LM317 Voltage Regulator Calculator

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.

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