LM317 Guide: Pinout, Resistor Values and Heat Limits

An LM317 circuit can produce exactly the expected voltage on an unloaded bench supply and still fail when its real load starts. The resistor ratio is only one part of the design: pin orientation, minimum current, input headroom and heat determine whether that voltage remains usable. This guide connects those checks so engineers and buyers can distinguish a workable substitute from a familiar part number.

The reference device here is Texas Instruments' LM317, using its April 2025 revision Z datasheet. Other manufacturers' LM317 versions, different current grades and adjustable modules need their own checks. The illustrations are original explanatory drawings, and the numerical examples are calculations rather than bench measurements.

1. Which LM317 does this guide describe?

The LM317 is an adjustable positive linear regulator. TI specifies a nominal 1.25V reference between OUTPUT and ADJUST, with an adjustable output range extending to 37V under suitable conditions. It is not a negative regulator, a switching converter or a regulator that can automatically reach zero volts.

Revision Z separates legacy and new silicon in several tables, while the TO-220 versions are identified as legacy devices. That distinction matters when copying a ripple-rejection figure or thermal number from a product page. Keep the complete manufacturer part number, package code and applicable datasheet together throughout qualification.

The TI LM317 datasheet is the source for device-specific statements below. Application calculations are identified separately so that an illustrative result is not mistaken for a guaranteed specification.

2. LM317 pinout: the tab is OUTPUT

For TI's TO-220 KCS/KCT packages, pin 1 is ADJUST, pin 2 is OUTPUT and pin 3 is INPUT. The metal tab is also OUTPUT. Read the drawing with its stated viewing direction: turning a package over reverses the apparent left-to-right order.

A grounded chassis is therefore not automatically a safe mounting surface for a bare tab. If the application requires electrical isolation, the mounting system must provide it, and its thermal resistance belongs in the cooling calculation. Check insulating washers, screw clearance and continuity before applying power.

The same ADJUST/OUTPUT/INPUT function names do not make every three-terminal regulator interchangeable. A fixed regulator can have a different pin sequence and a ground-connected tab. SOT-223, TO-263 and TO-220 packages also need separate footprints even when their main pins share numbers.

3. LM317 resistor calculator: include adjustment current

Connect R1 from OUTPUT to ADJUST and R2 from ADJUST to ground. The reference acts across R1, so the divider differs from an ordinary feedback network whose upper resistor runs to a ground-referenced feedback input. Keep those resistor names consistent when moving between a schematic, calculator and purchasing list.

VOUT = VREF × (1 + R2/R1) + IADJ × R2. TI gives 1.25V as the typical reference and 50µA as typical adjustment current, with limits and conditions in the electrical table. Typical values are suitable for a nominal calculation; a tolerance analysis must use the applicable limits.

For a nominal 5V target with R1 = 120Ω, ignoring adjustment current initially gives R2 = 360Ω. Restoring a typical 50µA adds 18mV, producing about 5.018V. This is a nominal estimate, not a statement that the assembled circuit will stay within 18mV of 5V.

Solving for a chosen target instead gives R2 = (VOUT − VREF)/(VREF/R1 + IADJ). Real resistor series rarely contain that exact result, so choose practical values and recalculate. Reference limits, resistor tolerance, temperature drift, load regulation and wiring losses then complete the error budget.

These are examples of available resistor values, not precision-voltage recommendations. For a programmable supply, consider what happens if the potentiometer wiper loses contact. An adjustment fault must not silently raise the rail above the connected load's permitted voltage.

4. Why 240Ω is not a universal minimum-load solution

A regulator still needs its specified operating current when the external load sleeps or disconnects. The feedback network contributes current because approximately VREF/R1 flows through R1. For a 1.25V reference, 240Ω draws about 5.21mA, while 120Ω draws about 10.42mA.

TI's electrical table lists a 10mA maximum minimum-load requirement at a 40V input-output differential over the specified temperature range. That is a particular test condition, not a claim that every operating point consumes exactly 10mA. It does explain why reproducing a 240Ω typical circuit is insufficient proof of unloaded regulation across the full envelope.

