78xx ics and their
description
"7805", "7812", and "79xx"
Ic
7805 circuit diagram
An assortment of 78xx ICs
The 78xx (sometimes L78xx, LM78xx, MC78xx...) 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 ICs have three terminals and are commonly found in the
TO-220 form 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
(though smaller or larger packages may have a lower or higher current rating).
Advantages
78xx series ICs do not require additional components to
provide a constant, regulated source of power, making them easy to use, 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 current. 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.
Disadvantages
The input voltage must always be higher than the output
voltage by some minimum amount (typically 2.5 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 difference 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. Buck converters may be preferred over 78xx regulators
because they are more efficient and do not require heat sinks, but they are
more expensive.
Individual devices in the series
TS7805
linear voltage regulator in a TO-220 variant package
with electrically isolated tab
There are common configurations for 78xx ICs, including 7805
(5 V), 7806 (6 V), 7808 (8 V), 7809 (9 V), 7810 (10 V), 7812 (12 V), 7815 (15
V), 7818 (18 V), and 7824 (24 V) versions. The 7805 is the most common, as its
regulated 5-volt supply provides a convenient power source for most TTL
components.
Part number Output
voltage (V) Minimal input voltage
(V)
7805 +5 7.3
7806 +6 8.3
7808 +8 10.5
7810 +10 12.5
7812 +12 14.6
7815 +15 17.7
7818 +18 21.0
7824 +24 27.1
Less common are lower-power versions such as the LM78Mxx
series (500 mA) and LM78Lxx series (100 mA) from National Semiconductor. Some
devices provide slightly different voltages than usual, such as the LM78L62
(6.2 volts) and LM78L82 (8.2 volts) as well as the STMicroelectronics L78L33ACZ
(3.3 volts)
Unrelated devices
The LM78S40 from National Semiconductor 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
(designed as a drop-in replacement for 78xx chips), and not a linear regulator
like the 78xx ICs.
