I have started a project for my home lighting, I'm trying to build a DC LED module which would change the color temperature from 3000K to 1800K as dimmed with reducing current. I'd like to do it mostly passively without any current sense or threshold circuitry. I also want it to run on my current TRIAC dimmable constant current driver.
The most sensible way I could find to do it is to use two different temperatures of LEDs, route all the current to warm array in the beginning of dimming range, later ramp up the current of the cool array for the rest of the dimming range. Maybe, preferably, completely turn off the warm array at the end of the dimming range.
Firstly, I'd like to share my current "progress" with you, if you can call adding a resistor as such:
I was able to come up with a very simple circuit to do it, I kept the number of warm LEDs in series low, thus the total forward voltage, put a series resistor to them, then parallelly connected the cool array with higher number of LEDs in series. The LTSpice schematic and DC current source sweep analysis results given below:
The red curve is for the cool array, green for the warm array.
However as you might have quickly realized, the series resistor dissipates up to 800 mW of power at the maximum brightness, and also the warm LEDs stay on at the end of the cycle(not really a big issue though). For a total of 4.8 Watts power consumed in the circuit, 0.8 W is directly wasted on the resistor.
I've seen some other Warm-to-Dim passive DC LED modules online, some add a transistor, some just use the resistor approach. Here is an example of such arrangement with transistors, but i wasn't able to figure out the circuitry from the pictures:
I can just speculate that the transistor assisted circuitry is much more efficient since the transistor or MOSFET just switch current instead of dissipating the unwanted amount.
I failed on designing some transistor or MOSFET circuits, I tried to sense the current on the warm LED branch, then feed it to the transistor so the cool branch could turn on after some set current.
Maybe me being an amateur hobby electronics guy, originally a mechanical engineer, couldn't see the obvious thing they've done there, or couldn't come up with a very simple transistor arrangement to achieve it.
Can you recommend me a circuit that:
- Use minimum number of components even at the expense of performance
- Doesn't waste as much current as the resistor circuit I was able to build
- Have it's crossover current adjustable with the value of a resistor
- And possibly have the warm LED branch completely turned of at the maximum current level?