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In electronics, a voltage regulator is a power supply circuit designed to supply a steady, constant voltage output.
A linear regulator is a voltage regulator based on a transistor operating in its "linear region". That transistor acts like a variable resistor.
(In contrast, a switching regulator is based on a transistor forced to act as an on/off switch).
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.
- Series regulators are the more common form. The series regulator works by providing a path from the supply voltage to the load through a variable resistance (the main transistor is in the "top half" of the voltage divider).
- The shunt regulator works by providing a path from the supply voltage to ground through a variable resistance (the main transistor is in the "bottom half" of the voltage divider). 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.
All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the drop-out voltage. 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.
When one wants a voltage higher than the available input voltage, no linear regulator will work (not even a LDO). Once is forced to use a switching regulator.
Using a linear regulator
The most common linear regulators are three-terminal integrated circuits in P1d packages/TO-220 package. (The TO-220 package is the same kind that many medium-power transistors commonly come in: three legs in a straight line protruding from a black plastic molded case with a metal backplate which has a hole for bolting to a heatsink).
After one connects the appropriate pins to 0v and incoming power, the regulated output voltage appears on the output pin.
When one wants a fixed voltage of +3v, +5v, -5v, +9v, -9v, +12, -12v, +15v, or -15v,
and one expects the load to typically require less than 7 amperes,
one uses one of the commonly-available "fixed" three-terminal linear regulators.
The "78" series (7805, 7812, etc.) regulate positive voltages while the "79" series (7905, 7912, etc.) regulate negative voltages. 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.
When one wants some other fixed voltage,
and one expects the load to typically require less than 7 amperes,
one uses one of the commonly-available "adjustable" three-terminal linear regulators and a variable resistor. The "317" series (+1.2v) regulates positive voltages while the "337" series (-1.2v) regulates negative voltages.
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 packages with more than three pins, including dual in-line packages.
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