1. POWER SUPPLIES
Equipment containing electronic circuits require voltages of between 5 and 30 volts dc. We
therefore need to convert the 240 volt ac mains supply to a suitable value before it can be used to
power the equipment, this is the job of the power supply.
TRANSFORMER
The transformer converts the hazardous ac mains supply voltage to an isolated low voltage
suitable for rectification into a dc voltage used by the electronic circuits.
Transformers used in power supplies are of two types of construction,
1. Laminated iron core and bobin for 50Hz linear power supplies.
2. Solid ferrite core and bobin for switch mode power supplies.
Comprising at least two windings mounted in the bobin, primary and secondary winding. The
primary winding connects to the mains input voltage and the secondary winding provides the
output voltage and are isolated from each other providing a safety barrier.
RECTIFIER CIRCUITS
There are three main classes of rectifier,
1. Half-Wave.
2. Full-Wave.
3. Full-Wave Bridge.
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transformer rectifier smoothing
circuit
stabilizing
circuit
dc
output
ac
input
Power Supply Block Diagram

2. Half Wave Rectification
During the first half cycle diode D conducts since its anode is positive with respect to it cathode.
During the second half cycle its anode is negative with respect to the cathode and the diode
blocks the flow of current. Therefore only the positive half cycles will be available to the load at
the frequency of the mains supply.
The voltage Vo will have an amplitude equal to the peak value of the secondary voltage Vi minus
one diode volt drop (0.7V), however only half the power will be available due to the missing half
cycle of the negative part of the input voltage.
Assuming the secondary voltage Vi to be 12V rms then Vo will be,
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I
D
Vi
Vo
0 time
+
-
-
Vi
Vo
ac
mains
R
+
Half-Wave
VVVV Dio 268.167.0)12414.1(2 =−×=−×=

3. Full Wave Rectification
The full wave rectifier circuit uses two diodes and a transformer with a centre-tapped secondary.
With the centre tap being the reference point at 0V, then when A is at its positive peak B will be at
its negative peak diode D1 will conduct. When A is at its negative peak B will be at its positive
peak and diode D2 will conduct.
In full wave rectification both halves of every cycle of input voltage are available to the load at a
frequency of twice the supply frequency. This circuit consists of two half-wave rectifiers supplying
the load on alternate half cycles of input voltage Vi.
The voltage Vo will have an amplitude equal to the peak value of the secondary voltage Vi minus
one diode volt drop (0.7V), however double the power of the half-wave version will be available
due to the additional half cycle from the negative part of the input voltage.
Assuming the secondary voltage Vi to be 12V rms then Vo will be,
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VVVV Dio 268.167.0)12414.1(2 =−×=−×=
B
-
time
Vi Vo
0
+
-
+
Vi
ac
mains
D2
D1o
RVo
Full-Wave
A
I
I

4. Full Wave Bridge Rectification
The full wave bridge rectifier circuit uses four diodes and, for a single rail output, an untapped
transformer secondary. When A is at its positive peak B will be at its negative peak diodes D1 and
D2 will conduct. When A is at its negative peak B will be at its positive peak and diodes D3 and D4
will conduct.
In full wave rectification both halves of every cycle of input voltage are available to the load at a
frequency of twice the supply frequency.
The voltage Vo will have an amplitude equal to the peak value of the secondary voltage Vi minus
two diode volt drops(1.4V), due to the current path through out going and return. As with the half-
wave the output will fluctuate at twice the mains frequency.
Assuming the secondary voltage Vi to be 12V rms then Vo will be,
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Vi
Vo
ac
mains
D2 D3
D1D4
R
Full-Wave Bridge
time
Vi Vo
0
+
-
A
B
VVVV Dio 57.154.1)12414.1(2 =−×=−×=

5. Dual Rail Output
An adaptation of the full-wave bridge circuit is that of the dual rail power supply commonly used
to power analogue electronic systems and computer circuits since they usually require both
positive and negative supply rails.
With the addition of a centre-tapped secondary two supply rails can be achieved using the centre
tap as the 0V reference.
Smoothing Circuits
The varying dc output voltage from the rectifier although adequate for charging a battery is not
suitable for electronic equipment and must be smoothed to a steady dc level.
The simplest way to obtain a smoothed output is to connect a high value capacitor across it,
called a reservoir capacitor. Its value can range from 100μF to 10,000μF depending on the
current and smoothing required.
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-Vi
R
D2
+
-
0V
+Vo
ac
mains
D3
D1
D4
B
A
-Vo
R
+Vi
Dual Rail Power Supply
Positive Output Negative Output
time
Vi Vo
0
+
-
time0
+ Vi
-
Vo

6. During the part of the positive half-cycle C is charging to near the peak value of the ac secondary
voltage. During the rest of the cycle C is supplying the load by partly discharging into it and Vo
falls until the next half-cycle tops up the charge on C.
The small variation in the smoothed dc output is called the ripple voltage and in the half-wave has
the same frequency as the mains supply.
Since the half-wave uses only the positive half-cycles the discharge period is twice as long as the
full-wave causing the ripple voltage to be greater.
Capacitor and Diode Ratings
Capacitor. The smoothing action of the reservoir capacitor is due to its large capacitance making
the time constant CR large, (R is the resistance of the load), compared with the time for one cycle
of ac mains. The larger C is the better the smoothing but the peak charging current pulses are
greater.
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Connection of Reservoir Capacitor
I
D
-
Vi
Vo
ac
mains
R
+
C
C charging C discharging
0 time
VO
-
Half-Wave Output
0 time
Vo
C charging C discharging
Full Wave output

