As long as there is sufficient excess voltage, the series resistor (for each LED or LED string) effective forms a constant current regulator. Strictly, the calculation of its value should take into account the Vf max and min for the LED string, and this may reveal a problem if the excess voltage is low. In this case a constant current driver can be useful.wh7qq wrote: ↑Fri Dec 08, 2017 10:18 pmSorry this posting is a bit late...my ISP crapped out just as I was writting it last evening. What you don't know is how consistent (or not) the 3535smd is from part to part, or even if you would get LEDs off the same die. Trying to drive all 5 off the same power bus is going to be a problem for this reason.: The LED with the lowest Vforward is probably going to hog the current and might even fry itself in the process. You will need to have a constant current driver for each LED. Search on google for "constant current LED driver". They are available in discrete packages from various manufacturers.
Correct. You will also want to consider the effects of different colour temperatures of white LED.wh7qq wrote: ↑Fri Dec 08, 2017 10:18 pmYou don't tell us what you are using the RPi for in this case. If you are doing some kind of PWM to control the LEDs, it is a whole additional can of worms because LED's do not have persistence and syncing them to the actual shutter opening will be difficult. LEDs are on and off much faster than we can perceive so we can't see their on and off but your camera's image sensor can.
Yes of course the PSU must be able to supply enough current for both LEDs and Pi, but there is unlikely to be a problem with wire voltage drop if the Pi's supply is separately regulated (eg UBEC) and the wires from the UBEC to Pi are short and thick. This applies to any Pi PSU.wh7qq wrote: ↑Fri Dec 08, 2017 10:18 pmI would not try to power the RPi from the same supply as the LED's because there may not be enough current left over after the LEDs, especially if you have a USB bus with anything drawing serious current. Even small wires will drop too much voltage just coming from the supply. RPi power bricks are really cheap.
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