The 78xx  is a family of self-contained fixed linear voltage regulator integrated circuits. The 78xx family is commonly used in electronic circuits requiring a regulated power supply due to their ease-of-use and low cost. For ICs within the family, the xx is replaced with two digits, indicating the output voltage (for example, the 7805 has a 5 volt output, while the 7812 produces 12 volts). The 78xx line are positive voltage regulators: they produce a voltage that is positive relative to a common ground. There is a related line of 79xx devices which are complementary negative voltage regulators. 78xx and 79xx ICs can be used in combination to provide positive and negative supply voltages in the same circuit.

78xx series ICs do not require additional components to provide a constant, regulated source of power, making them , as well as economical and efficient uses of space. Other voltage regulators may require additional components to set the output voltage level, or to assist in the regulation process. Some other designs (such as a switched-mode power supply) may need substantial engineering expertise to implement.
78xx series ICs have built-in protection against a circuit drawing too much power. They have protection against overheating and short-circuits, making them quite robust in most applications. In some cases, the current-limiting features of the 78xx devices can provide protection not only for the 78xx itself, but also for other parts of the circuit.

The input voltage must always be higher than the output voltage by some minimum amount (typically 2 volts). This can make these devices unsuitable for powering some devices from certain types of power sources (for example, powering a circuit that requires 5 volts using 6-volt batteries will not work using a 7805).
As they are based on a linear regulator design, the input current required is always the same as the output current. As the input voltage must always be higher than the output voltage, this means that the total power (voltage multiplied by current) going into the 78xx will be more than the output power provided. The extra input power is dissipated as heat. This means both that for some applications an adequate heatsink must be provided, and also that a (often substantial) portion of the input power is wasted during the process, rendering them less efficient than some other types of power supplies. When the input voltage is significantly higher than the regulated output voltage (for example, powering a 7805 using a 24 volt power source), this inefficiency can be a significant issue.

Pin No Function Name
1                                                              Input voltage (5V-18V)                                        Input
2                                                              Ground (0V)                                                         Ground
3                                                              Regulated output; 5V (4.8V-5.2V)                     Output 
<p>Thanks, I needed this info.</p><p>Zapp</p>
<p>if i give input of 7v,100mA then output is 5v.<br>Now my question is that if iincrease the output current to 100mA to 500mA then what will i do?<br>plz reply fast</p>
I'm afraid that this test is not legally posted here, as it was copied from Wikipedia (see this version of the article: http://en.wikipedia.org/w/index.php?title=78xx&amp;oldid=501688220) which is not compatibly licensed. It cannot be re-released under a non-commercial license. Additionally, it does not acknowledge its source, which is a violation of the license of the original material.
Under disadvantages I think you forgot to mention that the 78XX series are fixed regulators. So if you want a variety of different output voltages you have to stock a number of different components. Oh yeah, fixed regulators are not very adjustable either. I think there is some hack where you can float the ground like the adjustable regulators are adjusted but don't quote me on that ... But if you need a fast 5 or 12 volts the 78XX regulators can't be beat!
for that we used variable resister for very adjustable output... <br> <br>IF U PLACE 7815 IC and put variable resister across 3 and 2 pin then u get all the range from 0 to 15...
if you place a variable resistor between pin 3 and pin 2? where do place the ground? if i am not mistaken, with a variable resistor across pin 3 and pin 2, you have wired your regulator as a current regulator, for variable output best to use a LM317 3 pin regulator.

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