1. A presentation of eSyst.org

2. LINEAR POWER SUPPLY
Block diagram and functions of a transformer,
rectifier, filter, voltage regulator and voltage
divider.
Types of rectifier, filter and regulator circuits
A presentation of eSyst.org

3. Power Supply
All electronic circuits need a power source to work.
For electronic circuits made up of transistors and/or ICs,
this power source must be a DC voltage of a specific
value.
A battery is a common DC voltage source for some types
of electronic equipment especially portables like cell
phones and iPods.
Most non-portable equipment uses power supplies that
operate from the AC power line but produce one or more
DC outputs.
A presentation of eSyst.org

4. Power Supply Characteristics
 The input is the 120 volt 60 Hz AC
power line.
 The power supply converts the AC
into DC and provides one or more
DC output voltages.
 Some modern electronic circuits
need two or more different voltages.
 A good example of a modern power
supply is the one inside a PC that
furnishes 12, 5, 3.3 and 1.2 volts.
A presentation of eSyst.org

5. Components of a Power Supply
Main circuits in most power supplies.
A presentation of eSyst.org

6. Transformer
A transformer is commonly used to step the input AC voltage level
down or up. Most electronic circuits operate from voltages lower
than the AC line voltage so the transformer normally steps the
voltage down by its turns ratio to a desired lower level.
For example, a transformer with a turns ratio of 10 to 1 would convert
the 120 volt 60 Hz input sine wave into a 12 volt sine wave.
A presentation of eSyst.org

7. Rectifier
The rectifier converts the AC sine wave into a pulsating
DC wave.
There are several forms of rectifiers used but all are
made up of diodes.
Rectifier types and operation will be covered later.
A presentation of eSyst.org

8. Filter
The rectifier produces a DC output but it is pulsating
rather than a constant steady value over time like that
from a battery.
A filter is used to remove the pulsations and create a
constant output.
The most common filter is a large capacitor.
A presentation of eSyst.org

9. Regulator
The regulator is a circuit that helps maintain a fixed or
constant output voltage.
Changes in the load or the AC line voltage will cause the
output voltage to vary.
Most electronic circuits cannot withstand the variations
since they are designed to work properly with a fixed
voltage.
The regulator fixes the output voltage to the desired level
then maintains that value despite any output or input
variations.
A presentation of eSyst.org

10. How Rectifiers Work
The simplest form of rectifier is
the half wave rectifier shown.
Only the transformer, rectifier
diode, and load (RL) are shown
without the filter and other
components.
The half wave rectifier produces
one sine pulse for each cycle of
the input sine wave.
When the sine wave goes
positive, the anode of the diode
goes positive causing the diode
to be forward biased. The diode
conducts and acts like a closed
switch letting the positive pulse
of the sine wave to appear
across the load resistor.
A presentation of eSyst.org

11. How Rectifiers Work (continued)
When the sine wave goes
negative, the diode anode will be
negative so the diode will be
reverse biased and no current
will flow.
No negative voltage will appear
across the load. The load
voltage will be zero during the
time of the negative half cycle.
See the waveforms that show
the positive pulses across the
load. These pulses need to be
converted to a constant DC.
A presentation of eSyst.org

12. Bridge Rectifier
Another widely used rectifier is
the bridge rectifier. It uses four
diodes.
This is called a full wave rectifier
as it produces an output pulse
for each half cycle of the input
sine wave.
On the positive half cycle of the
input sine wave, diodes D1 and
D2 are forward biased so act as
closed switches appearing in
series with the load.
On the negative half cycle, diode
D1 and D2 are reverse biased
and diodes D3 and D4 are
forward biased so current flows
through the load in the same
direction.
A presentation of eSyst.org

13. How the Filter Works
A large capacitor is connected
across the load resistor. This
capacitor filters the pulses into a
more constant DC.
When the diode conducts, the
capacitor charges up to the peak
of the sine wave.
Then when the sine voltage
drops, the charge on the
capacitor remains. Since the
capacitor is large it forms a long
time constant with the load
resistor. The capacitor slowly
discharges into the load
maintaining a more constant
output.
The next positive pulse comes
along recharging the capacitor
and the process continues.
A presentation of eSyst.org

14. The Regulator
Most regulators are ICs .
These are feedback control circuits that actually monitor
the output voltage to detect variations.
If the output varies, for whatever reason, the regulator
circuit automatically adjusts the output back to the set
value.
Regulators hold the output to the desired value.
Since ripple represents changes in the output, the
regulator also compensates for these variations
producing a near constant DC output.
A presentation of eSyst.org

15. RC pi Filter
A presentation of eSyst.org
ii) RC pi Filter
•C1 performs the same function that it did
in the single capacitor filter. It is used to
reduce the percentage of ripple to a
relatively low value.
•C2 offers infinite impedance (resistance)
to the dc component of the output
voltage. Thus, the dc voltage is passed to
the load, but reduced in value by the
amount of the voltage drop across R2.
However, R2 is generally small compared
to the load resistance. Therefore, the drop

