LED Gurus - Dimming a single LED on a string?

Been wanting to use this graphic all day.

 

I was actually trying (and failing) to post that in a different thread. Sorry about that.

 

I don't think by have the pot in parallel to a strings of LEDs in series you are going to get what you are wanting and if you put the pot in front of the LED then everything behind it would be dimmed.

I think the only way to make this work would be to have the 2 blue LEDs in parallel to the reds. But let me do some looking around. Might be able to come up with something else.

 

Is this what your thinking of?


 

 

Had a thought: LEDS are constant current devices, correct? Using a potentiometer as a voltage divider is also a current divider....may have to think of something else.

 

I'm not an electrical engineer or electronic technologist, but I don't see anything wrong with the concept.
At a quick glance, your driver appears to be constant current within a certain voltage range. If you put a pot in parallel with each of the two blue LED's you should be able to divert some of the current around the LED as you suggest. As that happens, the voltage across the blue LED's/pots will change, which in turn will change the voltage drop over the whole string (in a perfect world, if you have your pot adjusted to zero resistance then the voltage drop across it and the LED would be zero). That should be OK in this case; your driver will compensate for voltage to maintain the constant current.
It could be a problem if you had a pot around every LED and all the pots were adjusted to zero resistance. In that case it would be the same as shorting out your driver...and I doubt that is a good thing--not sure how many safeguards are built in. If you wanted a pot around every LED, then a person may be able to size a resistor to put in series with each pot to ensure there is a minimum voltage drop across each LED--it could be sufficiently low enough that the LED goes out, but enough that the voltage does not drop below the drivers working range.
Of course, any resistors or pots should be sized appropriately to handle any possible currents or voltage.
Good luck! And let us know your results if you try it /DesktopModules/ActiveForums/themes/_default//emoticons/smile.gif

 

I don't know if it will work but I'm pretty sure though that as you dim the blues you will brighten the reds at the same time cause they will receive the extra current that was going to the blues so you would have to make sure you have enough reds on the string that when you dim the blues you don't overload the reds

 

Actually the more I think about it neither way will work. if you run them all in series no matter where you put the pot adjusting it would effect the whole string. It also wouldn't work running the reds in series and the blues in parallel to reds cause the trick to running LEDs in parallel strings is that both strings need to be balanced that's why you can't run different led in parallel strings unless you run the same combination on each string so half the reds and one blue on one string and the other reds and blues on the second string which means you couldn't dim just the blues. I think the only way to get this to work would be to run the reds on the constant current driver and the blues on a dimmable driver like the 60-48d but you would have to run least 4 led on this driver or come up with some fancy circuit board that can compensate for the unbalanced string which I have no idea on how to build