4--- Voltage Regulators

Voltage regulators produce fixed DC output voltage from variable DC (a small amount of AC on it). Normally we get fixed output by connecting the voltage regulator at the output of the filtered DC. It can also used in circuits to get a low DC voltage from a high DC voltage (for example we use 7805 to get 5V from 12V). There are two types of voltage regulators

1st type of regulator is the

Fixed voltage regulators(78xx,79xx)
In fixed voltage regulators there is another classification
1. +ve voltage regulators
2. -ve voltage regulators

and the 2nd type is the

Variable voltage regulators(LM317)



1. Positive Voltage Regulator includes 78xx voltage regulators. The most commonly used ones are 7805 and 7812. 7805 gives fixed 5V DC voltage if input voltage is in the range 7.5V to 20V.






2. Negative Voltage Regulator includes 79xx voltage regulators. The most commonly used ones are once again the 05 and 12.









This is the circuit diagram of 5V/9V/12V fixed power supply (Negative) at 1 Ampere current rating.

L1 = Step down transformer with i/p of 230 AC 50 Hz and output of (XX ) - 0- (XX)) volts(rms).
XX = Required DC output voltages.

Output voltage (DC Volts)------------------Transformer rating (rms Volts)
5------------------------------------------230: 5-0-5
9------------------------------------------230:9-0-9
12-----------------------------------------230:12-0-12
15------------------------------------230:15-0-15

The current rating has to be more than 1 Amp.


D1, D2 = Diodes 1N4003
D3 = Diode 1N4003/ 1N4001 (optional)

C1 = 1000 Micro Farad aluminum electrolytic capacitor(For loads less than 100mA you can sustitute with 220 microfards capacitor). Voltage rating = 2.5 times of Output Voltage.

C2 = 10 Micro Farad aluminum electrolytic capacitor


IC1 = 7905 for -5V DC output
=7909 for -9V DC output
=7912 for -12V DC output
=7915 for -15V DC output




A very important thing to be noted here is that 7805 gives fixed -5V DC voltage if input voltage is in the range -7V to -20V.
Also to be noted is that voltage regulators like 7805 acts as a current regulator and allows 1 A current through them.


But now if we want to use 7805 and get negative 5 volt we got to have a -7V. So how to get it?? One of the simplest solutions is to use the batteries as in the figure.





We can also get a positive and a negative voltage from the idea that every voltage is relative. There is no negative voltage in reality. 5 V can be treated positve if 0 V is considered the ground, and can be treated as -2 if +7 V is treated as ground. Likewise if we got a 12 V power supply and then divide the volatage using resistances and then use the 6V as the common ground for the whole circuit, we can use the original ground as the negative 6 volt.

We can also use a centre tapped transforme for the same purpose.




In the category of Variable Voltage Regulator most common is LM317 although other variable voltage regulators are available. The advantage of variable voltage regulator is that we can get a variable voltage supply by just varying a resistance only.





The basic circuit diagram for variable volatage regulator LM317 is given in the picture below





One disadvantage about LM317is that its very sensitive to temperature. The regulated voltage given as output by it changes with temperature. As we know IC's get warm after been used for a while. This results in the change in the output voltage after 317 is been used in the circuit(as it gets warm )after sometime. So constant value of voltage is not achieved. By the way we can also use a nice heat-sink attached to it so as to check it from getting warm.