I'm trying to build a variable power supply that has a fixed range between 1.8V to 2.4V. This is to be driving 5 red 1 watt LED's.

Below you can see I plan to use a LM350 (3A version of LM317) and have calculated the bottom resistor value (call it R2) to get both 1.8V and 2.4V from the LM350.

Min brightness - 1.8V output - 106Ω

Max brightness - 2.4V output - 221Ω

I'm trying to replace the single resistor (R2) with a resistor/pot network that will vary the resistance between 106Ω and 221Ω. I've only managed to find a 1K pot (I assumed the lower the better).

I plan to put the pot in series with a 106Ω resistor (giving the minimum resistance when the pot is fully turned clockwise and is effectively ground), and, in parallel with a 130Ω resistor (which when added to the 1k pot gives 115Ω). When the pot is fully turned anticlockwise, the 115Ω + 106Ω series resistor gives the full 221Ω.

The problem (which I'm sure you've noticed) is according to my calculations the pot + 130Ω resistor won't produce a smooth change between 0Ω to 115Ω across its full rotation. It'll be more like a logarithmic scale, and resistance will go up very quickly to start and then barely change. Using a log pot (as opposed to linear) might help a bit but I think I' might be going about this the wrong way.

I've not built this yet (LM350's due to arrive) but any pointers before I do would be greatly appreciated!! Thanks Once I'm done I'll post more info as if it works this should be quite a cool project...