LM317 Voltage Calculator

LM317 Voltage Calculator

A handy online output voltage calculator for the LM317, LM338, and other linear voltage regulators

electric circuit | electronics | general

The LM317 (LM317T), and high current LM338 (LM338T) are voltage regulators which can take an input voltage of 3-40 Volts DC, and output a fixed output voltage from 1.2 to 37 Volts DC.

The output voltage from the LM317 and LM338 is set using two resistors (R1 and R2) with their values chosen according to the following equation:

VOUT = 1.25 * ( 1 + R2/R1 )

REUK LM317/LM338 Voltage Calculator

Below is our automated voltage calculator for the LM317 and LM338 regulators. Simply enter your target output voltage Vout and chosen value for R1 (100 to 1000 Ohms), click Calculate, and the required value of R2 will be displayed.

Vout: Target Output Voltage 1.5 to 37.0 Volts

R1: R1 Resistor Value 100-1000 Ohms

R2: Click Calculate to display the calculated R2 value here.

The results given above can be used to put together a voltage regulator, but the calculated value of R2 will not be a standard/stock resistor value. Since the accuracy of the voltage regulator itself will be up to 5% out, and resistor values are not exact, it is common to use a Potentiometer (aka variable resistor or trim pot) for some or all of R2. This enables the output voltage to be manually fine-tuned to provide the exact voltage required.

Output Voltage Tables for Stock R1 and R2 Values

Where a very accurate voltage is not required, it is much easier to use standard (stock) resistors to put together an LM317 or LM338 voltage regulator. Below are tables of output voltages for different combinations of the most commonly found resistors.

R1 vs R2 Grid

R2\R1	150	180	220	240	270	330	370	390	470
68	1.82	1.72	1.64	1.60	1.56	1.51	1.48	1.47	1.43
82	1.93	1.82	1.72	1.68	1.63	1.56	1.53	1.51	1.47
100	2.08	1.94	1.82	1.77	1.71	1.63	1.59	1.57	1.52
120	2.25	2.08	1.93	1.88	1.81	1.70	1.66	1.63	1.57
150	2.50	2.29	2.10	2.03	1.94	1.82	1.76	1.73	1.65
180	2.75	2.50	2.27	2.19	2.08	1.93	1.86	1.83	1.73
220	3.08	2.78	2.50	2.40	2.27	2.08	1.99	1.96	1.84
240	3.25	2.92	2.61	2.50	2.36	2.16	2.06	2.02	1.89
270	3.50	3.13	2.78	2.66	2.50	2.27	2.16	2.12	1.97
330	4.00	3.54	3.13	2.97	2.78	2.50	2.36	2.31	2.13
370	4.33	3.82	3.35	3.18	2.96	2.65	2.50	2.44	2.23
390	4.50	3.96	3.47	3.28	3.06	2.73	2.57	2.50	2.29
470	5.17	4.51	3.92	3.70	3.43	3.03	2.84	2.76	2.50
560	5.92	5.14	4.43	4.17	3.84	3.37	3.14	3.04	2.74
680	6.92	5.97	5.11	4.79	4.40	3.83	3.55	3.43	3.06
820	8.08	6.94	5.91	5.52	5.05	4.36	4.02	3.88	3.43
1000	9.58	8.19	6.93	6.46	5.88	5.04	4.63	4.46	3.91
1200	11.25	9.58	8.07	7.50	6.81	5.80	5.30	5.10	4.44
1500	13.75	11.67	9.77	9.06	8.19	6.93	6.32	6.06	5.24
1800	16.25	13.75	11.48	10.63	9.58	8.07	7.33	7.02	6.04
2200	19.58	16.53	13.75	12.71	11.44	9.58	8.68	8.30	7.10
2700	23.75	20.00	16.59	15.31	13.75	11.48	10.37	9.90	8.43
3300	28.75	24.17	20.00	18.44	16.53	13.75	12.40	11.83	10.03

