LM317 Constant Current Calculator

Calculator Inputs

Calculation mode

Target current

Current unit

Known sense resistor

Resistance unit

Reference voltage

Reference tolerance (%)

Resistor tolerance (%)

Input voltage

Load voltage

Regulator dropout voltage

Nearest resistor series

Wattage safety factor

Parallel load branches

Ambient temperature (°C)

Thermal resistance (°C/W)

Formula Used

Current from resistor: I = Vref / Rset

Resistor from current: Rset = Vref / I

Sense resistor power: Presistor = Vref × I

Required input voltage: Vin(min) = Vload + Vref + Vdropout

Maximum load voltage: Vload(max) = Vin - Vref - Vdropout

Regulator power: Pregulator ≈ (Vin - Vload - Vref) × I

How to Use This Calculator

Select whether you want resistor value or current value.
Enter the target current or known sense resistor.
Set the LM317 reference voltage. The common value is 1.25 V.
Add resistor tolerance and reference tolerance for worst case current.
Enter supply voltage, load voltage, and dropout voltage.
Choose a preferred resistor series for a nearby standard value.
Review heat, headroom, resistor wattage, and branch current.
Use the CSV or PDF button to save the result.

Example Data Table

Target Current	Rset at 1.25 V	Resistor Power	Typical Use
20 mA	62.5 Ω	0.025 W	Indicator LED testing
100 mA	12.5 Ω	0.125 W	Small LED string
350 mA	3.57 Ω	0.438 W	Power LED driver
1 A	1.25 Ω	1.25 W	Bench current source

About the LM317 Constant Current Method

An LM317 can work as a simple current regulator. In this setup, the device tries to keep about 1.25 volts between its output and adjust pins. A resistor placed between those pins sets the current. The load then receives nearly the same current, as long as the supply voltage is high enough.

Why the Sense Resistor Matters

The sense resistor is the key part of the design. A smaller value gives more current. A larger value gives less current. The resistor must also handle heat. Its power is found from the reference voltage multiplied by current. Extra wattage margin is wise because resistors warm up, drift, and may sit near other hot parts.

Voltage Headroom

A current source cannot force current without enough voltage. The supply must cover the load voltage, the 1.25 volt sense resistor drop, and the regulator dropout. This calculator checks that available headroom. If headroom is negative, the circuit will leave regulation. Current will fall below the target value.

Heat and Practical Limits

Power inside the regulator depends on the voltage it must drop. High supply voltage and low load voltage create more heat. The thermal estimate helps you see whether a heat sink may be needed. Always compare the estimated junction temperature with the device rating from your chosen part data.

Tolerance and Real Current

The nominal formula is simple, but real parts are not perfect. The reference voltage has tolerance. The resistor also has tolerance. Together, these limits create a current range. This is important for LED strings, battery charging tests, sensor loops, and bench loads.

Using the Results

Choose a target current first. Then review the calculated resistor, nearest standard value, resistor wattage, regulator power, and maximum load voltage. Pick parts with margin. Test the circuit with a meter before connecting sensitive loads. For high current designs, check package limits, wiring resistance, board copper, and airflow. The calculator gives a strong starting point, but final validation should happen on the real circuit. It is also useful when comparing resistor series options. Nearby standard values can shift current slightly. Reviewing that shift before purchase reduces rework. Keep lead lengths short when currents are high during safe final assembly.

FAQs

What does the LM317 current formula use?

It uses the reference voltage divided by the sense resistor. The usual reference value is about 1.25 V. Current is I = 1.25 / Rset when the regulator has enough input voltage.

Why is my calculated resistor very low?

Higher current needs lower resistance because Rset equals Vref divided by current. Low resistance values can require high wattage parts. Check resistor power before choosing a small value.

What happens if input voltage is too low?

The regulator leaves constant current operation. The load current drops below the target value. Increase input voltage, reduce load voltage, or use a regulator with lower dropout.

How much resistor wattage should I choose?

Use a resistor rated above the calculated power. A safety factor of two is common for many bench designs. More margin helps when airflow is poor or ambient temperature is high.

Can this calculator help with LED drivers?

Yes. It can estimate current, resistor value, voltage headroom, and heat for simple LED current sources. Always confirm LED voltage and power limits before testing.

Does resistor tolerance affect output current?

Yes. A higher resistor value lowers current. A lower resistor value raises current. The calculator combines resistor tolerance and reference tolerance to show a worst case range.

Why is regulator power important?

The regulator turns extra voltage into heat. High input voltage or low load voltage increases this heat. Check the thermal estimate and add a heat sink when needed.

Can I place loads in parallel?

The regulator controls total current. Parallel loads share that current only if they are well matched. The branch result shows ideal sharing, but real branches may not split evenly.