Self Switching Power Supply

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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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Block Diagram







Hardware Requirements

Transformer

Rectifier

Filter

Regulator

Relay

Transistor (BC 547 & BC 557)

Resistor

Capacitor

1n 4007







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.







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.







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.







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.







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.







Working Principle A regulated power supply circuit that though a fixed-voltage regulator U1-

LM7805 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.

http://www.edgefxkits.com/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.







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.

Working Principle







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.







Advantages

Used as protector for electronic Circuits

Can obtain different voltages

Act as protector for monitors

Less costly







Disadvantages

If once the supply breaks then it is powered on manually.







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.







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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Self Switching Power Supply

Engineering

Transcript of Self Switching Power Supply