Linear regulator

In electronics, 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 times the voltage dropped across 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 power dissipated by the regulating device is equal to the power supply output current times the voltage drop in the regulating device.
  • 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. This form is very common for voltage reference circuits.

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 an LDO). In this situation, a switching regulator must be used.


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.

Common solid-state series voltage regulators are the LM78xx (for positive voltages) and LM79xx (for negative voltages), and common fixed voltages are 5 V (for transistor-transistor logic circuits) and 12 V (for communications circuits and peripheral devices such as disk drives). In fixed voltage regulators the reference pin is tied to ground, whereas in variable regulators the reference pin is connected to the centre point of a fixed or variable voltage divider fed by the regulator's output. A variable voltage divider (such as a potentiometer) allows the user to adjust the regulated voltage.

Fixed regulators

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 may use 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.

Adjustable regulators

For output voltages not provided by standard fixed regulators and load currents of less than 7 amperes, commonly-available "adjustable" three-terminal linear regulatorsare available. An adjustable regulator generates a fixed low nominal voltage between its output and its 'adjust' terminal (equivalent to the ground terminal in a fixed regulator). The "317" series (+1.2v) regulates positive voltages while the "337" series (-1.2v) regulates negative voltages.

The adjustment is performed by constructing a potential divider with its ends between the regulator output and ground, and its centre-tap connected to the 'adjust' terminal of the regulator. The ratio of resistances determines the output voltage using the same feedback mechanisms described earlier.

Other devices

More complex regulators are available in packages with more than three pins, including dual in-line packages.


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