Sorted List of Output Voltages with R1 and R2

1.43V R1 = 470, R2 = 68 1.47V R1 = 470, R2 = 82 1.48V R1 = 370, R2 = 68 1.51V R1 = 330, R2 = 68 1.51V R1 = 390, R2 = 82 1.52V R1 = 470, R2 = 100 1.53V R1 = 370, R2 = 82 1.56V R1 = 330, R2 = 82 1.57V R1 = 270, R2 = 68 1.57V R1 = 470, R2 = 120 1.57V R1 = 390, R2 = 100 1.59V R1 = 370, R2 = 100 1.60V R1 = 240, R2 = 68 1.63V R1 = 330, R2 = 100 1.63V R1 = 270, R2 = 82 1.64V R1 = 390, R2 = 120 1.64V R1 = 220, R2 = 68 1.65V R1 = 470, R2 = 150 1.66V R1 = 370, R2 = 120 1.68V R1 = 240, R2 = 82 1.71V R1 = 330, R2 = 120 1.71V R1 = 270, R2 = 100 1.72V R1 = 220, R2 = 82 1.72V R1 = 180, R2 = 68 1.73V R1 = 470, R2 = 180 1.73V R1 = 390, R2 = 150 1.76V R1 = 370, R2 = 150 1.77V R1 = 240, R2 = 100 1.81V R1 = 270, R2 = 120 1.82V R1 = 150, R2 = 68 1.82V R1 = 330, R2 = 150 1.82V R1 = 180, R2 = 82 1.83V R1 = 390, R2 = 180 1.84V R1 = 470, R2 = 220 1.86V R1 = 370, R2 = 180 1.88V R1 = 240, R2 = 120 1.89V R1 = 470, R2 = 240 1.93V R1 = 330, R2 = 180 1.93V R1 = 150, R2 = 82 1.94V R1 = 270, R2 = 150 1.96V R1 = 390, R2 = 220 1.97V R1 = 470, R2 = 270 1.99V R1 = 370, R2 = 220 2.02V R1 = 390, R2 = 240 2.03V R1 = 240, R2 = 150 2.06V R1 = 370, R2 = 240 2.08V R1 = 330, R2 = 220 2.10V R1 = 220, R2 = 150 2.12V R1 = 390, R2 = 270 2.13V R1 = 470, R2 = 330 2.16V R1 = 330, R2 = 240 2.16V R1 = 370, R2 = 270 2.19V R1 = 240, R2 = 180 2.23V R1 = 470, R2 = 370 2.25V R1 = 150, R2 = 120 2.27V R1 = 270, R2 = 220 2.27V R1 = 330, R2 = 270 2.29V R1 = 470, R2 = 390 2.29V R1 = 180, R2 = 150

2.31V R1 = 390, R2 = 330 2.36V R1 = 270, R2 = 240 2.37V R1 = 370, R2 = 330 2.40V R1 = 240, R2 = 220 2.44V R1 = 390, R2 = 370 2.50V R1 = 470, R2 = 470 2.57V R1 = 370, R2 = 390 2.61V R1 = 220, R2 = 240 2.65V R1 = 330, R2 = 370 2.66V R1 = 240, R2 = 270 2.73V R1 = 330, R2 = 390 2.74V R1 = 470, R2 = 560 2.75V R1 = 150, R2 = 180 2.76V R1 = 390, R2 = 470 2.78V R1 = 270, R2 = 330 2.78V R1 = 220, R2 = 270 2.84V R1 = 370, R2 = 470 2.92V R1 = 180, R2 = 240 2.96V R1 = 270, R2 = 370 2.97V R1 = 240, R2 = 330 3.03V R1 = 330, R2 = 470 3.05V R1 = 390, R2 = 560 3.06V R1 = 270, R2 = 390 3.06V R1 = 470, R2 = 680 3.08V R1 = 150, R2 = 220 3.13V R1 = 220, R2 = 330 3.14V R1 = 370, R2 = 560 3.18V R1 = 240, R2 = 370 3.25V R1 = 150, R2 = 240 3.28V R1 = 240, R2 = 390 3.35V R1 = 220, R2 = 370 3.37V R1 = 330, R2 = 560 3.43V R1 = 270, R2 = 470 3.43V R1 = 390, R2 = 680 3.43V R1 = 470, R2 = 820 3.47V R1 = 220, R2 = 390 3.50V R1 = 150, R2 = 270 3.54V R1 = 180, R2 = 330 3.55V R1 = 370, R2 = 680 3.70V R1 = 240, R2 = 470 3.82V R1 = 180, R2 = 370 3.83V R1 = 330, R2 = 680 3.84V R1 = 270, R2 = 560 3.88V R1 = 390, R2 = 820 3.91V R1 = 470, R2 = 1000 3.92V R1 = 220, R2 = 470 3.96V R1 = 180, R2 = 390 4.00V R1 = 150, R2 = 330 4.02V R1 = 370, R2 = 820 4.17V R1 = 240, R2 = 560 4.33V R1 = 150, R2 = 370 4.36V R1 = 330, R2 = 820 4.40V R1 = 270, R2 = 680 4.43V R1 = 220, R2 = 560 4.44V R1 = 470, R2 = 1200 4.46V R1 = 390, R2 = 1000 4.50V R1 = 150, R2 = 390 4.51V R1 = 180, R2 = 470 4.63V R1 = 370, R2 = 1000

