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A LM317 in a TO-220 power transistor packing unit shell with a heat sink screwed on to it.

The idea[]

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.

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 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.

The transistor (or other device) is used as one half of a 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 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 multiplied by the voltage difference between input and output voltage. 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 quieter. Some designs of linear regulators use only transistors, diodes and resistors, which are easier to fabriacate 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 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 regulators (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). In this situation, a switching regulator of the "boost" type 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.

The LM317 and LM337 were an attempt to make a medium to heavy duty answer to the problem. 

  • The LM317 is a popular adjustable linear voltage regulator. It was designed by Robert 'Bob' C. Dobkin in 1976 while he worked at National Semiconductor.
  • The LM337 (a negative complement to the LM317) regulates voltages below, rather than above, the reference. It was designed by Robert "Bob" Pease (1940 – 2011).


The then Western standard silicon\germanium etching process. The metal tab is a heat sink, but it can be attached to a larger metal heat sink block or the side of a metal machine housing to dissipate it's heat to stop it burning out.

Operation without a heat sink with an ambient temperature at 50 ⁰C such as on a hot summer day inside a box, a maximum power dissipation of (TJ-TA)/RθJA = ((125-50)/80) = 0.98 W can be permitted. (A piece of shiny sheet metal of Aluminum with the dimensions 6 x 6 cm and 1.5 mm thick, results in a thermal resistance that permits 4.7 W of heat dissipation).


A constant current source circuit constructed with LM317.

Schematic of LM317 in a typical voltage regulator configuration, including decoupling capacitors to address input noise and output transients.

As linear regulators, the LM317 and LM337 are used in DC to DC converter applications.

Linear regulators inherently waste as much current as they supply. When this current is multiplied by the voltage difference between input and output, a significant amount of heat results. Therefore the use of an LM317 commonly also requires a heat sink. For large voltage differences, the energy lost as heat can ultimately be greater than that provided to the circuit. This is the trade-off for using linear regulators which are a simple way to provide a stable voltage with few additional components. The alternative is to use a switching voltage regulator which is usually more efficient but has a larger footprint and requires a larger number of associated components.

In packages with a heat-dissipating mounting tab, such as TO-220, the tab is connected internally to the output pin which may make it necessary to electrically isolate the tab or the heat sink from other parts of the application circuit. Failure to do this may cause the circuit to short.

Operation without a heat sink with an ambient temperature at 50 ⁰C such as on a hot summer day inside a box, a maximum power dissipation of (TJ-TA)/RθJA = ((125-50)/80) = 0.98 W can be permitted. (A piece of shiny sheet metal of Aluminum with the dimensions 6 x 6 cm and 1.5 mm thick, results in a thermal resistance that permits 4.7 W of heat dissipation).

In a constant voltage mode with an input voltage source at VIN at 34 V and a desired output voltage of 5 V, the maximum output current will be PMAX / (VI-VO) = 0.98 / (34-5) = 32 mA.

For a constant current mode with an input voltage source at VIN at 12 V and a forward voltage drop of VF=3.6 V, the maximum output current will be PMAX / (VI - VF) = 0.98 / (12-3.6) = 112 mA.

Voltage regulator[]

Schematic of LM317 in a typical voltage regulator configuration, including decoupling capacitors to address input noise and output transients. The LM317 has three pins: INput, OUTput, and ADJustment. The device is conceptually an op amp with a relatively high output current capacity. The inverting input of the amp is the adjustment pin, while the non-inverting input is set by an internal bandgap voltage reference which produces a stable reference voltage of 1.25 V.

A resistive voltage divider between the output and ground configures the op amp as a non-inverting amplifier so that the voltage of the output pin is continuously adjusted to be a fixed amount, the reference voltage, above that of the adjustment pin. Ideally, this makes the output voltage:

Vout = Vref (1 + RL/RH) Because some quiescent current flows from the adjustment pin of the device, an error term is added:

Vout = Vref (1 + RL/RH) + IQRL To make the output more stable, the device is designed to keep the quiescent current at or below 100µA, making it possible to ignore the error term in nearly all practical cases. 

