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LM317 Regulator

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The LM317 is one of the most popular adjustable positive voltage regulators in electronics, capable of supplying 1.25V to 37V at up to 1.5A. This calculator determines the required R2 resistor value for your desired output voltage, and calculates power dissipation to determine heat sink requirements.

Vout = 1.25 × (1 + R2/R1) · R2 = R1 × ((Vout ÷ 1.25) − 1)
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How to use this calculator

Step-by-Step Instructions

1. **Enter your desired output voltage** Must be at least 1.25V (the LM317’s internal reference).
2. **Set R1** The standard value is 240Ω. You can change this but 240Ω is the most widely used value.
3. **Enter input voltage and load current**(optional) These are used to calculate power dissipation and determine if you need a heat sink.
4. **Click Calculate** The required R2 value, power dissipation, and dropout warnings are displayed.

Worked Example 5V Output

Vout = 5V, R1 = 240Ω R2 = 240 × ((5/1.25) − 1) = 240 × 3 = **720Ω**

With Vin=9V, Iload=0.5A Power = (9−5) × 0.5 = **2W** heat sink recommended.

Pro Tips

  • The LM317 requires at least 2V headroom (Vin must be at least Vout + 2V). The calculator warns you if this condition is not met.
  • If power dissipation exceeds 1W, use a heat sink. Above 2W, use an adequate heat sink with thermal paste.
  • Add 0.1µF capacitor on input and 1µF on output for stable operation (as per the LM317 datasheet).