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In
electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, a linear regulator is a
voltage regulator A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components ...
used to maintain a steady voltage. The resistance of the regulator varies in accordance with both the input voltage and the load, resulting in a constant voltage output. The regulating circuit varies its resistance, continuously adjusting a
voltage divider In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the in ...
network to maintain a constant output voltage and continually dissipating the difference between the input and regulated voltages as
waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ...
. By contrast, a ''
switching regulator A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic componen ...
'' uses an active device that switches on and off (oscilates) to maintain an average value of output. Because the regulated voltage of a linear regulator must always be lower than input voltage, efficiency is limited and the input voltage must be high enough to always allow the active device to drop some voltage. Linear regulators may place the regulating device in parallel with the load ( shunt regulator) or may place the regulating device between the source and the regulated load (a series regulator). Simple linear regulators may only contain as little as a
Zener diode A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached. Zener diodes are manufactured with a great varie ...
and a series resistor; more complicated regulators include separate stages of voltage reference, error amplifier and power pass element. Because a linear
voltage regulator A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components ...
is a common element of many devices, single-chip regulators ICs are very common. Linear regulators may also be made up of assemblies of discrete solid-state or
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
components. Despite their name, linear regulators are
non-linear circuits A network, in the context of electrical engineering and electronics, is a collection of interconnected components. Network analysis is the process of finding the voltages across, and the currents through, all network components. There are many ...
because they contain non-linear components (such as Zener diodes, as shown below in the simple shunt regulator) and because the output voltage is ideally constant (and a circuit with a constant output that doesn't depend on its input is a non-linear circuit.)


Overview

The transistor (or other device) is used as one half of a
voltage divider In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the in ...
to establish the regulated output voltage. The output voltage is compared to a reference voltage to produce a control signal to the transistor which will drive its gate or base. With negative feedback and good choice of compensation, the output voltage is kept reasonably constant. Linear regulators are often inefficient: since the transistor is acting like a resistor, it will waste electrical energy by converting it to heat. In fact, the power loss due to heating in the transistor is the
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
multiplied by the
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
difference between input and output voltage. The same function can often be performed much more efficiently by a
switched-mode power supply A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Lik ...
, but a linear regulator may be preferred for light loads or where the desired output voltage approaches the source voltage. In these cases, the linear regulator may dissipate less power than a switcher. The linear regulator also has the advantage of not requiring magnetic devices (inductors or transformers) which can be relatively expensive or bulky, being often of simpler design, and cause less
electromagnetic interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
. Some designs of linear regulators use only transistors, diodes and resistors, which are easier to fabricate into an integrated circuit, further reducing their weight, footprint on a PCB, and price. All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the dropout 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, before the output voltage begins sagging below the rated output. Its dropout 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 A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
power supplies, so-called ''
low dropout regulator A low-dropout regulator (LDO regulator) is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. The advantages of an LDO regulator over other DC-to-DC voltage regul ...
s'' (LDOs) must be used. When the output regulated voltage must be higher than the available input voltage, no linear regulator will work (not even a
Low dropout regulator A low-dropout regulator (LDO regulator) is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. The advantages of an LDO regulator over other DC-to-DC voltage regul ...
). In this situation, a
boost converter A boost converter (step-up converter) is a DC-to-DC power converter that steps up voltage (while stepping down current) from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) containing at least two semi ...
or a
charge pump A charge pump is a kind of DC-to-DC converter that uses capacitors for energetic charge storage to raise or lower voltage. Charge-pump circuits are capable of high efficiencies, sometimes as high as 90–95%, while being electrically simple c ...
must be used. Most linear regulators will continue to provide some output voltage approximately the dropout voltage below the input voltage for inputs below the nominal output voltage until the input voltage drops significantly. Linear regulators exist in two basic forms: shunt regulators and series regulators. Most linear regulators have a maximum rated output current. This is generally limited by either power dissipation capability, or by the current carrying capability of the output transistor.


Shunt regulators

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 directly to ground, making this form usually less efficient than the series regulator. It is, however, simpler, sometimes consisting of just a voltage-reference
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A 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. A shunt regulator can usually only sink (absorb) current.


Series regulators

Series regulators are the more common form; they are more efficient than shunt designs. The series regulator works by providing a path from the supply voltage to the load through a variable resistance, usually a transistor (in this role it is usually termed the series pass transistor); it is in the "top half" of the voltage divider - the bottom half being the load. The power dissipated by the regulating device is equal to the power supply output current times the ''voltage drop'' in the regulating device. For efficiency and reduced stress on the pass transistor, designers try to minimize the voltage drop but not all circuits regulate well once the input (unregulated) voltage comes close to the required output voltage; those that do are termed Low Dropout regulators, A series regulator can usually only source (supply) current, unlike shunt regulators.


