B.L.D.E.A’S SSM Polytechnic, Vijayapur.
Department of Mechatronics Engineering
CHAPTER -1. PowER SuPPliES
By-N.T.BIRADAR. Lect

UNIT 1.Power Supplies
A power supply is an electronic device that supplies electric energy to an electrical load.
The primary function of a power supply is to convert one form of electrical energy to
another.
1.1 Exlain the Block diagram of Linear Power supply.
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 A battery is a common DC
voltage source for some types of electronic equipment especially portables like cell phones
and iPods.For electronic circuits made up of transistors and/or ICs, this power source must be
a DC voltage of a specific value.All electronic circuits need a power source to work.Power
Supply
Main circuits in most power supplies. A presentation of Components of a Power Supply is
as follows
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 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.Transformer
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.
Filter: is a large capacitor. A filter is used to remove the pulsations and create a constant
output.The rectifier produces a DC output but it is pulsating rather than a constant steady
value over time like that from a battery.

Regulator: fixes the output voltage to the desired level then maintains that value despite any
output or input variations. Most electronic circuits cannot withstand the variations since they
are designed to work properly with a fixed voltage.Changes in the load or the AC line
voltage will cause the output voltage to vary.The regulator is a circuit that helps maintain a
fixed or constant output voltage Regulator.
1.2 Explain the operation of Half Wave Rectifier.
Fig.1.2 Half Wave Rectifier
A rectifier is an electronic device that converts AC voltage into DC voltage. In other
words, it converts alternating current to direct current. A rectifier is used in almost all the
electronic devices. Mostly it is used to convert the main voltage into DC voltage in the power
supplysection. By using DC voltage supply electronic devices work.
In a single-phase half-wave rectifier, either negative or positive half of the AC voltage flows,
while the other half of the AC voltage is blocked. Hence the output receives only one half of
the AC wave. A single diode is required for a single-phase half-wave rectification and three
diodes for a three-phase supply. Half wave rectifier produces more amount of ripple content
than full wave rectifiers and to eliminate the harmonics it requires much more filtering.
During the positive half cycle, when the secondary winding of the upper end is positive with
respect to the lower end, the diode is under forward bias condition and it conducts current.
During the positive half cycles, the input voltage is applied directly to the load resistance
when the forward resistance of the diode is assumed to be zero. The wave forms of output
voltage and output current are same as that of the AC input voltage.
During the negative half cycle, when the secondary winding of the lower end is positive with
respect to the upper end, the diode is under reverse bias condition and it does not conduct

current. During the negative half cycle, the voltage and current across the load remains zero.
The magnitude of the reverse current is very small and it is neglected. So, no power is
delivered during the negative half cycle.
1.3 Explain Bridge rectifier.
A bridge rectifier circuit is a common part of the electronic power supplies.
Many electronic circuits require rectified DC power supply for powering the
various electronic basic components from available AC mains supply. We can find this
rectifier in a wide variety of electronic AC power devices like home appliances, motor
controllers, modulation process, welding applications, etc
Fig1.3 Bridge rectifier
During the Positive half cycle of the input AC waveform diodes D1 and D2 are forward
biased and D3 and D4 are reverse biased. When the voltage, more than the threshold level of
the diodes D1 and D2, starts conducting – the load current starts flowing through it, as shown
as red lines path in the diagram below.
During the negative half cycle of the input AC waveform, the diodes D3 and D4 are
forward biased, and D1 and D2 are reverse biased. Load current starts flowing through the
D3 and D4 diodes when these diodes starts conducting as shown in the figure.
We can observe that in both the cases, the load current direction is same, i.e., up to down as
shown in the figure – so unidirectional, which means DC current. Thus, by the usage of a
bridge rectifier, the input AC current is converted into a DC current. The output at the load
with this bridge wave rectifier is pulsating in nature, but for producing a pure DC requires

additional filter like capacitor. The same operation is applicable for different bridge rectifiers,
but in case of controlled rectifiers thyristors triggering is necessary to drive the current to
load.
1.4 Necessity of filters.
• We know that rectifiers are used to convert AC to DC, but not a pure DC. There
would be considerable AC component in their output, called ‘ripple’, in addition to
the desired d.c. component.
• Most sophisticated electronic systems need pure DC supply to drive, or power them.
To construct a good power supply which gives pure DC output, we need to remove or
filter out the AC component from the output of rectifiers.
1.5 Explain L type and Pi type filters.
This filter circuit employs a choke and a capacitor. The circuit of an inverted L-type filter
circuit is shown in fig. below.
Fig 1.4 L type filters
Since the choke offers high reactance to high frequencies, it blocks them and the
capacitor C short them to ground as it offers negligible reactance to the high frequencies.
Thus only low frequencies below cutoff frequency fc are allowed to pass through without
significant attenuation.In this filter, the output is taken across the capacitor C.These
components have two elements: 1 capacitor and 1 inductance, and are second-order filters

