78xx

The 78xx (also sometimes known as LM78xx) series of devices is a family of self-contained fixed linear voltage regulator integrated circuits. The 78xx family is a very popular choice for many electronic circuits which require a regulated power supply, due to their ease of use and relative cheapness. When specifying individual ICs within this family, the "xx" is replaced with a two-digit number, which indicates the output voltage the particular device is designed to provide (for example, the 7805 has a 5 volt output, while the 7812 produces 12 volts). The 78xx line are positive voltage regulators, meaning that they are designed to 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 both positive and negative supply voltages in the same circuit, if necessary.

78xx ICs have three terminals and are most commonly found in the TO220 form factor, although smaller surface-mount and larger TO3 packages are also available from some manufacturers. These devices typically support an input voltage which can be anywhere from a couple of volts over the intended output voltage, up to a maximum of 35 or 40 volts, and can typically provide up to around 1 or 1.5 amps of current (though smaller or larger packages may have a lower or higher current rating).

Advantages

The 78xx series has several key advantages over many other voltage regulator circuits which have resulted in its popularity:

* 78xx series ICs do not require any additional components to provide a constant, regulated source of power, making them easy to use, as well as economical, and also efficient uses of circuit board real estate. By contrast, most other voltage regulators require several additional components to set the output voltage level, or to assist in the regulation process. Some other designs (such as a switching power supply) can require not only a large number of components but also substantial engineering expertise to implement correctly as well.

* 78xx series ICs have built-in protection against a circuit drawing too much power. They also have protection against overheating and short-circuits, rendering them "essentially indestructible" under most circumstances. 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 it is used in, preventing other components from being damaged as well.

Disadvantages

The 78xx devices have a few drawbacks which can make them unsuitable or less desirable for some applications:

* 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 which 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.

* Even in larger packages, 78xx integrated circuits cannot supply as much power as many designs which use discrete components, and therefore are generally not appropriate for applications which require more than a few amps of current.

Manufacturers

Semiconductor manufacturers producing 78xx ICs, or variants thereof, include:

* National Semiconductor
* Fairchild Semiconductor
* Datel
* Korea Electronics Company (KEC)
* Semelab
* Unisonic Technologies
* Bay Linear
* NTE Electronics
* Power Mate (PDuke)
* Motorola

Individual Devices in the Series

There are several common configurations for 78xx ICs, including 7805 (5 volt), 7806 (6 volt), 7808 (8 volt), 7809 (9 volt), 7810 (10 volt), 7812 (12 volt), 7815 (15 volt), 7818 (18 volt), and 7824 (24 volt) versions. The 7805 is very commonly used, as its regulated 5 volt supply can provide an easy and useful power source for most TTL components.

Some manufacturers also produce less common variations on the 78xx design, including lower-power versions such as the LM78Mxx series (500mA) and LM78Lxx series (100mA) from National Semiconductor. Some devices also provide slightly different voltages than usual, such as the LM78L62 (6.2 volts) and LM78L82 (8.2 volts).

Unrelated Devices

Despite similar names, it should be noted that the LM78S40 device from National Semiconductor is not part of the usual 78xx family, and does not use the same design. It is intended to be used as a component in switching regulator designs, and is not a linear regulator like other 78xx devices. Likewise, the 7803SR from Datel is actually a full switching power supply module (designed as a drop-in replacement for 78xx chips), and not actually a linear regulator like the 78xx ICs.

ee also

* LM317 — A similar linear regulator chip with a configurable output voltage.
* DC to DC converter — A class of devices which convert one DC voltage level to another. Linear regulators (and thus 78xx devices) are a form of DC to DC converter.

External links

* [http://www.national.com/ds/LM/LM340.pdf Datasheet of LM340/78xx from National Semiconductor(pdf)]