LM317 Calculator
Calculate resistor values for LM317 voltage regulator circuit. Determine R1 and R2 values to achieve desired output voltage.
Minimum: 1.25V (reference voltage)
Typical: 240Ω (standard value)
How to Use This Calculator
- Enter your desired output voltage (must be at least 1.25V, the reference voltage).
- Enter R1 value (typically 240Ω, the standard recommended value).
- The calculator displays the required R2 value and the closest standard resistor value.
- Use these resistor values in your LM317 circuit to achieve the desired output voltage.
LM317 Voltage Formula
LM317 output voltage is calculated from R1 and R2:
Where 1.25V is the internal reference voltage. Example: R1 = 240Ω, R2 = 720Ω: Vout = 1.25 × (1 + 720/240) = 1.25 × 4 = 5V. To solve for R2: R2 = R1 × ((Vout / 1.25) - 1).
Full Description
The LM317 is a popular adjustable linear voltage regulator that can provide output voltages from 1.25V to 37V. It's widely used in power supplies, battery chargers, and voltage regulation circuits. The LM317 uses two external resistors (R1 and R2) to set the output voltage according to a simple formula based on its internal 1.25V reference voltage.
R1 is typically 240Ω (a standard resistor value), though values between 100Ω and 1000Ω work. R2 is calculated based on the desired output voltage. The formula ensures that the voltage across R1 is always 1.25V (the reference), and the voltage divider created by R1 and R2 sets the output voltage. The LM317 maintains this voltage by adjusting its internal resistance.
This calculator helps you determine the resistor values needed for your LM317 circuit. Enter the desired output voltage and R1 value, and it calculates R2 and suggests the closest standard resistor value. Use it when designing power supplies, voltage regulators, or understanding how the LM317 works. The LM317 is a versatile and reliable voltage regulator for many applications.
Frequently Asked Questions
What is the LM317?
The LM317 is a popular adjustable voltage regulator that can provide output voltages from 1.25V to 37V. It uses two external resistors (R1 and R2) to set the output voltage. The formula is: Vout = 1.25V × (1 + R2/R1).
What value should I use for R1?
R1 is typically 240Ω (standard value). It should be between 100Ω and 1000Ω. Lower values provide better regulation but waste more power. Higher values are more efficient but may cause instability. 240Ω is the recommended value.
How do I calculate R2?
R2 = R1 × ((Vout / 1.25V) - 1). For example, to get 5V output with R1 = 240Ω: R2 = 240 × ((5/1.25) - 1) = 240 × 3 = 720Ω. Use the closest standard resistor value.
What are standard resistor values?
Standard E12 series: 100, 120, 150, 180, 200, 220, 240, 270, 300, 330, 360, 390, 430, 470, 510, 560, 620, 680, 750, 820, 910, and multiples. E24 series has more values. Use the closest standard value to the calculated R2.