LM317 Output Voltage Calculator: Set Any Voltage from 1.25V to 37V

The LM317 sits in a short list of components that every electronics builder should know inside out. It's adjustable, it's stable, it handles up to 1.5A, and it's been manufactured continuously for over 50 years because nothing has replaced it for straightforward linear regulation.

The output voltage is set by just two resistors. The maths is simple, but getting the right standard resistor values — and understanding the trade-offs — is where most people trip up.

The voltage equation

V_out = 1.25 × (1 + R2/R1) + (I_adj × R2)

In practice, the adjustment pin current (I_adj) is typically 50–100µA and is small enough to ignore in most calculations:

V_out ≈ 1.25 × (1 + R2/R1)

Rearranging to find R2 for a target voltage:

R2 = R1 × ((V_out / 1.25) - 1)

The LM317 datasheet recommends R1 = 240Ω as the standard value. This sets about 5mA through the resistor divider, which is high enough to swamp the adjustment current and maintain accuracy.

Common output voltages and resistor values

Using a potentiometer for adjustable output

For an adjustable supply, replace the fixed R2 with a potentiometer in series with a small fixed resistor. The fixed resistor prevents the output from going below your minimum desired voltage when the pot is at zero.

For 1.25V to 12V range:

R1 = 240Ω

R2 = 680Ω fixed + 5kΩ potentiometer

At pot = 0: V_out = 1.25 × (1 + 680/240) = 4.79V — not 1.25V. To get down to 1.25V, don't use a series fixed resistor; connect the pot directly between ADJ and GND, accepting that the minimum is determined by wiper resistance.

For a wide range, a 10-turn potentiometer gives much finer adjustment than a single-turn pot. For a bench supply, this makes a real practical difference when you're trying to set exactly 3.3V.

Input voltage requirements

The LM317 requires its input to be at least 3V above its output — this is the dropout voltage. So for a 5V output, you need at least 8V input.

For a 12V output, you need at least 15V input. For a 30V output (the LM317's maximum), you'd need 33V input — which exceeds the IC's 40V absolute maximum input voltage. In practice, the LM317's upper limit for output voltage depends on your available input voltage.

The input-output voltage difference also determines heat dissipation. A large difference with high current means significant heating. This is the primary limitation of linear regulators compared to switching regulators — the excess voltage becomes heat rather than useful output power.

Improving regulation with capacitors

The datasheet calls for:

• 0.1µF ceramic on the input (suppresses oscillation from long input leads)
• 1µF on the adjustment pin to ground (improves ripple rejection by ~15dB — well worth the cost of a single capacitor)
• 10µF electrolytic on the output (improves transient response)

A protection diode from output to input (anode toward input, cathode toward output) prevents the output capacitor from discharging back through the LM317 if the input is shorted — without it, a large output capacitor can destroy the IC in this condition.