In our article Using the LM317 with LED Lighting we looked at how the LM317T (and high current equivalent LM338T) can be used to supply a fixed current to a load – in the case of the article to LED bulbs.
The output current from the circuit shown above is set using a single resistor according to the following equation:
| Resistor Required (Ohms) = 1.25 (Volts) / nominal LED current (Amps) |
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 fitted (click here to see our article LM317T Heatsinking 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. We also have a guide to making an Adjustable Voltage LM317 Power Supply here.