Download Self Switching Power Supply

Self Switching Power
Supply
Self Switching Power Supply
Introduction
 Embedded system requires a regulated power supply. This power supply
circuit gives a variable regulated supply and switches off in no load
condition.
 Through an arrangement of voltage regulator 7805 and a potentiometer
the output voltage is varied from 3.7V to 8.7V. Another feature of this
power supply is, when no load is there it automatically switches off. It is
achieved through an arrangement of transistors and relay.
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Self Switching Power Supply
Block Diagram
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Self Switching Power Supply
Hardware Requirements
 Transformer
 Rectifier
 Filter
 Regulator
 Relay
 Transistor (BC 547 & BC 557)
 Resistor
 Capacitor
 1n 4007
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Self Switching Power Supply
Power Supply
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Self Switching Power Supply
Step down Transformer
 Its secondary voltage is less than its primary voltage.
 It is designed to reduce the voltage from the primary winding to
the secondary winding.
 The transformer converts high-voltage, low-current power into
low-voltage, high-current power.
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Self Switching Power Supply
Rectifier
 A bridge rectifier is an arrangement of four or more diodes in a bridge
circuit configuration which provides the same output polarity for either
input polarity. It is used for converting an alternating current (AC) input
into a direct current (DC) output.
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Self Switching Power Supply
Filters
 A filter is a capacitor which filters out a certain frequency or range of
frequencies from a circuit.
 Usually capacitors filter out very low frequency signals.
 These are signals that are very close to 0Hz in frequency value.
 These are also referred to as DC signals.
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Self Switching Power Supply
Voltage Regulator (LM 7805)
 A voltage regulator is designed to automatically maintain a fixed
voltage level.
 7805 is a voltage regulator integrated circuit.
 It is a member of 78xx series of fixed linear voltage regulator ICs.
 The xx in 78xx indicates the fixed output voltage it is designed to
provide.
 7805 provides +5V regulated power supply.
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Self Switching Power Supply
Relay
 A relay is an electromagnetic switch operated by a relatively small
electric current.
 The purpose of relay is to control the movement of the gate ( whether
to open or close).
 Relay is used to control the Electrical Devices
 A relay is able to control an output circuit of higher power than the
input circuit.
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Self Switching Power Supply
Working Principle
 A regulated power supply circuit that though a fixed-voltage regulator U1LM7805 not only gives a variable but also auto switch off features This is
achieved by connecting a potentiometer between common terminal of
regulator IC and ground. For every 100-ohm increment in the in- circuit
value of the resistance of potentiometer RV1, the output voltage increases
by 1 volt.
 Thus, the output varies from 3.7V to 8.7V (taking into account 1.3-volt drop
across diodes D7 and D8).
 Another important feature of the supply is that it switches itself off when
no load is connected across its output terminals.
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Self Switching Power Supply
Working Principle
 This is achieved with the help of transistors Q1 and Q2, diodes D7 and
D8, and capacitor C2. When a load is connected at the output, potential
drop across diodes D7 and D8 (approximately 1.3V) is sufficient for
transistors Q2 and Q1 to conduct.
 As a result, the relay gets energized and remains in that state as long as
the load remains connected. At the same time, capacitor C2 gets
charged to around 7-8 volt potential through transistor Q2. But when
the load( a lamp here in series with S2 ) is disconnected, transistor Q2
is cut off. However, capacitor C2 is still charged and it starts discharging
through base of transistor Q1.
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Self Switching Power Supply
Working Principle
 After some time (which is basically determined by value of C2), relay RL1 is
de-energized, which switches off the mains input to primary of transformer
TR1.
 To resume the power again, switch S1 Push button should be pressed
momentarily. Higher the value of capacitor C2 more will be the delay in
switching off the power supply on disconnection of the load, and vice versa.
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Self Switching Power Supply
Working Principle
 A transformer with a secondary voltage of 12V-0V, 250mA was used, it can
nevertheless be changed as per user’s requirement (up to 30V maximum.
and 1-ampere current rating). For drawing more than 300mA current, the
regulator IC must be fitted with a small heat sink over a mica insulator.
When the transformer’s secondary voltage increases beyond 12 volts
(RMS), potentiometer RV1 must be re-dimensioned. Also, the relay
voltage rating should be predetermined.
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Self Switching Power Supply
Advantages
 Used as protector for electronic Circuits
 Can obtain different voltages
 Act as protector for monitors
 Less costly
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Self Switching Power Supply
Disadvantages
 If once the supply breaks then it is powered on manually.
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Self Switching Power Supply
Applications
 This circuit is used in different electronic circuits, electronic
equipment's.
 This circuits is used for damage less uses.
 By this circuit we can produces various voltages.
 It can be used as supply voltage for different electronic equipment's.
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Self Switching Power Supply
Conclusion
 Almost all electronic equipements include a circuit that converts AC
voltage of main supply into DC voltage. This part of the equipments is called
power supply. In General, at the input of the power supply, there is a power
transformer. A diode circuit called rectifier follows it. The output of the
rectifier goes to a smoothing filter and then to a voltage regulator circuit.
The rectifier is the heart of the power supply.
 This power supply is used as a variable one. In laboratory applications we
need variable voltages in different level. So this circuit is very much helpful
in electronic experimental applications. Also the auto switching facility less
the energy wastage.
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