Simple shunt regulator

The image shows a simple shunt voltage regulator that operates by way of the
Zener diode A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached. Zener diodes are manufactured with a great varie ...
's action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the
Zener breakdown In electronics, the Zener effect (employed most notably in the appropriately named Zener diode) is a type of electrical breakdown, discovered by Clarence Melvin Zener. It occurs in a reverse biased p-n diode when the electric field enables tunn ...
region. The
resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
''R''1 supplies the Zener current I_\mathrm as well as the load current ''I''R2 (''R''2 is the load). ''R''1 can be calculated as R1 = \frac, where V_\mathrm is the Zener voltage, and ''I''R2 is the required load current. This regulator is used for very simple low-power applications where the currents involved are very small and the load is permanently connected across the Zener diode (such as
voltage reference A voltage reference is an electronic device that ideally produces a fixed (constant) voltage irrespective of the loading on the device, power supply variations, temperature changes, and the passage of time. Voltage references are used in power supp ...
or
voltage source A voltage source is a two-terminal device which can maintain a fixed voltage. An ideal voltage source can maintain the fixed voltage independent of the load resistance or the output current. However, a real-world voltage source cannot supply unli ...
circuits). Once ''R''1 has been calculated, removing ''R''2 will allow the full load current (plus the Zener current) through the diode and may exceed the diode's maximum current rating, thereby damaging it. The regulation of this circuit is also not very good because the Zener current (and hence the Zener voltage) will vary depending on V_\mathrm and inversely depending on the load current. In some designs, the Zener diode may be replaced with another similarly functioning device, especially in an ultra-low-voltage scenario, like (under forward bias) several normal diodes or LEDs in series.


Simple series regulator

Adding an
emitter follower In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit the base terminal o ...
stage to the simple shunt regulator forms a simple series voltage regulator and substantially improves the regulation of the circuit. Here, the load current IR2 is supplied by the transistor whose base is now connected to the Zener diode. Thus the transistor's base current (IB) forms the load current for the Zener diode and is much smaller than the current through ''R''2. This regulator is classified as "series" because the regulating element, viz., the transistor, appears in series with the load. ''R''1 sets the Zener current (IZ) and is determined as R1 = \frac where, VZ is the Zener voltage, IB is the transistor's base current, K = 1.2 to 2 (to ensure that ''R''1 is low enough for adequate IB) and I_\mathrm = \frac where, IR2 is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and hFE(min) is the minimum acceptable DC current gain for the transistor. This circuit has much better regulation than the simple shunt regulator, since the base current of the transistor forms a very light load on the Zener, thereby minimising variation in Zener voltage due to variation in the load. Note that the output voltage will always be about 0.65 V less than the Zener due to the transistor's ''V''BE drop. Although this circuit has good regulation, it is still sensitive to the load and supply variation. This can be resolved by incorporating negative feedback circuitry into it. This regulator is often used as a "pre-regulator" in more advanced series voltage regulator circuits. The circuit is readily made adjustable by adding a potentiometer across the Zener, moving the transistor base connection from the top of the Zener to the pot wiper. It may be made step adjustable by switching in different Zeners. Finally it is occasionally made microadjustable by adding a low value pot in series with the Zener; this allows a little voltage adjustment, but degrades regulation (see also capacitance multiplier).


Fixed regulators

"Fixed" three-terminal linear regulators are commonly available to generate fixed voltages of +3.3 V, and plus or minus 5 V, 6 V, 9 V, 12 V, or 15 V, when the load is less than 1.5 A. The "
78xx 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 ...
" series (7805, 7812, etc.) regulate positive voltages while the "
79xx 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 ...
" series (7905, 7912, etc.) regulate negative voltages. Often, the last two digits of the device number are the output voltage (e.g., a 7805 is a +5 V regulator, while a 7915 is a −15 V regulator). There are variants on the 78xx series ICs, such as 78L and 78S, some of which can supply up to 2 A., Datasheet of L78xx Showing a model that can output 2 A


Adjusting fixed regulators

By adding another circuit element to a fixed voltage IC regulator, it is possible to adjust the output voltage. Two example methods are: # A Zener diode or resistor may be added between the IC's ground terminal and ground. Resistors are acceptable where ground current is constant, but are ill-suited to regulators with varying ground current. By switching in different Zener diodes, diodes or resistors, the output voltage can be adjusted in a step-wise fashion. # A potentiometer can be placed in series with the ground terminal to increase the output voltage variably. However, this method degrades regulation, and is not suitable for regulators with varying ground current.


