LM317 constant current source and four LEDs.
Fig. 1

LM317 Constant Current Source for Lighting LEDs

by Lewis Loflin


In the above links I've explored using a constant current source (CCS) and basic theory. Here I'll concentrate on the use of a constant current source for lighting light emitting diodes or LEDs. Understanding series circuits and voltage drops will help with this subject.

Briefly in a series circuit the current through all series components is the same, while the sum of the voltage drops across each series component will add back to the source voltage.

In Fig. 1 we use a LM317 in its constant current mode. The value of R3 divided into 1.25 volts will determine load current through each LED in the string. In this case current is 1.25 / 4.7 =~ 266mA. Each LED has a slightly differing voltage drop which is normal and why we want to use a CCS. The total voltage drop across the 3 LEDs equals 12.2 volts while the drop across Us and R3 is 6.8 volts. They add back to equal the 19 volt power source.

Note that total voltage drops are limited to the power supply voltage and we should maintain drop across U3 and R3 of three volts leaving a drop for whatever load of 16V in this case. Drop more than 16V and the CCS will not function.

Fig. 2

In Fig. 2 I've removed one LED and the associated voltage drop, but the drop across the other three LEDs remains the same. That missing voltage drop adds to the voltage drop across U3 and R3.

Fig. 3

In Fig. 3 I've changed Vin to 24V and has no effect on the LED current, which is still set by R3 at 266mA. In all three cases above the voltage drops across U3 and R3 become useless heat.

The most efficient way energy wise is to use a switching voltage regulator to reduce Vin to 3-5 volts above the load voltage required.