LM317 Output Voltage Calculator (Resistor Values for the Adjustable Regulator)
The output voltage of an LM317 from its two resistors, the R2 needed for the voltage you want, the input voltage it requires and the heat it has to dissipate.
Results
The dashed line marks the value you entered.
What this tool does
The LM317 holds 1.25 V between its output and its ADJ pin, so a current of 1.25 V ÷ R₁ flows through R₁ and then through R₂, which adds up to V_out = 1.25 × (1 + R₂ ÷ R₁) plus a small extra term from the roughly 50 µA that leaves the ADJ pin. R₁ is normally 120–240 Ω, enough to keep the regulator's minimum load current flowing. The input must be above the output by the dropout voltage, about 2 V, and the difference times the load current is turned into heat, so a 12 V input for 5 V at 1 A means 7 W and a proper heatsink. The part is rated up to 1.5 A.
Formula
V_out = 1.25 V × (1 + R₂ ÷ R₁) + I_ADJ × R₂ · R₂ = (V_out − 1.25) ÷ (1.25 ÷ R₁ + I_ADJ) · P = (V_in − V_out) × I
Variables
| Symbol | Meaning | Unit |
|---|---|---|
r1 | R1 (between output and ADJ) | Ω |
r2 | R2 (between ADJ and ground) | Ω |
vt | Output voltage you want | V |
ia | ADJ pin current | µA |
vin | Input voltage | V |
il | Load current | A |
vd | Dropout voltage | V |
VO | Output voltage with R1 and R2 | V |
RN | R2 needed for the wanted voltage | Ω |
VM | Lowest input voltage that works | V |
PD | Power the regulator dissipates | W |
Worked example
- R1 (between output and ADJ)240 Ω
- R2 (between ADJ and ground)720 Ω
- Output voltage you want9 V
- ADJ pin current50 µA
- Input voltage12 V
- Load current0.5 A
- Dropout voltage2 V
- Output voltage with R1 and R25.036 V
- R2 needed for the wanted voltage1474 Ω
- Lowest input voltage that works7.04 V
- Power the regulator dissipates3.48 W
Limitations
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.