Find Resistors for an LM317

LM317 (LM117-series), the ubiquitous 3-Terminal, Adjustable Voltage Regulator

Choose Two Resistors and Find the Output Voltage

Referring to Application Note 181 and the LM317 Datasheet:

• V_OUT may be from 1.2V to 37V at 1.5 A
• V_REF is a typical value of around 1.25 V
• I_ADJ is a typical value of around 50 µA

V_OUT = V_REF(1 + R2 / R1) + I_ADJ * R2

R1 (typical range 120Ω and 240Ω)	120 130 150 160 180 200 220 240 270
R2	10 11 12 13 14 15 16 18 20 22 24 27 30 33 36 39 43 47 51 56 62 68 75 82 91 100 110 120 130 150 160 180 200 220 240 270 300 330 360 390 430 470 510 560 620 680 750 820 910 1000 1100 1200 1300 1500 1600 1800 2200 2400 2700 300 3300 3600 3900 4300 4700 5100 5600 6200 6800 7500 8200 9100 10000 11000 12000 13000 15000 16000 18000 20000 22000 24000 27000 30000 33000 36000 39000 43000 47000 51000 56000 62000 68000 75000 82000 91000
Output Voltage	Vout =

Choose an Output Voltage and R1, and Find R2

Similarly, the equation (1) in the LM317 Datasheet can be solved for R2, allowing us to select an output value and a sample R1 and solve for R2. For this, we use a slightly less precise version of the equation, expressing only the radio of R2 to R1 and the Reference Voltage (see: Answers/FAQs).

V_OUT = V_REF(1 + R2 / R1), Or solving for R2 = R1 * (V_OUT / V_REF - 1)

The 50 µA biasing current is small compared to 5 mA and causes only a small error in actual output voltages.

Vout
R1 (typical range 120Ω and 270Ω)	120 130 150 160 180 200 220 240 270
R2	R2 =

Heat Dissipation Calculation for LM317

Equation (3) in the LM317 Datasheet can estimate the heat dissipation. I_L is the load current. I_G is the current consumed by the LM317, measured at the Adjustment Pin. The datasheet shows 'Adjustment Pin Current' to be a typical value of 50µA.

P_D = ((V_IN − V_OUT) × I_L) + (V_IN × I_G)

VIN	V
VOUT	V
IL	A
PD	PD =

For more technical data, see: National Semiconductor, LM317 - 3-Terminal Adjustable Regulator