I'm trying to design a lighting circuit for an indoor garden, and I'm not understanding things that seem like they're really basic, so I need some help. The schematic I'm referencing is:
I have a 24V input source which I'd like to limit to 20V. LA is a series array of LEDs such that the total voltage drop across the series is ~19.8V. R1 and Z1 are designed to limit the circuit to 20V. The portion of the circuit after that (the area above the "Repeat 14x" label) is a constant-current circuit as adapted from this instructable, and will be repeated a total of 14 times. Each of those will draw approximately 0.02A.
My confusion stems from the presence of RA and ZA, that, in tweak 3 of the constant current instructable, limits the input to the gate of QB to about 5V. Doesn't it also limit the entire circuit to 5V, including the drop across the LED array? RB was calculated to be 27 ohm 1/4 W from the formulas provided in the constant current instructable; do I only need QA, QB, and RB in the circuit, and then connect the gate of QB and collector of QA to the R1/Z1 circuit?
Device value specifics:
R1 - 140 ohm, 1/4 watt resistor
Z1 - 20V 1W zener diode
RA - 100k ohm, 1/4 watt resistor
ZA - 4.7V 500mW zener diode
QA - NPN transistor (Fairchild 2N5088BU)
QB - N-channel MOSFET (Infineon IPP057N06N3 G)
LA - 6x3.2V LED in series (e.g Cree C512A-WNS)
RB - 27 ohm, 1/4 watt resistor