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| In [[electronics]], a '''linear regulator''' is a system used to maintain a steady voltage. The resistance of the regulator varies in accordance with the load resulting in a constant output voltage. The regulating device is made to act like a variable [[resistor]], continuously adjusting a [[voltage divider]] network to maintain a constant output voltage, and continually dissipating the difference between the input and regulated voltages as waste heat. By contrast, a ''[[switching regulator]]'' uses an active device that switches on and off 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.
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| Linear regulators may place the regulating device between the source and the regulated load ( a series regulator), or may place the regulating device in parallel with the load (shunt regulator). Simple linear regulators may only contain a [[Zener diode]] and a series resistor; more complicated regulators include separate stages of voltage reference, error amplifier and power pass element. Because a linear voltage regulator is a common element of many devices, [[integrated circuit]] regulators are very common; linear regulators may also be made up of assemblies of discrete solid-state or [[vacuum tube]] components.
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| ==Overview==
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| The transistor (or other device) is used as one half of a [[Voltage divider|potential divider]] 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 [[Frequency compensation|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 (electricity)|current]] times the [[voltage]] dropped across the transistor. The same function can often be performed much more efficiently by a [[switched-mode power supply]], 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 being [[electromagnetic interference|quieter]]. Some designs of linear regulators use only transistors, diodes and resistors, which are easier to fab into an integrated circuit, further reducing their weight, footprint on a PCB, and price.
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| Linear regulators exist in two basic forms: series regulators and shunt regulators.
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| 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.
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| 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-power electronics|low-power]]ed circuits where the wasted current is too small to be of concern. This form is very common for voltage reference circuits.
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| 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 5V, but can only maintain this if the input voltage remains above about 7V, before the output voltage begins sagging below the rated output. Its dropout voltage is therefore 7V - 5V = 2V. When the supply voltage is less than about 2V above the desired output voltage, as is the case in low-voltage [[microprocessor]] power supplies, so-called ''[[low dropout regulator]]s'' (LDOs) must be used.
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| When the output regulated voltage must be higher than the available input voltage, no linear regulator will work, (not even a [[Low dropout regulator]]). In this situation, a [[switched-mode power supply|switching regulator]] of the "boost" type must be used.
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| ==Simple Zener regulator==
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| [[Image:Zener reg.gif|left|frame|Simple Zener voltage regulator]]
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| The image shows a simple Zener voltage regulator. It is a shunt regulator and operates by way of the [[Zener diode]]'s action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the [[Zener breakdown]] region. The [[resistor]] R1 supplies the Zener current I<sub>Z</sub> as well as the load current I<sub>R2</sub> (R2 is the load). R1 can be calculated as -
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| <math>R1 = \frac{V_{S} - V_{Z}}{I_{Z} + I_{R2}}</math> where, V<sub>Z</sub> is the Zener voltage, and I<sub>R2</sub> is the required load current.
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| 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]] or [[voltage source]] circuits). Once R1 has been calculated, removing R2 will cause the full load current (plus the Zener current) to flow 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<sub>S</sub> and inversely depending on the load current. In some designs, the Zener diode may be replaced with another similarly-functioning device, especially in ultra-low-voltage scenario, like (under forward bias) several normal diodes or LEDs in series.<ref>When I designed my AM pocket radio powered by a 3.7V lithium-ion battery, the 1.5-1.8V power supply required by the TA7642 chip is provided using a Zener regulator using an red LED (with a forward voltage of 1.7V) in forward in place of the Zener diode. This LED also doubled as the power indicator.</ref>
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| ==Simple series regulator==
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| [[Image:Series reg.gif|left|frame|Simple series voltage regulator]]
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| Adding an [[Common collector|emitter follower]] stage to the simple Zener regulator forms a simple series voltage regulator and substantially improves the regulation of the circuit. Here, the load current I<sub>R2</sub> is supplied by the transistor whose base is now connected to the Zener diode. Thus the transistor's base current (I<sub>B</sub>) forms the load current for the Zener diode and is much smaller than the current through R2. This regulator is classified as "series" because the regulating element, viz., the transistor, appears in series with the load.
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| R1 sets the Zener current (I<sub>Z</sub>) and is determined as -
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| <math>R1 = \frac{V_{S} - V_{Z}}{I_{Z} + K.I_{B}}</math> where, V<sub>Z</sub> is the Zener voltage, I<sub>B</sub> is the transistor's base current and K = 1.2 to 2 (to ensure that R1 is low enough for adequate I<sub>B</sub>).
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| <math>I_{B} = \frac{I_{R2}}{h_{FE(min)}}</math> where,
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| I<sub>R2</sub> is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and h<sub>FE(min)</sub> is the minimum acceptable DC current gain for the transistor.
