.B. is ready for mounting the components.
Fig: (6.2) PCB layout of the regulated dual voltage DC power supply
The component mounting view of the P.C.B. is shown in Fig. 6.3.
Fig: (6.3) Component Mounting on PCB
The components and their values used to build the circuit are given in the following table.
Components Value
• Capacitors
C1, C2
C3,C4
C5, C6
• Diodes
D1, D2, D3, D4
• ICs
IC1
2200µ F/35V
100 nF = 0.1µ F
10µ F/25V
1N 4001 diodes
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 15/161
IC2
• Miscellaneous
Transformer
Switch
LM 7812T, positive voltage regulator
LM 7912T, negative voltage regulator
220V AC Pri: to 17V-0-17V, 1A Sec:
ON/OFF switch
UNIT: 7
TEST AND RESULTS
This Unit will discuss some of the tests carried out on the final circuit which was
discussed in chapter (6).
The equipments used is analyzing circuit is vital in yielding the correct information about
the advantages and any design. During the course of final test, the equipments used were a digital
multi-meter (DMM). This was used to measure the line regulation, the load regulation, and
output DC voltages. The results of line regulation and load regulation are shown in Fig. 7.1 and
Fig. 7.2 respectively. The study of Fig. 7.1 shows that the output of the power supply remains
constant for the input of line voltage from 160V A.C. Similarly the study of Fig. 7.2 shows that
the output voltages of the power supply remain constant and the current decreases with increase
in load resistance.
8/3/2019 Dual Power Supply Final
http://slidepdf.com/reader/full/dual-power-supply-final 16/16
Conclusion:
The circuit used to fabricate the dual power supply using ICs 7812 and 7912 work well.
It can be used for the study of electronic circuits.
REFERENCES
(1) Floyd, Thomas L.
"Electronic Devices"
Fourth Editions, Prentice Hall, Inc, 1996
(2) Wason Kamal
"Electronics Projects"
Second Edition, EFY Enterprises Pvt Ltd, 1999
(3) http://www.electronicsforu.com