LM317 (Adjustable Regulator) Symbol
Definition: The LM317 symbol represents a three-terminal adjustable positive linear voltage regulator IC that delivers an output voltage continuously variable from 1.25 V to 37 V, set by two external resistors connected to its IN, OUT, and ADJ pins, capable of supplying up to 1.5 A output current; in schematics it is drawn as a labelled three-pin rectangular block or TO-220/TO-92 outline following general IC conventions, designated U or VR followed by a number.
Also known as: LM317T, adjustable voltage regulator, variable linear regulator, positive adjustable regulator, 317 regulator.
What the LM317 (Adjustable Regulator) symbol means
The LM317 symbol identifies the Texas Instruments (originally National Semiconductor) LM317 series adjustable positive voltage regulator, one of the most widely used linear regulator ICs in electronics. The three-pin device maintains a fixed 1.25 V reference voltage (VREF) between its OUT and ADJ terminals; by connecting a resistor divider from OUT to ADJ to GND, the user sets the output voltage to any value from 1.25 V up to 37 V. The symbol in a schematic communicates that the output voltage is not fixed by the IC itself but is set by the values of the two external resistors R1 and R2.
In a circuit diagram the LM317 symbol marks the supply-regulation node for circuits requiring a custom, non-standard voltage that is not served by a fixed regulator series. The IN pin connects to the unregulated input supply, the OUT pin delivers the regulated output to the load, and the ADJ pin connects to the voltage-setting resistor network. The LM317's load regulation, line regulation, and temperature coefficient are sufficient for the vast majority of general-purpose linear regulation tasks.
How to identify the LM317 (Adjustable Regulator) symbol
The LM317 symbol is a three-terminal rectangular block or triangular IC outline with three labelled pins: IN on the left (or top-left), OUT on the right (or top-right), and ADJ at the bottom (or bottom-centre). When drawn in a TO-220 package outline, the three leads from left to right are ADJ, OUT, IN (note: this differs from 78xx regulators where the order is IN, GND, OUT). The label 'LM317' or 'LM317T' appears inside or beside the block. Schematic tools often add a note showing the output voltage formula: VOUT = 1.25 × (1 + R2/R1).
Function in a circuit
The LM317 contains an internal band-gap voltage reference that maintains a precise 1.25 V between the OUT and ADJ pins. The error amplifier continuously adjusts the pass transistor to enforce this 1.25 V differential. The output voltage is set by the formula VOUT = 1.25 × (1 + R2/R1) + IADJ × R2, where R1 is connected between OUT and ADJ (typical value 240 Ω), R2 connects from ADJ to ground (variable), and IADJ is the small bias current flowing into the ADJ pin (typically 50–100 µA, often negligible). A capacitor on the ADJ pin (10 µF) reduces noise on the output. The LM317 includes built-in safe-area protection, current limiting (1.5 A), and thermal shutdown (junction temperature limit 150 °C).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated schematic symbol for the LM317 or any specific regulator IC. In IEC-style schematics the device is drawn as a general rectangular function block with three pins labelled IN, OUT, and ADJ, carrying the part-number designation 'LM317'. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 (ANSI Y32.2) defines no dedicated symbol for the LM317. The device follows the general IC block drawing convention: a rectangle with input pins on the left (IN), output on the right (OUT), and reference or control pins at the bottom (ADJ). The reference designator is U or VR followed by a number. |
| Key difference | IEC and ANSI representations are identical for the LM317 block; neither standard assigns a specific glyph. The TO-220 package outline symbol (showing the three physical leads) is drawn the same way in both conventions. |
Terminals / pins
| Pin | Name |
|---|---|
| in | IN |
| out | OUT |
| adj | ADJ |
Typical values
Input voltage range: 3 V to 40 V; Output voltage range: 1.25 V to 37 V (adjustable); Output current: 1.5 A (TO-220 package with heat sink), 100 mA (TO-92); Minimum input-output differential (dropout): 3 V; Internal reference voltage: 1.25 V (±1%); Line regulation: 0.01%/V typical; Load regulation: 0.1% typical; IADJ: 50–100 µA; Quiescent current: ~5 mA; Thermal shutdown: 150 °C junction temperature.
