The first cut design of a PCB to control the 50W LED by digital current control is complete. Making Gerbers now. 8-bit control gives 256 levels of brightness from the LED.
It is important when making lights for biological systems to use CURRENT CONTROL ... not voltage/PWM. An LED controled by a PWM signal only APPEARS to brighten or dim. It's not really ... it's just fast blinking all on/all off . The strobing effect is undesirable by lots of plants and animals. On the AVERAGE, the PWM signal looks analog, but it's really just strobing digital.
I arrange eight mosfets in parallel - each controlling 1/2 the current of its most-significant neighbor, and twice the crrent of its least-significant neighbor. The most current is controlled by the most-significant bit in the control word/byte.
To control the 50W LED, the most-significant MOSFET turns 25W on or off (b7). Then next MSb turns 12.5 W on/off (b6), then 6.25 W, then 3.125 W ... etc etc.
control bit power controlled
7 25 W
6 12.5 W
5 6.25 W
4 3.125 W
3 1.5625 W
2 781 mW
1 390 mW
0 195 mW
Voltage is a constant 36 VDC, regardless of current.
Power level on each MOSFET is controlled by a current limiting resistor on the drain. The sources of ALL MOSFETS are tied straight to the anode of the 50 W LED.
36V / 1.4 Amps = 27.5 Ohms resistor on the most-significant MOSFET (a high power beastie, I put two 55 Ohm 12Watt R's there in parallel)
Number 2 MOSFET requires 55 ohms
Number 3 MOSFET: 110 ohms
#4: 220 ...
... you see the trend, I think