# Variable color temperature question



## videoman (Feb 15, 2012)

Project requires 2 XM-L leds, one 2700K the other 6000K . each with it's own 3A driver. The drivers ( DC bucks 8-18V DC 3A rated )have provisions for a rotary potentiometer for dimming ( 50K pot) via 2 solder holes on the driver. I have access to many different pots if needed including dual gang, triple gang, linear and log or audio taper ones. My goal is once I set the color temperature with ONE pot (most likely ganged to both leds for the "mix"), I must have another pot for the overall dimming of the mixed in color. I need the lamp to have only 2 pots, one for color setting and one for dimming. I imagine using 2 dual gang 100K linear pots, one connected to both drivers and the other pot connected to this pot in a parallel config. Is this the only was possible. ?


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## AnAppleSnail (Feb 15, 2012)

I have used pots to throttle direct-drive LEDs, and find a log taper to be most like visual output. I think you'll want to have one log taper controlling both as your main dimmer, and another pot between each driver and the one dimmer. Otherwise you can only go from a 50/50 split (Dimmer at anything, second pot at 0%) to 100/0 (second pot at infinite resistance).


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## videoman (Feb 15, 2012)

I have tried log pots as main dimmer and the problem with them was that the brightness gradually increases up to around the 3/4 way mark on the rotary and then zooms up way too rapidly to full brightness on the remaining 1/4 turn to full. Bu using a linear pot, the increase is gradual all the way to full with better control and predictability on brightness, as far as I can judge with my eyes. In my trial design for the above, say for a video camera light that needs a color temperature swing from tungsten indoor to daylight blue, I mix the ratio with a ganged 100k dual pot so that as one pot increases the other decreases and hence the mix. At halfway mark BOTH leds are half dim and color temperature is neutral around 4000K mark. As the pot is turned counterclockwise, then the 2700K led will receive less current and the blue led more so that now we have a cooler tint. Once color is set, the dimming would be another 100K dual gang in parallel with the first so that the ratio will remain the same to both drivers( or almost the same, as 2 linear variable pots in parallel will produce a slightly log taper as the resultant ) hoping little or unnoticed color shift during dimming. The drivers each are designed for 50K pot, that is why I chose 100K pots as the parallel would be the 50K required at full brightness.I must really solder them together and see if this setup will work, hoping little, if any color shift during dimming.


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## AnAppleSnail (Feb 15, 2012)

videoman said:


> I must really solder them together and see if this setup will work, hoping little, if any color shift during dimming.



I think with the variables involved hashing it out in real prototyping is the best way to go. Good luck!


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## videoman (Feb 15, 2012)

Thanks, I will post back with results as soon after I have a working unit, like in a week or so.


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