Current regulator[]

The device can be configured to regulate the current to a load, rather than the voltage, by replacing the low-side resistor of the divider with the load itself. The output current is that resulting from dropping the reference voltage across the resistor. Ideally, this is:

Iout = Vref/RH Accounting for quiescent current, this becomes:

Iout = (Vref/RH) + IQ LM317 can also be used to design various other circuits like 0 V to 30 V regulator circuit, adjustable regulator circuit with improved ripple rejection, precision current limiter circuit, tracking pre-regulator circuit, 1.25 V to 20 V regulator circuit with minimum program current, adjustable multiple on-card regulators with single control, battery charger circuit, 50 mA constant current battery charger circuit, slow turn-on 15 V regulator circuit, ac voltage regulator circuit, current-limited 6 V charger circuit, adjustable 4 V regulator circuit, high-current adjustable regulator circuit and many more.

Compared to the 78xx/79xx ranges[]

The LM317 is an adjustable analogue to the popular 78xx fixed regulators. Like the LM317, each of the 78xx regulators is designed to adjust the output voltage until it is some fixed voltage above the adjustment pin (which in this case is labeled "ground").

The mechanism used is similar enough that a voltage divider can be used in the same way as with the LM317 and the output follows the same formula, using the regulator's fixed voltage for Vref (e.g. 5 V for 7805). However, the 78xx devices' quiescent current is substantially higher and less stable. Because of this, the error term in the formula cannot be ignored and the value of the low-side resistor becomes more critical. More stable adjustments can be made by providing a reference voltage that is less sensitive than a resistive divider to current fluctuations, such as a diode drop or a voltage buffer. The LM317 is designed to compensate for these fluctuations internally, making such measures unnecessary.

The LM337 relates in the same way to the fixed 79xx regulators.


Symbol Parameter Value Unit
Vout - 1.25 37 V
Vin – Vout difference 3 40 V
TJ Operating junction temperature range 0-125 °C 
IO(MAX) Maximum output current 1.5 A
IL(MIN) Minimum load current 3.5 mA typical, 12 mA maximum -
PD Power dissipation Internally Limited W  
RθJA Thermal resistance, Junction to ambient 80 °C/W  
RθJC Thermal resistance, Junction to case 5 °C/W  
LM317 adjustable linear voltage regulator.
Category. Statistic.
Designed in. 1976.
Made in. Mid 1970s.
Transistors per chip. 1.
Power supply. Medium and high.
Still in use. Yes.
Nationality. American.


Part pinout of LM317T showing its constant voltage reference. The package is TO-220.

The package is a plastic\epoxy TO-220. with a metal heat sink on the back.

Soviet plastic quality issues[]

They had equivalent linear voltage regulators, but the cases in the USSR were either epoxy, resin, metal or ceramic due to the crappy nature of Soviet plastic and packs castings. It was on occasions gritty, crumbly or perished very quickly in extreme temperature situations. Casing results were also poor and some rather irregular casting shapes thus occurred.

Also see[]


  1. http://www.fairchildsemi.com/ds/LM/LM317.pdf
  2. https://web.archive.org/web/20131217221518/
  3. http://www.fairchildsemi.com/ds/LM/LM317.pdf
  4. http://www.digikey.co.uk/product-search/en/programmers-development-systems/evaluation-boards-linear-voltage-regulators-ldos/2622524?k=linear%20regulator&WT.srch=1&mkwid=aTNxfOdP&pcrid=3921043272&pkw=%2Blinear%20%2Bregulator&pmt=bb&pdv=c
  5. http://www.linear.com/product/LM317
  6. https://www.arrow.com/en/products/power-management/regulators-and-controllers/linear-regulator?utm_source=bing&utm_medium=cpc&utm_term=%2blinear+%2bregulator&utm_campaign=int+-+category+-+power+management
  7. https://images.search.yahoo.com/search/images;_ylt=AwrBTz5X7XNXOswAVb1XNyoA;_ylu=X3oDMTEyY3M1aWxwBGNvbG8DYmYxBHBvcwMxBHZ0aWQDVUkyQzJfMQRzZWMDc2M-?p=LM317+Adjustable+Linear+Voltage+Regulator&fr=yset_chr_cnewtab
  8. https://en.wikipedia.org/wiki/Linear_regulator
  9. https://en.wikipedia.org/wiki/LM317
  10. https://en.wikipedia.org/wiki/Heat_sink