LM317 Voltage and Resistor Calculator

Adjustable Voltage Regulator Output Tool

Input Fields

Resistor R1

R1

Ω

Resistor connected between output and adjust pin

Resistor R2

R2

Ω

Resistor between adjust pin and ground

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Your Results

Output Voltage

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LM317 Output Voltage Formula

$$V_{out} = V_{ref} \cdot \left(1 + \frac{R2}{R1}\right) + I_{adj} \cdot R2$$

Where:

• $$V_{out}$$ = output voltage (V)
• $$V_{ref}$$ = reference voltage ≈ 1.25 V
• $$R1$$ = resistor between output and adjust pin (Ω)
• $$R2$$ = resistor between adjust pin and ground (Ω)
• $$I_{adj}$$ = adjustment pin current (typically ≈ 50 µA)

If $$I_{adj}$$ is negligible (which is often the case), the formula simplifies to:
$$V_{out} ≈ 1.25 \cdot \left(1 + \frac{R2}{R1}\right)$$

Calculation Example

Given:

• R1 = 240 Ω
• R2 = 1.2 kΩ

Calculation:

1. $$V_{out} ≈ 1.25 \cdot \left(1 + \frac{1200}{240} \right)$$
2. $$= 1.25 \cdot (1 + 5) = 1.25 \cdot 6 = 7.5~V$$

The LM317 is a widely used voltage regulator that allows for precise control of output voltage using two resistors. By setting appropriate R1 and R2 values, users can generate custom voltages from a higher input. This tool simplifies the calculation process and is helpful in designing power supplies, battery chargers, and LED drivers. It can also be used in reverse to calculate R2 for a desired output voltage.