Variable regulators

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). This family of devices includes low power devices like LM723 and medium power devices like
LM317 The LM317 is a popular adjustable positive linear voltage regulator. It was designed by Bob Dobkin in 1976 while he worked at National Semiconductor. The LM337 is the negative complement to the LM317, which regulates voltages below a reference. ...
and L200. Some of the variable regulators are available in packages with more than three pins, including
dual in-line package In microelectronics, a dual in-line package (DIP or DIL), is an electronic component package with a rectangular housing and two parallel rows of electrical connecting pins. The package may be through-hole mounted to a printed circuit board (P ...
s. They offer the capability to adjust the output voltage by using external resistors of specific values. For output voltages not provided by standard fixed regulators and load currents of less than 7 A, commonly available adjustable three-terminal linear regulators may be used. The LM317 series (+1.25 V) regulates positive voltages while the LM337 series (−1.25 V) 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. Single IC dual tracking adjustable regulators are available for applications such as op-amp circuits needing matched positive and negative DC supplies. Some have selectable current limiting as well. Some regulators require a minimum load.


Protection

Linear IC voltage regulators may include a variety of protection methods: *
Current limiting Current limiting is the practice of imposing a limit on the current that may be delivered to a load to protect the circuit generating or transmitting the current from harmful effects due to a short-circuit or overload. The term "current limiting" is ...
such as constant-current limiting or foldback * Thermal shutdown *
Safe operating area For power semiconductor devices (such as BJT, MOSFET, thyristor or IGBT), the safe operating area (SOA) is defined as the voltage and current conditions over which the device can be expected to operate without self-damage. SOA is usually presented ...
protection Sometimes external protection is used, such as crowbar protection.


Using a linear regulator

Linear regulators can be constructed using discrete components but are usually encountered in
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
forms. The most common linear regulators are three-terminal
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s in the
TO-220 The TO-220 is a style of electronic package used for high-powered, through-hole components with pin spacing. The "TO" designation stands for "transistor outline". TO-220 packages have three leads. Similar packages with two, four, five or seven ...
package. Common voltage regulators are the LM
78xx 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 ...
-series (for positive voltages) and LM79xx-series (for negative voltages). Robust automotive voltage regulators, such as LM2940 / MIC2940A / AZ2940, can handle reverse battery connections and brief +50/-50V transients too. Some
Low-dropout regulator A low-dropout regulator (LDO regulator) is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. The advantages of an LDO regulator over other DC-to-DC voltage regul ...
(LDO) alternatives, such as MCP1700 / MCP1711 / TPS7A05 / XC6206, have a very low quiescent current of less than 5 µA (approximately 1,000 times less than the LM78xx series) making them better suited for battery-powered devices. Common fixed voltages are 1.8 V, 2.5 V, 3.3 V (for low-voltage
CMOS logic Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
circuits), 5 V (for transistor-transistor logic circuits) and 12 V (for communications circuits and peripheral devices such as
disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...
s). 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 A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrume ...
allows the user to adjust the regulated voltage.


See also

*
Brokaw bandgap reference Brokaw bandgap reference is a voltage reference circuit widely used in integrated circuits, with an output voltage around 1.25 V with low temperature dependence. This particular circuit is one type of a bandgap voltage reference, named after Paul Br ...
*
List of LM-series integrated circuits The following is a list of LM-series integrated circuits. Many were among the first analog integrated circuits commercially produced since late 1965; some were groundbreaking innovations. As of 2007, many are still being used. The LM series orig ...
*
Low-dropout regulator A low-dropout regulator (LDO regulator) is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. The advantages of an LDO regulator over other DC-to-DC voltage regul ...
*
Voltage regulator A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components ...


References


External links


ECE 327: Procedures for Voltage Regulators Lab
— Gives schematics, explanations, and analyses for Zener shunt regulator, series regulator, feedback series regulator, feedback series regulator with current limiting, and feedback series regulator with current foldback. Also discusses the proper use of the
LM317 The LM317 is a popular adjustable positive linear voltage regulator. It was designed by Bob Dobkin in 1976 while he worked at National Semiconductor. The LM337 is the negative complement to the LM317, which regulates voltages below a reference. ...
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
bandgap voltage reference A bandgap voltage reference is a temperature independent voltage reference circuit widely used in integrated circuits. It produces a fixed (constant) voltage regardless of power supply variations, temperature changes, or circuit loading from a devi ...
and bypass
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s.
ECE 327: Report Strategies for Voltage Regulators Lab
— Gives more-detailed quantitative analysis of behavior of several shunt and series regulators in and out of normal operating ranges.
ECE 327: LM317 Bandgap Voltage Reference Example
— Brief explanation of the temperature-independent bandgap reference circuit within the LM317.

at ''Hyperphysics'' {{DEFAULTSORT:Linear Regulator Voltage regulation