1. What is the LM317 Constant Current Circuit?

The LM317 can be configured as a constant current source by using a single resistor between the output and adjustment pins. This simple circuit provides a stable current regardless of load variations (within limits).

2. How Does the Calculator Work?

The calculator uses the LM317 constant current formula:

Where:

• \( R \) — Current-setting resistor (Ω)
• \( V_{ref} \) — Reference voltage (typically 1.25V)
• \( I_{max} \) — Desired output current (A)

Explanation: The LM317 maintains 1.25V (typical) between the output and adjustment pins, so the current is determined by Ohm's Law (I = V/R).

3. Importance of Current Limiting

Details: Constant current circuits are essential for LED driving, battery charging, and other applications where maintaining a specific current is more important than maintaining a specific voltage.

4. Using the Calculator

Tips: Enter the reference voltage (typically 1.25V) and your desired maximum current. The calculator will determine the required resistor value.

5. Frequently Asked Questions (FAQ)

Q1: What's the typical V_ref for LM317?
A: The typical value is 1.25V between output and adjustment pins, but it can vary between 1.2V-1.3V depending on the specific device.

Q2: What's the maximum current possible?
A: The LM317 can typically handle up to 1.5A, but heat dissipation becomes a major concern at higher currents.

Q3: How accurate is this current source?
A: Accuracy depends on the resistor tolerance and the V_ref stability (which varies with temperature and current).

Q4: Can I use this for precision applications?
A: For precision applications, consider using a dedicated current source IC or op-amp based circuit.

Q5: How do I calculate power dissipation?
A: Power dissipation in the resistor is I²R. In the LM317, it's (V_in - V_out) × I.