Where the LM317 (Adjustable Regulator) symbol is used
- Bench power supply and laboratory adjustable power supply circuits requiring continuously variable output voltage from 1.25 V to 30 V
- Battery charger circuits where the charging voltage must be precisely set to the specific terminal voltage of NiMH, SLA, or NiCd batteries
- LED current regulator: the LM317 configured with a single resistor from OUT to ADJ sets a stable constant current through an LED string regardless of supply voltage
- Audio preamplifier and op-amp supply: adjustable LM317 and LM337 (negative regulator) pair generates symmetric ±Vcc rails for differential amplifier circuits
- Motor speed control: the LM317 with a variable resistor (R2 = potentiometer) provides an adjustable DC voltage to control the speed of small DC motors
- Custom-voltage power supply for specialised ICs or legacy circuits that require non-standard supply voltages (e.g. 2.5 V, 3.0 V, 6.3 V)
Example
In a variable bench power supply schematic, an LM317T (U1) receives 18 V from a bridge rectifier and filter capacitor at its IN pin; OUT connects to the load positive rail through a 0.1 µF bypass capacitor; ADJ connects to a 240 Ω fixed resistor (R1) to OUT and a 5 kΩ potentiometer (R2) to GND. Turning the potentiometer from 0 Ω to 5 kΩ varies VOUT from 1.25 V to 1.25 × (1 + 5000/240) = 27.3 V. A 0.01 µF capacitor on ADJ suppresses output noise. The LM317 symbol in the schematic is labelled 'U1 / LM317T' with the VOUT formula annotated alongside.
Key facts
- The LM317 is a three-terminal adjustable positive linear voltage regulator with an internal 1.25 V reference; output voltage is set by the formula VOUT = 1.25 × (1 + R2/R1), continuously adjustable from 1.25 V to 37 V.
- The three pins are IN (input supply), OUT (regulated output), and ADJ (adjustment); the TO-220 package lead order from left to right is ADJ, OUT, IN — different from the 78xx fixed regulator lead order.
- Maximum output current is 1.5 A (TO-220) with adequate heat sinking; the device includes built-in current limiting, safe-area protection, and thermal shutdown at 150 °C junction temperature.
- The LM317 requires a minimum input-to-output differential (dropout) of approximately 3 V; if VIN drops within 3 V of VOUT, regulation is lost.
- The LM317 symbol in schematics is a labelled three-pin rectangular block (general IC convention); neither IEC 60617 nor IEEE 315-1975 defines a dedicated glyph for this device.
- The reference designator for the LM317 in schematics is U (integrated circuit) or VR (voltage regulator) followed by a number: U1, VR1.
- The LM317 is directly complemented by the LM337 negative adjustable regulator (−1.25 V to −37 V output, same pinout convention); together they form symmetric bipolar supply circuits for dual-rail op-amp and audio applications.
Frequently asked questions
What does the LM317 symbol look like in a schematic?
The LM317 symbol is a three-terminal rectangular block or triangular IC outline with three labelled pins: IN (input supply, left), OUT (regulated output, right), and ADJ (adjustment/reference, bottom). The label 'LM317' or 'LM317T' appears inside or beside the block. Some schematics also show the output voltage formula VOUT = 1.25 × (1 + R2/R1) as an annotation.
What does the LM317 do in a circuit?
The LM317 is an adjustable positive linear voltage regulator. It takes an unregulated DC input voltage (VIN, up to 40 V) and delivers a stable, regulated output voltage (1.25 V to 37 V) set by two external resistors. The output voltage remains stable despite changes in input voltage or load current, within the device's specified regulation limits.
How do I calculate the output voltage of an LM317?
The output voltage is set by the formula VOUT = 1.25 × (1 + R2/R1), where R1 is the resistor between OUT and ADJ (typically 240 Ω) and R2 is the resistor from ADJ to GND. For example, with R1 = 240 Ω and R2 = 2.16 kΩ, VOUT = 1.25 × (1 + 2160/240) = 1.25 × 10 = 12.5 V. A potentiometer as R2 allows continuous adjustment.
What is the pin order of the LM317 in a TO-220 package?
In the TO-220 package the three leads from left to right (with the tab and label facing you) are: ADJ (pin 1), OUT (pin 2), IN (pin 3). This is the opposite of the common 78xx fixed regulator lead order (IN, GND, OUT), so care must be taken when substituting or replacing regulators on a PCB.
What is the minimum dropout voltage of the LM317?
The LM317 requires a minimum input-to-output differential (dropout voltage) of approximately 3 V. For example, to maintain a regulated 5 V output, the input voltage must be at least 8 V. If VIN falls below VOUT + 3 V, the pass transistor comes out of its linear region and regulation is lost. For applications with tight VIN-VOUT margin, an LDO (low-dropout regulator) with 100–600 mV dropout is preferable.
Does the LM317 need a heat sink?
A heat sink is required whenever the power dissipated — (VIN - VOUT) × IOUT — exceeds the device's safe operating temperature limit for the ambient environment. The TO-220 package without a heat sink has a junction-to-ambient thermal resistance of about 50 °C/W; at 1 A with a 5 V dropout that is 5 W, causing a 250 °C rise — far above the 150 °C thermal shutdown limit. A heat sink reducing thermal resistance to 5–10 °C/W is required for currents above 200–300 mA at typical dropout voltages.
Can the LM317 be used as a constant current source?
Yes. When configured with only R1 between OUT and ADJ and no R2 to ground, the LM317 forces 1.25 V across R1, delivering a constant current of IOUT = 1.25 / R1 through the load connected in series with the OUT pin. For example, R1 = 62.5 Ω sets IOUT = 20 mA — ideal for LED current regulation or battery charging at a fixed current.
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