The theoretical insertion loss is 40 dB per decade. For optimum performance the filter must
be placed with the inductor situated on the side which has the lower impedance. These filters
are ideal for circuits with imbalanced source / load impedances
1.6 Explain Pi type filters.
This filter circuit employs a choke and two capacitors. The circuit of an inverted pi-type
filter circuit is shown in fig. below
Fig 1.5 Pi type filters
Here a second capacitor C2 is added in the circuit to improve the filtering action by
grounding higher frequencies.The inductor or choke is always connected in series between
the input and output and the capacitors are grounded in parallel.The output voltage is taken
across the capacitor C2 .The capacitor-input filter operates in three steps:
• The capacitor C1 offers low reactance to the AC component of the rectifier output
while it offers infinite resistance to the DC component. As a result the
capacitor shunts an appreciable amount of the AC component while the DC
component continues its journey to the inductor L.
• The inductor L offers high reactance to the AC component but it offers almost zero
resistance to the DC component. As a result the DC component flows through the
inductor while the AC component is blocked.
• The capacitor C2 shunts the AC component which the inductor had failed to block.
As a result only the DC component appears across the load RL.
1.7 Explain the Zener diode regulator.

However, the Zener Diode or “Breakdown Diode”, as they are sometimes referred too, are
basically the same as the standard PN junction diode but they are specially designed to have a
low and specified Reverse Breakdown Voltage which takes advantage of any reverse voltage
applied to it.The Zener diode behaves just like a normal general-purpose diode consisting of
a silicon PN junction and when biased in the forward direction, that is Anode positive with
respect to its Cathode, it behaves just like a normal signal diode passing the rated
current.However, unlike a conventional diode that blocks any flow of current through itself
when reverse biased, that is the Cathode becomes more positive than the Anode, as soon as
the reverse voltage reaches a pre-determined value, the zener diode begins to conduct in the
reverse direction.
Fig.1.6 Zener diode regulator
The current now flowing through the zener diode increases dramatically to the maximum
circuit value (which is usually limited by a series resistor) and once achieved, this reverse
saturation current remains fairly constant over a wide range of reverse voltages. The voltage
point at which the voltage across the zener diode becomes stable is called the “zener
voltage”, ( Vz ) and for zener diodes this voltage can range from less than one volt to a few
hundred volts.

The resistor, RS is connected in series with the zener diode to limit the current flow
through the diode with the voltage source, VS being connected across the combination. The
stabilised output voltage Vout is taken from across the zener diode. The zener diode is
connected with its cathode terminal connected to the positive rail of the DC supply so it is
reverse biased and will be operating in its breakdown condition. Resistor RS is selected so to
limit the maximum current flowing in the circuit.
With no load connected to the circuit, the load current will be zero, ( IL = 0 ), and all the
circuit current passes through the zener diode which in turn dissipates its maximum power.
Also a small value of the series resistor RS will result in a greater diode current when the
load resistance RL is connected and large as this will increase the power dissipation
requirement of the diode so care must be taken when selecting the appropriate value of series
resistance so that the zener’s maximum power rating is not exceeded under this no-load or
high-impedance condition.
1.8 Explain Series voltage regulator.
The series voltage regulator or series pass voltage regulator uses a variable element
placed in series with the load. By changing the resistance of the series element, the voltage
dropped across it can be varied to ensure that the voltage across the load remains
constant.The advantage of the series voltage regulator is that the amount of current drawn is
effectively that used by the load, although some will be consumed by any circuitry associated
with the regulator. Unlike the shunt regulator, the series regulator does not draw the full
current even when the load does not require any current. As a result the series regulator is
considerably more efficient.One of the simplest implementations of this concept is to use a
single pass transistor in the form of an emitter follower configuration, and a single Zener
diode drive by a resistor from the unregulated supply. This provides a simple form of
feedback system to ensure the Zener voltage is maintained at the output, albeit with a voltage
reduction equal to the base emitter junction voltage - 0.6 volts for a silicon transistor
Fig1.7 Series voltage regulator
The Zener diode will generally need a minimum of around 10mA for a small Zener to keep
its regulated voltage. The resistor should then be calculated to provide the base drive current and
the minimum Zener current from a knowledge of the unregulated voltage, Zener voltage and the
current required. [ (Unregulated voltage - Zener voltage ) / current ]. A small margin should be