Check the lower reference limit and upper resistor tolerance when calculating the smallest divider current. For example, 1.20V across a 120Ω resistor at its +1% limit gives about 9.90mA. A design relying on a guaranteed 10mA load would need another guaranteed contribution or a revised resistor choice.

Also account for the divider's standby consumption and resistor dissipation. R1 dissipates approximately VREF squared divided by R1; R2 carries the programming current plus adjustment current. Higher output settings can make the lower resistor's power rating important even when the reference resistor runs cool.

5. Input headroom: check the lowest point, not the label

The LM317 is a conventional linear regulator rather than a low-dropout part. TI recommends 3V input-output headroom for the stated operating goal, and its electrical conditions use a minimum 3V differential for key specifications. A typical dropout curve is not a substitute for a guaranteed boundary over the design's full load and temperature range.

For a 5V rail, an 8V regulator input is therefore a useful first screening point using that recommendation. Check the actual input pin during load steps and at the lowest supply condition. Cable loss, rectifier ripple and connector resistance can consume the apparent margin before it reaches the regulator.

The opposite extreme matters too: excess input voltage creates excess dissipation. A 12V adapter and a 24V industrial rail are not equivalent sources for a small linear regulator producing the same 5V output. Include startup, shutdown and fault differentials when checking absolute maximum limits; a high permitted differential does not imply full output current there.

6. Heat budget: where the 1.5A assumption breaks

Start with PD ≈ (VIN − VOUT) × IOUT, then refine the estimate for other currents and losses where necessary. In the following example, IOUT is 0.50A total delivered from the regulator's output, including feedback-network current. It is not an additional 0.50A load on top of that network, and adjustment-terminal power is omitted from this first estimate.

The efficiency column counts all output-terminal power, including divider consumption, and ignores adjustment-current effects. Useful-load efficiency will be lower. These rows describe a hypothetical operating demand; they do not establish that the device can sustain the requested current at each differential.

With those explicitly assumed conditions, the first-order budget is (110 − 50)/35 = approximately 1.71W. The 12V-to-5V case would imply about 172.5°C junction temperature if the simple steady-state model could continue unchanged. That result rejects the proposed operating point; it does not predict a valid steady temperature through protection or device failure.

To hold the same 110°C design target at 3.50W and 50°C ambient, total effective thermal resistance would need to be approximately 17.14°C/W or lower. A heat-sink calculation must allocate that budget between the junction-to-case path, mounting interface and sink-to-air path. Electrical isolation can increase the interface contribution.

Do not transplant a package's listed junction-to-ambient number into an unrelated board or enclosure. TI's thermal-metrics application note explains why test-board metrics require care. At large voltage drops, reducing the upstream voltage or evaluating a switching stage can be more practical than enlarging the sink.

7. Capacitors and reverse-discharge paths

In TI's LM317 application guidance, the output capacitor improves transient behavior but is not required for stability. Local input bypassing becomes important when the regulator is separated from the upstream filter capacitor. Do not turn that device-specific statement into a capacitor rule for every adjustable regulator.

An ADJUST bypass capacitor can improve ripple rejection, but it also stores energy. Consider an input supply that collapses while an output capacitor remains charged, and an output short while the ADJUST capacitor retains charge. Those conditions have different discharge paths.

D1 connects with its anode at OUTPUT and cathode at INPUT; D2 connects with its anode at ADJUST and cathode at OUTPUT. In normal positive-regulator operation these paths are reverse-biased. Under the relevant collapse or short condition, they can provide a discharge route around vulnerable internal paths.

Check the chosen capacitor sizes, supply discharge behavior and possible external backfeed before deciding the protection network. A diode symbol alone does not establish surge capability or safe behavior with another energized supply. Keep high-current discharge loops short and separate them from the quiet feedback connection.

8. Key differences: LM317A, LM1117 and switching options

A replacement decision needs a stated reason: tighter accuracy, less headroom, lower heat or a different package. Those goals lead to different candidates. Matching the familiar three-terminal outline is only an initial mechanical observation.

