Linear regulator

Linear regulators are so named because they contain a transistor operating in its "linear region", with the transistor acting like a variable resistor rather than an on/off switch (as in a switching regulator).

The transistor is used as one half of a potential divider to control the output voltage, and a feedback circuit compares the output voltage to a reference voltage in order to adjust the input to the transistor, thus keeping the output voltage reasonably constant.

This is inefficient: since the transistor is acting like a resistor, it will dissipate heat. In fact, the power loss due to heating in the transistor is the current through the regulator squared, times the resistance of the transistor.

The same function can be performed more efficiently by a switched-mode power supply, but the latter is more complex and the alternating currents in it tend to produce electromagnetic interference.

Linear regulators exist in two basic forms: series regulators and shunt regulators. The description above is of a series regulator, the more common form. The shunt regulator works by providing a path from the supply voltage to ground through a variable resistance. The current through the shunt regulator is diverted away from the load and flows uselessly to ground, making this form even less efficient than the series regulator. It is, however, simpler, sometimes consisting of just a voltage-reference diode, and is used in very low-powered circuits where the wasted current is too small to be of concern.

One important parameter of a series regulator is called the drop-out voltage. This is the minimum difference allowed between the input and output voltages. For example, a common regulator such as the 7805 has an output voltage of 5 V, but can only maintain this if the input voltage remains above about 7 V. Its drop-out voltage is therefore 7 V - 5 V = 2 V. When the supply voltage is less than about 2 V above the desired output voltage, as is the case in low-voltage microprocessor power supplies, so-called low dropout regulators (LDOs) must be used.

Using a linear regulator

The most common linear regulators are three-terminal integrated circuits in P1d packages[?] (the kind medium-power transistors commonly come in - three legs in a straight line protruding from a black ceramic square with a metal backplate which has a hole for bolting to a heatsink).

They are commonly available with output voltages of +5v, -5v, +9v, -9v, +12, -12v, +15v, and -15v, and maximum currents from 0.1 amperes upwards, but more than 2 amperes is rare. Often, the last two digits of the device number are the output voltage; eg, a 7805 is a +5v regulator, while a 7915 is a -15v regulator.

One pin is for connection to the 0v rail, one for the incoming power, and the last pin provides the regulated output voltage.

There are also adjustable regulators; the adjustment is performed by supplying a feedback path from the output terminal via a variable resistor, connected to the 'adj' terminal of the regulator, which replaces the 0v terminal - the other end of the variable resistor being connected to 0v.

Needless to say, more complex regulators are available in DIL packages[?].