This LM317 current regulator calculator finds the resistor that sets a constant output current, using the LM317’s 1.25 V reference. Enter your target current above to get the resistor value.
Formula
R = 1.25 ÷ Iout Iout = 1.25 ÷ R
Example
For 1.25 A: R = 1.25 ÷ 1.25 = 1 Ω. For 125 mA, R = 1.25 ÷ 0.125 = 10 Ω.
Size the resistor’s wattage for I²R, and keep within the LM317’s 1.5 A limit.
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Frequently Asked Questions
How do I set current with an LM317?
Place a resistor between OUT and ADJ. The current equals 1.25 divided by that resistor, since the LM317 holds 1.25 V across it.
What resistor gives 1 amp on an LM317?
A 1.25-ohm resistor sets about 1 amp: 1.25 volts divided by 1.25 ohms.
What is the LM317 current limit?
The LM317 supplies up to about 1.5 A, with adequate heat sinking.