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| This circuit has much better regulation than the simple Zener 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 0.65V less than the Zener due to the transistor's V<sub>BE</sub> 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.
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| The circuit is readily made adjustable by adding a pot 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.
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| {{See also|Capacitance multiplier}}
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| ==Using a linear regulator==
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| Linear regulators can be constructed using discrete components but are usually encountered in [[integrated circuit]] forms. The most common linear regulators are three-terminal [[integrated circuit]]s in the [[TO220]] package.
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| Common solid-state series voltage regulators are the LM[[78xx]] (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 drive]]s). In fixed voltage regulators the reference pin is tied to [[ground (electricity)|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.
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| ==Fixed regulators==
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| [[File:7800 IC regulatorsa.jpg|thumb|200px|An assortment of [[78xx]] series ICs]]
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| "Fixed" three-terminal linear regulators are commonly available to generate fixed voltages of plus 3 V, and plus or minus 5 V, 6V, 9 V, 12 V, or 15 V, when the load is less than 1.5 [[ampere]]s.
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| The "[[78xx]]" series (7805, 7812, etc.) regulate positive voltages while the "[[79xx]]" 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 1.5 Amps.<ref>[http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000444.pdf], Datasheet of L78xx Showing a model that can output 1.5 Amps</ref>{{Dead link|date=March 2013}}
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| ===Adjusting fixed regulators===
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| By adding an additional circuit element to a fixed voltage IC regulator, it is possible to adjust the output voltage. Two example methods are listed below.
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| # 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 zeners, diodes, or resistors, the output voltage can be adjusted in a step-wise fashion.
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| # 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.
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| ==Variable regulators==
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| {{Expand section|date=October 2012}}
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| Variable regulators come in the form of ICs, with a capability to adjust the output voltage by using external resistors of specific values. This family includes low power devices like LM723 and medium power devices like [[LM317]] and L200. Some of the variable regulators are available in packages with more than three pins, including [[dual in-line package]]s.
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| [[File:Low Drop Voltage Regulator.png|right|thumb|162 px|adjustable voltage regulator circuit showing 'adjust' terminal]]
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| For output voltages not provided by standard fixed regulators and load currents of less than 7 [[ampere]]s, commonly available adjustable three-terminal linear regulators may be used. 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 LM317 series (+1.25V) regulates positive voltages while the LM337 series (-1.25V) 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.
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| Single IC dual tracking adjustable regulators are available for applications such as op-amp circuits needing matched positive and negative DC supplies.{{Which|date=October 2012}} Some have selectable current limiting as well. Some regulators require a minimum load.
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| ==Protection== | |
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| Linear IC voltage regulators may include a variety of protection methods:
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| * [[current limiting]] such as [[current source|constant-current]] limiting or [[Foldback (power supply design)|foldback]]
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| * thermal shutdown
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| * [[Safe operating area|safe area]] protection
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| Sometimes external protection is used, such as [[Crowbar (circuit)|crowbar protection]]
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| ==See also==
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| {{Portal|Electronics}}
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| * [[Voltage regulator]]
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| * [[Bandgap voltage reference]]
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| * [[LM317]]
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| * [[Brokaw bandgap reference]]
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| * [[Switched-mode power supply]]
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| * [[Low-dropout regulator]]
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| == References ==
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| {{Reflist}}
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| ==External links==
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| * [http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html "Zener regulator"] at ''Hyperphysics''
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| * [http://www.youtube.com/watch?v=GSzVs7_aW-Y Linear voltage regulator tutorial video in HD] Includes practical examples.
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| * [http://www.tedpavlic.com/teaching/osu/ece327/lab3_vreg/lab3_vreg_lm317_example.pdf ECE 327: LM317 Bandgap Voltage Reference Example] — Brief explanation of the temperature-independent bandgap reference circuit within the LM317.
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| * [http://www.tedpavlic.com/teaching/osu/ece327/lab3_vreg/lab3_vreg_procedure.pdf 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]] [[integrated circuit]] [[bandgap voltage reference]] and bypass [[capacitor]]s.
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| ** [http://www.tedpavlic.com/teaching/osu/ece327/lab3_vreg/lab3_vreg_repstrat.pdf 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.
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| *[http://www.datasheetcatalog.org/datasheet/sipex/SPX1580.pdf "7A SPX1580 regulator"]
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| {{Electronic component}}
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| {{DEFAULTSORT:Linear Regulator}}
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| [[Category:Electrical power control]]
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| [[de:Spannungsregler]]
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| [[ja:リニアレギュレータ]]
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