added to the current to ensure that there is sufficient room for margin when the load, and hence
the transistor base is taking the full current.
The power dissipation capacity for the Zener diode should be calculated for the case when
the load current, and hence the base current is zero. In this case the Zener diode will need to take
the full current passed by the series resistor.
1.9 Explain IC Voltage Regulators: IC 723
Voltage regulators, the 723 Voltage Regulators IC. The functional diagram of the voltage
regulator is shown below. It consists of a voltage reference source (Pin 6), an error amplifier with
its inverting input on pin 4 and non-inverting input on pin 5, a series pass transistor (pins 10 and
11), and a current limiting transistor on pins 2 and 3. The device can be set to work as both
posistive and negaive voltage regulators with an output voltage ranging from 2 V to 37 V, and
output current levels upto 150 m A. The maximum supply voltage is 40 V, and the line and load
regulations are each specified as 0.01%.
The figure shown below is a positive voltage regulator with an IC 723. The output voltage
can be set to any desired positive voltage between (7-37) volts. 7 volts is the reference starting
voltage. All these variations are brought with the change of values in resistors R1 and R2 with
the help of a potentiometer. A darlington connection is made by the transistor to Q1 to handle
large load current. The broken lines in the image indicate the internal connections for current
limiting. Even foldback current limiting is possible in this IC. A regulator output voltage less
than the 7 V reference level can be obtained by using a voltage divider across the reference
source. The potentially divided reference voltage is then connected to terminal 5.
Fig.1.8 Voltage Regulators: IC 723
Another importat point to note about this IC is that the supply voltage at the lowest point on
the ripple waveform should be at least 3 V greater than the output of the regulator and greater
than Vref. If it is not so a high-amplitude output ripple is possible to occur.

1.10 Explain IC 78XX
Voltage Regulator is one of the most important and commonly used electrical
components. Voltage Regulators are responsible for maintaining a steady voltage across an
Electronic system.Voltage fluctuations may result in undesirable effect on an electronic
system, so to maintaining a steady constant voltage is necessary according to the voltage
requirement of a system.Let us assume a condition when a simple light emitting diode can
take a max of 3V to the max, what happens if the voltage input exceeds 3V ?, of course the
diode will burn out. This is also common with all electronic components like, led’s,
capacitors, diodes etc. The slightest increase in voltage may result in the failure of entire
system by damaging the other components too. For avoiding Damage in such
situations voltage regulator are used for regulated power supply.
Fig.1.9 IC 78XX
1.11Explain IC 79XX regulator
79xx voltage regulators are very commonly used in electronic circuits. The main purpose
of this IC is to supply required regulated negative voltage to the circuits. IC 79xx can supply
a constant negative voltage output, in spite of any voltage fluctuations in its input voltage. It
can be mainly found in the circuits in which integrated circuits that require +Vcc and – Vcc
are used

Fig.1.10 IC 79XX
IC 79xx is a three pin negative voltage controller IC. It is a small integrated circuit used
in a circuit to supply a constant negative input voltage. The number 79 indicates that it is a
negative voltage regulator and xx indicates the output voltage of the IC. ‘xx’ can be replaced
by the controlled output voltage provided by the regulator, for example, if it is 7905, then the
output voltage of the IC is -5 V. Similarly if it is 7912, then output voltage of the IC is -12
volts and so on. The name of the IC may vary based on the manufacturer as LM79xx, L79xx,
MC79xx etc.IC 79xx requires heat sink for its safe operation. Heat sink boosts heat
dissipation therefore the life of the device can be extended.
1.11 Explain Block diagram of UPS.
In order to protect a sensitive system from power losses and blackouts, an alternative
power source is required that can switch into operation immediately when disruption occurs.
An interruptible power supply (UPS) is just such an alternative source.

