LED Lighting Using a Dropping Resistor

[Paul Elliott]

Originally Posted by DeepFrz

I guess you didn't read the link showing the power drop across the LM317 when used as a current regulator. The bigger the voltage drop the higher the power dissipated. I suppose if you wire 3 or 4 LEDs in series after the output of the LM317 you could get away with it, but if you are using it to power 1 or 2 LEDs then you have a lot of power to dissipate.

Well, the 317 as a voltage regulator is still going to dissipate power -- how much depends on how many series LEDs and what your secondary regulation looks like. The total power dissipation will be the same in any case. Use two regulators, or one with a big heat sink, it's going to be the same power.

Besides, we haven't even gotten to the point in this discussion where we are considering the number of LEDs or power levels. Once we establish that we can work on the power dissipation issues. In my red LED night-lights a LM317 as a current regulator is going to be barely warm (actually, I used resistors).

[Lake-Effect]

We had a kick at this earlier. My contributions are here, here and, um, here.

[phorvati]

I've used just a resistor in each LED branch but i also put an appropriately rated zener in parallel with each of the three LEDs. This seems to work good for few years now. All the branches that did not have a zener ended up dimming noticeably in 2 years.