Why must R1 always be 240 ohms or less?

The LM317 requires a minimum load current of 3.5 to 5.0 mA to maintain internal regulation. Setting R1 = 240Ω guarantees 1.25V / 240Ω = 5.2 mA always flows through R1 even when no external load is connected, preventing Vout from drifting up to Vin.

What is the minimum voltage difference (dropout) required between Vin and Vout?

The LM317 is NOT a Low-Dropout (LDO) regulator. Its internal Darlington pass transistor requires at least 2.5V to 3.0V of headroom (Vin - Vout ≥ 3.0V) across temperature and full load to maintain clean DC regulation.

How hot does the LM317 get without a heatsink?

In a bare TO-220 package in free air, thermal resistance is 65°C/W. Dissipating just 1.5 Watts raises internal junction temperature by +97.5°C above room temperature, triggering thermal shutdown. Always mount a heatsink when dissipating more than 1 Watt.

Free LM317 Voltage & Constant Current Regulator Calculator

⚡ Regulator Mode & Parameters

Operating Mode

Input DC Voltage (Vin in V) Must be ≥ Vout + 3.0V

Load Current (Iout in A) LM317 max 1.5A | LM350 3A | LM338 5A

Upper Resistor R1 (Ω) Standard 240Ω (or 120Ω for high temp)

Target Output Vout (V) Desired regulated rail

📊 Calculated Resistor & Power Loss

Required Resistor (R2 / Rset) 1,488 Ω Use 1.5 kΩ standard E24

Regulator Thermal Dissipation 4.80 Watts (Vin - Vout) × Iout

Actual Regulated Output: 9.00 Volts

Dropout Voltage Headroom: 6.00 V (≥3.0V required)

Minimum Load Current Satisfied: Yes (5.2 mA via R1)

Required Heatsink Rating: Medium Heatsink (~12 °C/W)

✅ Safe Operating Region: 6V headroom provides rock-solid line regulation. Heatsink is required for 4.8W thermal loss.

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The Operating Chemistry of the Classic LM317

The LM317 is a legendary floating 3-terminal adjustable linear voltage regulator. It maintains a constant precision 1.25 Volt reference ((V_{ ext{ref}})) between its OUTPUT pin and ADJUST pin.

Voltage Regulator Mode Formula

By placing resistor (R_1) across the output and adjust pins, a constant reference current flows: (I_{R1} = rac{1.25 ext{V}}{R_1}). This current then flows through bottom resistor (R_2) to ground:

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

Since the internal adjust pin current (I_{ ext{adj}}) is tiny (approx 50 µA), it can be ignored when (R_1 le 240,Omega).

The Constant Current Source Mode

If you connect a single resistor (R_{ ext{set}}) between the OUTPUT and ADJUST pins, and take your load from the ADJUST pin, the LM317 forces exactly 1.25V across that resistor, creating a bulletproof constant current generator:

I_out = 1.25 / R_set → R_set = 1.25 / I_out

For example, driving a 350mA high-power LED requires: (R_{ ext{set}} = 1.25 / 0.350 = mathbf{3.57,Omega}) (with a power rating of (P = 1.25 imes 0.35 = 0.44 ext{ Watts})).