Voltag Regulators

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Voltage Regulator

Voltage regulator is a device which provides fix output voltage in spite of the variable input

voltage supplied. It is a three terminal device. Voltage regulator basically comes in two

different series: 78XX and 79XX.Voltage regulator under 78XX series are designed for

positive inputs i.e. if while 79XX series are designed for negative inputs. In market variety

of voltage regulators are available with output as 6V, 9V, 12V, 15V etc. Voltage regulator

can also withstand over current drawn due to short circuit or overheating. It will cut off the

circuit before damage occurs. One must take care while mounting the regulator because

reverse polarity may destroy the regulator.

Pin configuration of negative and positive voltage regulator are shown in fig.

As we know that output of regulator is fixed but with the help of voltage divider rule we can

use 5V regulator to deliver 12V. But reverse is not possible that is we cannot obtain 5V from12V regulator.

How we have calculated the value of resistor for different voltages-

Suppose value of resistor connected between com and output pin of regulator be

470ohm(R1).This means we have 10.6mA current (because V =5V and V = IR) available

between pin, com and output. There would be some stand-by current of about 2.5mA which

will be available between rotary switch and ground. Therefore total current available will be

approx. 13.1mA. Now, letssay we want 5V to 12V from this circuit. For minimum 5V, we

will directly get this from regulator output. But if you want maximum 12V, then apart from

5V,additional 7V would require selection of appropriate resistor.

Here R =?

V = 7V

I =13.1mA

Therefore V =I*R

R = 543ohm

Therefore, we have to connect resistor of 543ohm with 470ohm to get the desire output of

12V. But resistor of this value might not be easily available, so we can use resistor that has

values near to it viz. 560ohm.
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Now if you want to obtain different voltage between 5V and 12V use different values of

resistor like if you want to obtain 6V then-

V =6V

I = 10.6mA

R = 6V/10.6mA

R = 566ohm

But we have already connected resistor R1 of 470ohm so, for 6V we have to use resistor

value =100ohm( 566- 470ohm= 96 approx 100ohm). Similarly, you can calculate different

values of resistor for obtaining different voltages.

In this circuit we have used different resistors to obtain different values of voltage. You can

also use a variable resistor to obtain the different values of voltage with a single resistor.

IC 7805 (Voltage Regulator IC)

7805is avoltage regulator integrated circuit. It is a member of 78xx series of fixed linear

voltage regulator ICs. The voltage source in a circuit may have fluctuations and would not

give the fixed voltage output. The voltage regulator ICmaintains the output voltage at a

constant value. The xx in 78xx indicates the fixed output voltage it is designed to provide.

7805 provides +5V regulated power supply. Capacitors of suitable values can be connected at

input and output pins depending upon the respective voltage levels.

Pin Diagram:

Pin Description:


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Pin No Function Name

1 Input voltage (5V-18V) Input

2 Ground (0V) Ground

3 Regulated output; 5V (4.8V-5.2V) Output

IC 7905

7905 is a voltage regulatorintegrated circuit. It is a member of 79xx series of fixed linear

voltage regulator ICs. The voltage source in a circuit may have fluctuations and would not

give the fixed voltage output. The voltage regulator IC maintains the output voltage at a

constant value. The xx in 78xx indicates the fixed output voltage it is designed to

provide. 7905 provides a regulated supply of -5 V and 1A current. Its additional features

include internal thermal overload protection, short circuit protection and output transistor safeoperating area compensation.

Pin Diagram:

Pin Description:

Pin No Function Name

1 Ground (0V) Ground

2 Input voltage (5V-18V) Input

3 Regulated output; 5V (4.8V-5.2V) Output

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