first cut board design complete

A project log for Diurnal Reef Controller for Lighting and Tides

Exactly the right lighting and tidal conditions have to occur to induce the spawning of corals (and other sea creatures).

Philip CoxPhilip Cox 06/07/2019 at 21:240 Comments

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.

50W/36V=1.4 Amps

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