For ICs within the 78xx family, the xx is replaced with two digits, indicating the output voltage. 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 79xxdevices 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 ICs have three terminals and are commonly found in the TO-220form factor, although they are available in surface-mount, TO-92, and TO-3 packages. These devices support an input voltage anywhere from around 2.5 volts over the intended output voltage up to a maximum of 35 to 40 volts depending on the model, and typically provide 1 or 1.5 amperes of current.
Family members
78xx
There are common configurations for 78xx ICs, including 7805, 7806, 7808, 7809, 7810, 7812, 7815, 7818, and 7824 versions. The 7805 is the most common, as its regulated 5-volt supply provides a convenient power source for most TTL components. Less common are lower-power versions such as the LM78Mxx series and LM78Lxx series from National Semiconductor. Some devices provide slightly different voltages than usual, such as the LM78L62 and LM78L82 as well as the STMicroelectronics L78L33ACZ. The 7805 has been used in some ATX power supply designs for the +5 VSB output.
79xx
The 79xx devices have a similar "part number" to "voltage output" scheme, but their outputs are negative voltage, for example 7905 is −5 V and 7912 is −12 V. The 7905 and/or 7912 were popular in many older ATX power supply designs, and some newer ATX power supplies may have a 7912.
Unrelated devices
The LM78S40 from Fairchild is not part of the 78xx family and does not use the same design. It is a component in switching regulator designs and is not a linear regulator like other 78xx devices. The 7803SR from Datel is a full switching power supply module, and not a linear regulator like the 78xx ICs.
Advantages
While external capacitors are typically required, 78xx series ICs do not require additional components to set their output voltage. 78xx designs are simple in comparison to switch-mode power supply designs.
78xx series ICs have built-in protection against a circuit drawing too much current. They have protection against overheating and short-circuits, making them robust in most applications.
Disadvantages
The input voltage must always be higher than the output voltage by some minimum amount. This can make these devices unsuitable for powering some devices from certain types of power sources. For input voltages closer to the output voltage, a pin-compatible low-dropout regulator can be used instead.
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 going into the 78xx will be more than the output power provided. The difference is dissipated as heat. This means both that for some applications an adequate heatsink must be provided, and also that a 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, this inefficiency can be a significant issue. Buck converters may be preferred over 78xx regulators because they are more efficient and do not require heat sinks, though they might be more expensive.
When using them in 'LM317 mode', with an R1/R2 voltage divider, note that the bias/adjust current drawn is several orders of magnitude higher, so this extra current makes the simple R1/R2 voltage division calculation less accurate for higher values of R.
