LM317 Calculator
Calculate output voltage and resistor values for LM317 adjustable voltage regulator
LM317 Voltage Regulator Calculator
Typical range: 100Ω - 1kΩ (affects reference current)
Determines output voltage ratio with R1
Advanced Options
LM317 Calculation Results
Configuration Summary
R1: 240.00 Ω
R2: 1.20 kΩ
Output Voltage: 7.500V
Reference Voltage: 1.25V
Voltage Ratio: 6:1
Min Input Voltage: 10.50V
Formula Used
Design Considerations
Example Calculation
5V Power Supply Design
Goal: Design a 5V regulated power supply
Given: R1 = 240Ω (standard value)
Required R2: R2 = R1 × ((Vout/1.25) - 1)
Calculation: R2 = 240Ω × ((5V/1.25V) - 1) = 240Ω × 3 = 720Ω
Standard value: Use 750Ω (closest standard resistor)
Result Verification
Actual output: Vout = 1.25V × (1 + 750Ω/240Ω) = 5.16V
Reference current: Iref = 1.25V / 240Ω = 5.2mA
Power in R1: P = I²R = (5.2mA)² × 240Ω = 6.5mW
LM317 Specifications
Output Voltage
1.25V to 37V adjustable
Output Current
0.01A to 1.5A maximum
Input Voltage
Vout + 3V to 40V
Reference Voltage
1.25V ±1% typical
Design Tips
Use R1 = 240Ω for most applications
Add input/output capacitors for stability
Ensure adequate heat sinking
Minimum 3V dropout voltage
Use precision resistors for accuracy
Understanding the LM317 Voltage Regulator
What is the LM317?
The LM317 is a three-terminal adjustable positive voltage regulator capable of supplying more than 1.5A over an output voltage range of 1.25V to 37V. It's widely used in electronic circuits where a stable, adjustable DC voltage is required.
Key Features
- •Adjustable output voltage from 1.25V to 37V
- •Output current up to 1.5A
- •Built-in thermal overload protection
- •Short circuit protection
How It Works
Voltage Reference
The LM317 maintains a constant 1.25V between the output and adjust terminals
Voltage Division
External resistors R1 and R2 form a voltage divider that sets the output voltage
Regulation
Internal feedback maintains constant output despite input voltage and load variations
Formula Explanation
Basic Formula (IADJ neglected):
- Vout: Output voltage
- 1.25V: Internal reference voltage
- R1: Resistor from ADJ to ground
- R2: Resistor from OUT to ADJ
Complete Formula:
- IADJ: Adjustment pin current (~50µA)
- Effect: Usually negligible
- When important: High R2 values
- Typical error: <1% for most circuits
Common Applications
Variable Power Supplies
Adjustable bench power supplies for electronics testing and development
Battery Chargers
Constant voltage charging circuits for various battery types
LED Drivers
Constant current sources for LED lighting applications
Reference Voltages
Precision voltage references for analog circuits and ADCs
Motor Control
Speed control circuits for DC motors and fans
Audio Circuits
Bias voltage generation for audio amplifiers and preamps