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LM317 Current Calculator

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


LM317T adjustable linear voltage regulator


In our article Using the LM317 with LED Lighting (1) we looked at how the LM317T (2) (and high current equivalent LM338T (3)) can be used to supply a fixed current to a load - in the case of the article to LED bulbs.

LM317T Current Calculator
The output current from the circuit shown above is set using a single resistor (4) according to the following equation:

Resistor Required (Ohms) = 1.25 (Volts) / nominal LED current (Amps)


REUK LM317/LM338 Current Calculator

Below is our automated current calculator for the LM317 and LM338 linear voltage regulators. Simply enter your desired output current (measured in milliamps), and the value of the required resistor and its power rating will be displayed:

Target Output Current milliamps
Resistor Value: Ohms with a power rating greater than: Watts

Do not forget that the maximum continuous current which can be managed by the LM317T is 1.5 Amps, and that is with a decent heatsink (5) fitted (click here to see our article LM317T Heatsinking (6) for more details).

The LM338T is rated at 5 Amps of continuous current with 50ms bursts of up to 7 Amps. However with a large heatsink and/or cooling fan it can manage continuous currents of 8 Amps+.

LM317/LM338 Voltage Calculator

If you would like to use the LM317T or LM338T to output a fixed voltage rather than current, click here to visit our LM317 Voltage Calculator (7). We also have a guide to making an Adjustable Voltage LM317 Power Supply (8) here.

Web Link References

(1) http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm
(2) http://www.reuk.co.uk/LM317T-Voltage-Regulator-Chip.htm
(3) http://www.reuk.co.uk/buy-LM338T.htm
(4) http://www.reuk.co.uk/Resistor-Colour-Codes.htm
(5) http://www.reuk.co.uk/buy-TO220-BOLT-ON-HEATSINK.htm
(6) http://www.reuk.co.uk/LM317T-Heatsinking.htm
(7) http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
(8) http://www.reuk.co.uk/LM317-Adjustable-Power-Supply.htm

Article from REUK.co.uk:
http://www.reuk.co.uk/LM317-Current-Calculator.htm
Published: 24th September 2014
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