constant current Basic schematic help


Posted on Feb 7, 2014

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


constant current Basic schematic help
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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




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