16. RC pi Filter
• C2 offers very low impedance to the ac ripple frequency. Thus, the
ac ripple senses a voltage divider consisting of R2 and C2 between
the output of the rectifier and ground. Therefore, most of the ripple
voltage is dropped across R2.
• The RC filter has some disadvantages, however. First, the voltage
drop across R2 takes voltage away from the load. Second, power is
wasted in R2, R1 and is dissipated in the form of unwanted heat.
• The input capacitor (C1) has the greatest pulsating voltage applied
to it and is the most susceptible to voltage surges. As a result, it is
frequently subject to voltage breakdown and shorting. The shunt
capacitor (C1 and C2) in the filter circuit is not subject to voltage
surges because of the protection offered by the series filter resistor.
A presentation of eSyst.org

17. Definition of Ripple
The amount of ripple factor of the full wave rectified signal is smaller
than the half wave signal and provides a better filtered signal.
The amount of ripple factor of the full wave rectified signal is smaller
than the half wave signal and provides a better filtered signal.

18. THE CHOKE INPUT (L-C FILTER)•As is known, in an inductor filter, ripple increases with RL but decreases in
a capacitor filter.
•The combination of L and C filter makes the ripple independent of RL
a) Shows the filter circuit. b) The voltage variation.

19. CLC or Pi Filter
•The LC input filter is one of the most commonly
used filters.
•The input capacitor C1 is selected to offer very low
reactance to the ripple frequency. Hence, major part
of filtering is done by C1. Most of the remaining
ripple is removed by the combined action of L and
C2.
•L is a large value iron-core inductor (choke.) It has
a high value of inductance and, therefore, a high
value of XL, which offers a high reactance to the
ripple frequency. At the same time, C2 offers a very
low reactance to the ac ripple. L and C2 form an ac
voltage divider and, because the reactance of L is
much higher than that of C2, most of the ripple

20. • Aside from the voltage divider effect, the inductor improves filtering in
another way. You should recall that an inductor resists changes in
the magnitude of the current flowing through it. Consequently, when
the inductor is placed in series with the load, the inductor tends to
hold the current steady. This, in turn, helps to hold the voltage
across the load constant.
• Generally, this resistance is very low and the dc voltage drop across
the coil is minimal. Thus, the LC filter overcomes the disadvantages
of the RC filter.
• The LC filter has two disadvantages. The first is cost. The LC filter is
more expensive than the RC filter because its iron-core choke costs
more than the resistor of the RC filter. The second disadvantage is
size, since the iron-core choke is bulky and heavy. Thus, the LC filter
may be unsuitable for some applications but is still one of the most
widely used. A presentation of eSyst.org

21. a)ZENER DIODE AS VOLTAGE REGULATOR
•A zener shunt regulator is that
the diode dissipation is too
large in some application.

22. b) SERIAL TRANSISTOR
A presentation of eSyst.org
Q β = 50
•The way to reduce the diode zener shunt power
dissipation is called an amplifier zener regulator.
Transistor Q1 is the series control element.
•Zener diode DZ provides the reference voltage
Operation
•If the output voltage decreases, the increased base-
emitter voltage causes transistor Q1 to conduct more.
•Thereby raising the output voltage, maintaining the output
constant.
•If the output increases, the decreased base-emitter
voltage causes transistor Q1 to conduct less, reducing the
output voltage maintaining the output constant.

23. A simple regulator consists of a sampling circuit, an error amplifier, a
conduction element, and a voltage reference element.
The sampling regulator circuit (voltage divider) monitors the output
voltage by feeding sample voltage back to the error amplifier.
The reference voltage element (zener diode) acts to maintain a
constant reference voltage that used by the error amplifier.
The error amplifier’s output is then fed to the current-control element
(transistor), which used to control the load current.

24. Negative-feedback voltage regulator.
A presentation of eSyst.org
•Transistor Q1 acts like an emitter follower.
Transistor Q1 provides voltage gain in a
negative-feedback loop.
•Suppose the load voltage tries to increase. The
feedback voltage VF will increase. Since the
emitter voltage Q1 is held constant by the Zener
diode, more collector current flows through Q1
and through R3.
•This reduces the current throughQ1 and R3.
The higher voltage at the base of Q2 increases
the emitter voltage of Q2, and this almost
completely offsets the original decrease in load
voltage.

25. A presentation of eSyst.org
Voltage regulator
•The positive voltage regulator LM78 “xx” and
negative voltage regulator LM79xx digits represent
the output voltage such as 7805 (5V), 7806 (6 V),
7909 (-9V) etc.
•Can handle a maximum output current of 1.5A if
properly heat-sink.
•To remove unwanted input or output spikes/noise,
capacitors can be attached to the regulator’s input
and output terminals, as shown on figure above.

27. SIMPLE POWER SUPPLY

28. In Summary
All electronic circuits and equipment need a power supply, usually
one that supplies are very specific DC voltage.
A battery is a near perfect DC supply but it is used mainly in portable
applications.
Most equipment uses an AC to DC power supply.
In most AC to DC supplies, the 120 volt AC line is first filtered then
stepped up or down to the desired voltage level then rectified into
pulsating DC, then filtered to a constant DC. A regulator holds the
output to a desired level. A DC-DC converter may also be used to
generate another DC voltage.
The two most common rectifiers are the single diode half wave
rectifier and the four diode full wave bridge rectifier.