The LM317A datasheet specifies a 1% accuracy class with its own conditions. That does not remove resistor error or make every family package identical. The LM1117 datasheet describes an 800mA-class regulator and different capacitor requirements, so an adjustable LM1117 is not an automatic LM317 substitute.

A module is another product again: its potentiometer, capacitor ratings, protection and thermal construction all matter. Do not approve it from the regulator name printed in a listing. For related context, see the AMS1117-3.3 alternatives guide, while retaining the distinction between fixed 3.3V and adjustable circuits.

9. A migration and incoming-inspection checklist

Before placing a volume order, record the incumbent manufacturer and full MPN, required output accuracy, minimum and maximum load, input extremes and ambient conditions. Include the package drawing and heat-sink arrangement. This turns a broad request for an equivalent into a checkable engineering requirement.

1. Compare pin functions and tab connection using the same viewing direction.
2. Recalculate nominal voltage, tolerance corners, divider current and resistor power.
3. Check minimum headroom during load steps and maximum differential during faults.
4. Check capacitor requirements, discharge paths and startup or shutdown sequencing.
5. Validate thermal performance on the actual board and in its enclosure.
6. Record acceptance limits before testing samples or approving the substitute.

For incoming inspection, compare labels, full ordering codes and lot traceability against purchasing documents. Visual appearance and a correct unloaded voltage do not establish authenticity or full electrical compliance. Where risk warrants it, use a qualified test plan covering relevant electrical, thermal and packaging checks.

Use LM317 part-number search and LM317A part-number search to identify sourcing candidates, then confirm exact ordering codes through quotation. Search results are not a promise of stock or an approved-alternative list. The component availability guide explains why quantity, MOQ and delivery evidence should remain separate from technical qualification.

10. LM317 FAQ

Can an LM317 produce 5V from a 5V USB rail?

No, it cannot regulate 5V from an equal 5V input. It needs input-output headroom, including margin for wiring loss and supply variation. Evaluate an appropriate different power topology if the required output must remain at 5V.

Are 240Ω and 720Ω a guaranteed 5V combination?

No, they give 5V only in the simplified nominal equation. Typical adjustment current adds about 36mV, and reference and resistor tolerances remain. The approximately 5.21mA divider current also needs a minimum-load check.

Can the metal tab be bolted directly to ground?

Not for the illustrated TI TO-220 pinout: the tab is OUTPUT. Use an appropriate isolated mounting arrangement when the heat sink must be grounded, and include its thermal effect. Verify the specific package rather than assuming all regulators match.

Why does the unloaded output look correct but fall under load?

Several mechanisms can cause it, including insufficient headroom, wiring drop, current limiting or overheating. Measure input and output at the device pins and observe how the symptom changes with load and time. A normal open-circuit reading cannot distinguish those causes.

Does thermal shutdown make an undersized heat sink acceptable?

No, thermal protection is not a normal temperature-control strategy. Repeated shutdown can interrupt the powered system and does not demonstrate operation within the specified envelope. Reduce loss or improve the verified cooling path.

Can a potentiometer make the output reach zero volts?

No, a ground-referenced passive divider does not remove the positive reference offset. A true zero-volt adjustable supply needs additional circuitry or a suitable alternative architecture. Check behavior at startup and shutdown as well as the lowest steady setting.

11. Official sources and calculation boundaries

Cover photograph: Retired electrician, Wikimedia Commons, CC0 1.0; cropped, color-adjusted and typeset by Chip-seek. The photographed onsemi LM317T illustrates the package; electrical examples in this guide refer to TI devices.

• TI LM317, SLVS044Z, April 2025: pin assignment, electrical limits and application circuits.
• TI LM317A, SNVSAC2A, June 2020: accuracy and variant comparison.
• TI LM1117, SNVS774Q: current class and capacitor requirements.
• TI Semiconductor and IC Package Thermal Metrics, SPRA953D: interpreting thermal metrics.

References were checked on October 29, 2026. Resistor results use stated nominal or corner assumptions; thermal graphics use an explicitly assumed cooling path and omit adjustment-current power. None of the drawings or calculations represents a measured result or replaces qualification of the actual circuit.