SELF SWITCHING POWER SUPPLY SUBMITTED BY

ABSTRACT 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.

BLOCK DIAGRAM

HARD WARE EQUIPMENTS POWER SUPPLY BLOCK RELAY BC 547 TRNSISTOR BC 558 IN4007 RECTIFIER DIODE LED RESISTORS & CAPACITORS VOLTAGE REGULATOR

BC547 The BC547 transistor is an NPN Epitaxial Silicon Transistor. The BC547 transistor is a general-purpose transistor in small plastic packages. It is used in general-purpose switching and amplification BC847/BC547 series 45 V, 100 mA NPN general-purpose transistors. Whenever base is high, then current starts flowing through base and emitter and after that only current will pass from collector to emitter  

BC557 The BC557 transistor is an PNP Epitaxial Silicon Transistor The BC557 transistor is a general-purpose transistor in small plastic packages. It is used in general-purpose switching and amplification BC847/BC547 series 45 V, 100 mA PNP general-purpose transistors.

The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

RELAY IT IS A ELECTRO MAGNETIC SWITCH USED TO CONTROL THE ELECTRICAL DEVICES COPPER CORE MAGNETIC FLUX PLAYS MAIN ROLE HERE

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. 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. 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.

BIBILOGRAPHY The 8051 Microcontroller and Embedded systems” by Muhammad Ali Mazidi and Janice Gillispie Mazidi , Pearson Education. ATMEL 89S52 Data Sheets. www.atmel.com www.beyondlogic.org www.wikipedia.org www.howstuffworks.com www.alldatasheets.com

THANK ‘Q’