4.79V R1 = 240, R2 = 680 5.04V R1 = 330, R2 = 1000 5.05V R1 = 270, R2 = 820 5.10V R1 = 390, R2 = 1200 5.11V R1 = 220, R2 = 680 5.14V R1 = 180, R2 = 560 5.17V R1 = 150, R2 = 470 5.24V R1 = 470, R2 = 1500 5.30V R1 = 370, R2 = 1200 5.52V R1 = 240, R2 = 820 5.80V R1 = 330, R2 = 1200 5.88V R1 = 270, R2 = 1000 5.91V R1 = 220, R2 = 820 5.92V R1 = 150, R2 = 560 5.97V R1 = 180, R2 = 680 6.04V R1 = 470, R2 = 1800 6.06V R1 = 390, R2 = 1500 6.32V R1 = 370, R2 = 1500 6.46V R1 = 240, R2 = 1000 6.81V R1 = 270, R2 = 1200 6.92V R1 = 150, R2 = 680 6.93V R1 = 330, R2 = 1500 6.94V R1 = 180, R2 = 820 7.02V R1 = 390, R2 = 1800 7.10V R1 = 470, R2 = 2200 7.33V R1 = 370, R2 = 1800 7.50V R1 = 240, R2 = 1200 8.07V R1 = 330, R2 = 1800 8.08V R1 = 150, R2 = 820 8.19V R1 = 270, R2 = 1500 8.30V R1 = 390, R2 = 2200 8.43V R1 = 470, R2 = 2700 8.68V R1 = 370, R2 = 2200 9.06V R1 = 240, R2 = 1500 9.58V R1 = 330, R2 = 2200 9.77V R1 = 220, R2 = 1500 9.90V R1 = 390, R2 = 2700 10.03V R1 = 470, R2 = 3300 10.37V R1 = 370, R2 = 2700 10.63V R1 = 240, R2 = 1800 11.25V R1 = 150, R2 = 1200 11.44V R1 = 270, R2 = 2200 11.48V R1 = 330, R2 = 2700 11.67V R1 = 180, R2 = 1500 11.83V R1 = 390, R2 = 3300 12.40V R1 = 370, R2 = 3300 12.71V R1 = 240, R2 = 2200 13.75V R1 = 330, R2 = 3300 15.31V R1 = 240, R2 = 2700 16.25V R1 = 150, R2 = 1800 16.53V R1 = 270, R2 = 3300 16.59V R1 = 220, R2 = 2700 18.44V R1 = 240, R2 = 3300 19.58V R1 = 150, R2 = 2200 20.00V R1 = 220, R2 = 3300 23.75V R1 = 150, R2 = 2700 24.17V R1 = 180, R2 = 3300 28.75V R1 = 150, R2 = 3300

LM317/LM338 Current Calculator

If you would like to use the LM317T or LM338T to output a fixed current rather than voltage, click here to visit our LM317 Current Calculator.
Article Last Modified: 12:47, 12th Mar 2008

Comment on this Article

If you have any comments on this article, please email them to neil@reuk.co.uk.

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