7. The capacitor should have a voltage rating at least equal to the peak value of the transformer
secondary voltage at maximum line input.
Diode. Consideration must be given to the peak inverse voltage which is the maximum voltage
the diode can block when reverse biased and should be at least twice the peak to peak value of
the transformer secondary voltage at maximum line input. Also the peak current rating must be
greater than that required by the reservoir capacitor during charging.
Stabilizing Circuits
The circuits discussed have one major draw back in that they do not provide any form of control
on the output voltage which can change with mains and load variation,
To overcome this and provide a stable output voltage of a known value we need to add a
stabilizing circuit, or regulator.
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dc
output
load
bad
good
Load Regulation

8. Zener Diode Regulator
A basic form of regulator can be achieved using a zener diode and a resistor to limit the zener
current. When a zener diode is reverse biased the voltage across it is almost constant. This
property can be used to provide a stable voltage for a wide range of input voltage.
The total current I is divided between the zener current IZ and the load current IL.
Since R and VZ are constant then I is constant, if IL reduces due to increase in RL then IZ will
increase by the same amount. At IL = 0, then IZ = I, the zener diode must has the power rating to
safely carry this current.
Zener Diode Power rating,
and
Maximum Current rating,
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+
-
R
IL
VL
Z RL
IZ
I
Vi
Zener Diode Regulator
I
VV
R Zi −
=LZ III += and
Z
ZZ
V
P
I
IVP
max
max
max
=
=

9. Transistor Regulator
Adding a transistor to the zener diode circuit will remove the problems associated with the simple
zener regulator since the zener current is constant, neglecting small variations in IB. Also the load
current capability is increased by the HFE of the transistor.
Type No Pol. Vce (V) Ic (max) PTOT hFE (min) Case
BD135 npn 45 1.5A 12.5W 40@1.5A TO-126
BD136 pnp 45 1.5A 12.5W 40@1.5A TO-126
TIP31A npn 60 3A 40W 10@1A TO-220
TIP32A pnp 60 3A 40W 25@1A TO-220
2N3055 npn 60 15A 117W 20@4A TO-3
TIP2955 pnp 100 15A 90W 20@4A TO-218
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VZ
-VBEI
Zener Stabilized Transistor Regulator
IB
IZ
+IB
VO
VI
RL
IL
VZ
Example of Power Transistor Data

10. Integrated Circuit Voltage Regulator
Integrated circuit voltage regulators are available as three terminal devices with internal
protection from overload and overheating. They are available in both positive and negative fixed
voltages from ±5 to ±15 volts with current ratings from 100mA to 3 Amps. These devices have
complex internal circuitry and should be mounted on a heat sink to ovoid overheating and thermal
shutdown.
Type No Voltage Current Polarity Style
78L, 05, 12, 15 5, 12, 15 100mA positive TO92
79L, 05, 12, 15 5, 12, 15 100mA negative TO92
78, 05, 12, 15 5, 12, 15 1A positive TO220
79, 05, 12, 15 5, 12, 15 1A negative TO220
78S, 05, 12, 15 5, 12, 15 2A positive TO220
LM323K 5 3A positive TO3
LM387A 5 500mA positive TO220
LM338T 1.2 – 37 variable 5A positive TO3
LM337T 1.2 – 37 variable 5A negative TO3
Heatsinks
A common method of voltage regulation in linear power supplies uses either a transistor or three
terminal regulator to control the output voltage.
The process of regulation
dissipates power, which, in
turn generates heat
within the regulation device.
To remove this heat quickly
and efficiently the regulator is
mounted on a heatsink.
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Example of Heat Sinks
Integrated Circuit Regulator
RL
ac
mains
Vi
Vo
7805
Heat Sink
Common types of Voltage Regulator

11. Heat sinks are manufactured from aluminium and usually painted black so as to dissipate the
heat energy more efficiently and prevent overheating. They are rated in degrees per watt known
as the thermal resistance. For example a 2°CW-1
the temperature of the heatsink rises by 2°C
above its ambient temperature for every watt of power it has to remove. To dissipate 10W, the
rise will be 20°C.
Switch Mode Power Supplies
The power supplies described so far are known as linear power regulators since they operate on
a stable dc level and the control voltage is a continuously varying level. Although cost effective
and simple in their design and operation they are bulky and inefficient, as low as 50% and
requiring large heatsinks on the regulator.
Switch mode supplies work by chopping the dc level at a high frequency, 100 – 200kHz using fast
switching transistors and then pass this to a small high frequency ferrite cored transformer. The
switching frequency is pulse width modulated meaning that the ratio of ‘on’ time to ‘off’ time
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12. changes as the load or line input changes. These power supplies although complex in their
design are much smaller than the linear types and up to 85% efficient.
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Example of Switch Mode Power Supply (SMPS)