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I have a 12 V DC source that I'm wanting to run a Raspberry Pi Zero and a couple of 3 W IR LEDs. I'm not using the Pi to control the LEDs in any way. The LEDs are just to provide lighting at night for a camera running on the Pi.

I have a cc/cv buck converter to run constant current to the LEDs at 1600 mA. If I limit the output voltage to 5 V, could I run the Raspberry Pi off the same buck converter? Or do I need to do this in 2 parts with one converter to reduce 12 V to 5 V for the Pi, then run another separate converter to keep the constant current for the LEDs?

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    \$\begingroup\$ All will depend on how you are connecting you IR LEDs (what is their forward voltage?). You can't "limit the output to 5v" and have CC=1600 mA at the same time. Schematics for your IR LED connection please. \$\endgroup\$ – Ale..chenski Aug 2 '18 at 17:18
  • \$\begingroup\$ For linear CC mode the shared 5V is fine but possibly less efficient than a switched mode CC regulator. It depends on your power specs. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 2 '18 at 17:19
  • \$\begingroup\$ Ali, I don't have an easy way to draw up a schematic right now, but it's just a series circuit with 2 LEDs. The LEDs each have a forward voltage of 1.6v and a max current of ~1800ma. \$\endgroup\$ – mhaserodt Aug 2 '18 at 17:53
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Aside from various possible disaster scenarios, the Raspberry Pi power consumption varies widely perhaps from 0.4 to 1.5W. That would cause the light output from your IR LEDs to vary considerably, so I don't think it's a good idea.

I suggest running both converters from the 12V input rather than chaining them, for better efficiency.

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It's not terribly uncommon to chain together power supplies. For instance, many higher end electronics have a VSYS which is typically the switched voltage between 5V USB and battery voltage. That then gets bucked down (or even lowered with an LDO depending on what's being powered) to the voltages needed by the processor, sensors, flash etc.

I'm not sure what type of 12V supply you're using but if you're using a battery you will have to be careful about the quiescent current of the buck when it's not in use. From a cost perspective though it makes sense to run everything off the same constant voltage supply but you need to be sure that as Spehro mentions that the transient current from the processor may cause issues. If your supply is rated higher or equal to the max of the Pi and the LEDs you should be good.

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A CC LED driver doesn't have a definite voltage output. The voltage will be determined by V(f) of connected LEDs, in accord with their (individual) I-V curves.

If you LED has V(f) = 1.6 V (+- 0.1 I guess, depending on current and temperature), you can connect 3 LEDs in series to your CC driver. Then the total voltage would be 4.8 V (+-). This should be enough to power the Raspberry Zero.

As a matter of fact, LEDs can be used as "parametric voltage stabilizers" instead of Zeners, with about same quality of voltage regulation. So 3 LEDs should form a sufficient stabilizer, since RPi uses PMIC (PAM2306AYPKE), which is tolerant to 5.5V. So this should work if you put 3 LEDs in series.

The only small concern is that the Rpi might have variable power consumption, which will eat into your fixed CC budget, and steal some current from LEDs as Spehro Pefhany noted. It will have some effect on LED brightness.

If constant brightness of LEDs is important, you will need to use a separate DC-DC 12V-5V converter to feed your Rpi. Or make your Rpi code to consume constant power, and adjust the CC limit accordingly.

Of course if one of your LEDs suddenly dies (which do happen with them), your Rpi might be in trouble, so you might want to design some extra protection from overvoltage unless your CC doesn't have one. But you said you can set the driver to no more than 5 V, then you should be fine from all angles.

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