How do you calculate LM317 output voltage?

The LM317 output voltage is Vout = 1.25 × (1 + R2/R1). R1 is typically 240Ω (datasheet recommended). Choose R2 to set the desired output voltage. For example, for 5V output: R2 = 240 × (5/1.25 − 1) = 720Ω.

What is the minimum input voltage for LM317?

The LM317 requires a minimum dropout voltage of approximately 2V. This means the input voltage must be at least Vout + 2V for proper regulation. For a 5V output, the input must be at least 7V.

When does the LM317 need a heatsink?

The LM317 needs a heatsink when power dissipation exceeds about 1W. Power dissipation is P = (Vin − Vout) × Iout. For example, with 12V input, 5V output, and 500mA load: P = 7V × 0.5A = 3.5W — a heatsink is required.

What is the maximum output current of the LM317?

The LM317 can supply up to 1.5A of output current with adequate heatsinking. The internal current limiting protects the device, but thermal management is critical — the junction temperature must stay below 125°C.

LM317 Voltage Regulator Calculator

Calculate output voltage and resistor values for LM317 adjustable linear regulator circuits.

Component Values

R1 (typically 240Ω)

R2 (adjust resistor)

Input Voltage (Vin) (optional)

Output Current (Iout) (optional)

Results

Output Voltage (Vout)7.500 V

Reference Voltage1.25 V

Minimum Vin9.500 V

Power Dissipation2.25 W

HeatsinkRecommended (P > 1W)

Vout = 1.25 × (1 + R2/R1)

VOUT VS R2

Vout = 1.25 × (1 + R2/R1)

How does the LM317 work?

The LM317 is an adjustable three-terminal positive voltage regulator that can supply up to 1.5A with an output voltage range from 1.25V to 37V. It regulates by maintaining a constant 1.25V reference voltage between its output and adjustment pins. This means Vout = 1.25 × (1 + R2/R1).

R1 is typically fixed at 240Ω (the datasheet-recommended value). R2 is chosen to set the desired output voltage. A small current (~50µA) flows out of the adjustment pin, but it's small enough to ignore in most designs. For stable operation, place a 0.1µF ceramic capacitor close to the input pin and a 1µF capacitor on the output.

The LM317 needs at least 2V of headroom (dropout voltage) above the output. If Vin − Vout < 2V, regulation fails. The power dissipated as heat is P = (Vin − Vout) × Iout. Above ~1W you'll need a heatsink. For high-current applications with large Vin−Vout difference, consider a switching regulator instead.

Output Voltage (Vout)

Vout = 1.25 × (1 + R2/R1)

Power Dissipation

P = (Vin − Vout) × Iout

Key Points

Vout = 1.25V × (1 + R2/R1)
Minimum dropout: ~2V (Vin must be ≥ Vout + 2V)
Maximum output current: 1.5A with adequate heatsink
Add 0.1µF input and 1µF output bypass capacitors

Applications

Adjustable bench power supplies
Battery charger circuits
LED constant-current driver (with R sense)
Replacing fixed-voltage regulators for custom voltages

LM317 Adjustable Voltage Regulator

Vout = Vref × (1 + R2/R1) + Iadj × R2Vref = 1.25V | Iadj ≈ 50–100µA (negligible if R2 < 10kΩ)Simplified: Vout ≈ 1.25 × (1 + R2/R1) | R1 typically 240ΩCurrent limiter: I_limit = Vref / R_current = 1.25 / R

Output Voltage Reference (R1 = 240Ω)

Vout	R1	R2	Notes
1.25V	240Ω	0Ω	Minimum output (Vref)
1.5V	240Ω	96Ω	Use 100Ω
2.5V	240Ω	240Ω	2× R1
3.3V	240Ω	394Ω	Use 390Ω
5.0V	240Ω	720Ω	Use 680Ω + 39Ω
6.0V	240Ω	931Ω	Use 910Ω
9.0V	240Ω	1.49kΩ	Use 1.5kΩ
12.0V	240Ω	2.06kΩ	Use 2.0kΩ
15.0V	240Ω	2.64kΩ	Use 2.7kΩ

Practical Examples

5V regulated supply from 9V battery

R1=240Ω, R2=680Ω: Vout = 1.25×(1+680/240) = 4.79V ≈ 5V ✓

Add output cap 10µF + input cap 0.1µF. Max Vin=37V, Iout=1.5A.

Heat: P = (9–5)×0.5A = 2W → TO-220 heatsink needed for >200mA.

Current limiter for battery charging (500mA)

Connect as current source: R = 1.25V / 0.5A = 2.5Ω (use 2.4Ω + 0.1Ω trim).

Limits charge current regardless of battery voltage. Safe for Li-ion CC stage.

Design Tip

LM317 needs Vin – Vout > 3V (dropout voltage) to regulate. Power dissipation: Pd = (Vin – Vout) × Iout — use heatsink for > 1W. For very low dropout: use LM1117 (1.2V dropout) or LDO like MCP1700 (0.17V). Add 1µF tantalum on ADJ pin to improve ripple rejection by 15dB.

Did you know? The LM317 can provide output voltages from 1.25 V to 37 V with only two external resistors. Its internal reference voltage of 1.25 V is one of the most precisely specified parameters in analog electronics — typically within ±1%.