power supply 78xx

7/28/2019 Power Supply 78xx

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POWER SUPPLY

LM7805 Positive Voltage Regulator

The LM78XX series of three terminal positive regulators are available in

the TO-220 package and with several fixed output voltages, making them useful

in a wide range of applications. Each type employs internal current limiting,

thermal shut down and safe operating area protection, making it essentially

indestructible. If adequate heat sinking is provided, they can deliver over 1A

output current. Although designed primarily as fixed voltage regulators, these

devices can be used with external components to obtain adjustable voltages and

currents.

Features

Output current in excess of 0.5A

No external components

Internal thermal overload protection


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Internal short circuit current-limiting

Output transistor safe-area compensation

Available in TO-220, TO-39, and TO-252 D-PAK packages

Output voltages of 5V, 12V, and 15V

Power supply circuit :

From a 12V Adaptor, input is taken into the power supply unit via a DC Socket.

A 5V regulator circuit is designed using LM7805 as shown below which is

needed for microcontroller as supply voltage.

A 12V Bridge rectifier in between the DC Socket and LM7805 is used.

This Bridge continues to provide supply even in case of Power- pin polarity

inversion at the DC Socket.

- +

U 1

B R I D G E

1

2

3

4

U 2 L M 7 8 0 5

V I N1

G

N

D

2

V O U T3

+ C 2

1 0 u F / 6 3 V

1

2

R 1

1 K

1 2

C O N 1

3 P I N D C S O C K E T

1

2

3

L E D 1

L E D

1 2

C 4

0 . 1 u F1

2

+ 1 2 V

+ 1 2 V

+C 3

4 7 0 u F / 3 5 V

1

2

+ 5 V

The capacitor C3 is used to filter the Bridge Rectifier o/p

C4 is to filter the ripples at the input of LM7805.

C2 at the LM7805 o/p is again to get a smooth DC o/p.


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A general ckt and output waveform of a Bridge rectifier is as shown below:

The Positive Half-cycle

During the negative half cycle of the supply, diodes D3 and D4 conduct in

series, but diodes D1 and D2 switch "OFF" as they are now reverse biased. The

current flowing through the load is the same direction as before.

The Negative Half-cycle


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As the current flowing through the load is unidirectional, so the voltage

developed across the load is also unidirectional the same as for the previous two

diode full-wave rectifier, therefore the average DC voltage across the load is

0.637Vmax. However in reality, during each half cycle the current flows through

two diodes instead of just one so the amplitude of the output voltage is two

voltage drops ( 2 x 0.7 = 1.4V ) less than the input VMAX amplitude. The ripple

frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply)


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power supply 78xx

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