Fig 1.11 Block diagram of UPS
A UPS generally consists of a rectifier, battery charger, a battery bank and inverter circuit
which converts the commercial ac input into dc suitable for input to the battery bank and the
inverter. The rectifier should have its input protected and should be capable of supplying
power to the inverter when the commercial supply is either slightly below the normal voltage
or slightly above.
In case of On-line UPS, the battery operated inverter works continuously whether the
mains supply is present or not. Triac T1is on for all the times while Triac T2 has been
provided to bypass the UPS inverter, only when a fault develops in the UPS inverter. When
the mains supply fails, the UPS supplies power only until the batteries get discharged.
However, once the mains power resumes, the batteries will get charged again. The switching
times of these supplies is considered to be zero. Usually sealed maintenance free batteries are
used and the running time of the inverter is low (approximately 10 to 30 minutes).
In the case of Off-Line UPS, the inverter is off when the mains power is on and the
output voltage is derived directly from the mains. The inverter turns on only when the mains
supply fails. Its switching time is less than 5 ms. These UPS are generally used with PCs or
computers or other appliances where a small duration (5 ms or less) interruption in power
supply can be tolerated. Usually, sealed batteries or lead-acid batteries are used. The running
time of these supplies is also low (about 10 to 30 minutes).
1.12 Explain Block diagram of SMPS
The electric power is not normally used in the form in which it is produced or distributed.•
Practically all electronic systems require some form of Power conversion.• A device that
transfers electric energy from a source to a load using electronic circuits is referred to as
power supply.• A typical application of a power supply is to convert utility AC voltage into
regulated DC voltages required for electronic equipment. Block diagram of power supply
Parts of a power supply:

Fig.1.12 Block diagram of SMPS
Categories of Power SuppliesThere are two broad categories of power supplies:• Linear
regulated power supply• switched mode power supply (SMPS) In some cases one may use a
combination of switched mode and linear power supplies to gain some desired advantages of
both the types.
The electric power is not normally used in the form in which it is produced or
distributed.• Practically all electronic systems require some form of Power conversion.• A
device that transfers electric energy from a source to a load using electronic circuits is
referred to as power supply.• A typical application of a power supply is to convert utility AC
voltage into regulated DC voltages required for electronic equipment.
Categories of Power SuppliesThere are two broad categories of power supplies:• Linear
regulated power supply• switched mode power supply (SMPS) In some cases one may use a
combination of switched mode and linear power supplies to gain some desired advantages of
both the types. The AC voltage is connected to a transformer, which steps that ac voltage
down to the level for the desired dc output.• A diode rectifier then provides a full-wave
rectified voltage.• This is initially filtered by a simple capacitor filter to produce a dc
voltage.• This resulting dc voltage usually has some ripple or ac voltage regulation.• A
regulator circuit can use this dc input to provide a dc voltage that not only has much less
ripple voltage but also remains the same dc value even if the input dc voltage varies
somewhat or the load connected to the output dc voltage changes.• This voltage regulation is
usually obtained using one of the voltage regulator IC units.
Transformer• Transformer convert Ac electricity from one voltage to another with little
loss of power.• Transformers work only with AC & this is one of the reasons why mains
electricity is AC.TYPES OF TRANSFORMER• Step-up Transformer• Step-down
Transformer Step-up transformers increase voltage, step-down transformers reduce voltage.•
The input coil is called the primary & the output coil is called the secondary.• There is no
electrical connection between the two coils, instead they are linked by the alternating

magnetic field created in the soft iron core of the transformer.The two lines in the middle of
the circuit symbol represent the core.• Transformers waste very little power , so the power
out is almost equal to the power in. So, as voltage is stepped down current is stepped up.
RECTIFIER• In mains supplied electronic systems the AC input voltage must be
converted into a DC voltage with the right value & degree of stabilization.• Rectifier does
this work.• In other words a rectifier circuit is necessary to convert a signal having zero
average value into one that has a nonzero average.• Two types of rectifiers : a. Half wave
rectifier. b. Full wave rectifier.Figure above uses a center-tapped transformer with two
rectifier diodes.• Figure below uses a simple transformer & four rectifier diodes usually
known as a bridge rectifier.
SMOOTHING/FILTER• We need a way to smooth out the pulsations& get a much
cleaner dc power source for the load circuit.• This is done by a filter circuit.• In power
supply, a filter must remove or reduce the ac variations while still making the desired dc
available to the load circuitry.• Any given filter involve capacitors, inductors,&/resistors in
some combination.