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I need to supply 5v analog power to an adc. I am supplying the pi with Li-ion batteries, so the voltage falls over time, but does remain within range of the pi's required input.

If the input voltage falls below 5v, i.e. 4.9V would there still be 5V on the 5V pins?

I would test this myself, but I do not have the resources at the moment?

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    Although it is mostly about the 3.3V rail, this is probably of interest: raspberrypise.tumblr.com/post/144555785379/… See the graph under Load Regulation halfway down, with the observation, "unlike the 3.3V rail, there is a voltage drop on the 5V rail as load increases. This is expected as some 5V components are subject to changes in resistance and conductivity under load and there is no regulation to compensate for this". – goldilocks Nov 1 '16 at 17:47
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    Once again, you should really avoid using the 5V pin directly from the Pi. There is no good reason to ever use it really. And also by the sounds of things you also are going to have a big headache with your batteries... because you are powering the Pi directy from your "battery" bank is a very bad idea too. I sense a gamma of lack of understanding across the board ... ;( You should really think about picking up a book and trying to understand some of the concepts you are trying to work with. Good understanding of theory is 50% practical complete. – Piotr Kula Nov 1 '16 at 18:54
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    Seems to me requiring a 5V supply is "a good reason" to use it, at least if it is only for a few hundred mA or so. I think you need a good reason to not use it in that case. – goldilocks Nov 1 '16 at 18:58
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No.

The 5V pins are unregulated. It is safest to assume they will be at the same voltage as the input microUSB power supply.

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    And you should add that the total current the 5V pin can supply is whatever the Pi uses - 1Amp. So really not that much. – Piotr Kula Nov 1 '16 at 18:55
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    @ppumkin "Not that much" is very subjective. Since the context here is for an ADC it is probably plenty. – goldilocks Nov 1 '16 at 19:00
  • The point is... its unknown variable. And when you start to bolt on more and more you suddenly run into brown out problems, or questions like my WiFi doesn't work or my Pi wont boot up, while a Christmas tree hangs off the 5V PIN. – Piotr Kula Nov 1 '16 at 19:02
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    In fact, the 5V suply to the Pi should be regulated before it ever reaches the board – Chris H Nov 2 '16 at 9:19
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I assume the 5V you are supplying to the ADC is the reference voltage. As you might guess, any voltage level that serves as a "reference" should be reliable within a tight tolerance. Consequently, you want a stable voltage and you cannot expect that when powering the unregulated 5V rail from batteries that lose power over time.

If you were supplying the Pi from a conventional micro-USB power adapter, and any other loads were modest and not varying to a great extent, then you may have a stable 5V power to use as a reference.

If you need to use batteries (e.g., for mobility), you might try testing the operation of your ADC under varying voltages to determine the range where it works correctly. Then implement some form of power monitoring to alert you when the batteries need to be changed.

Another option, which is kludgy but might be acceptable for a non-commercial DIY project, is to connect a step-up voltage converter to the 3.3V rail that outputs 5V. The voltage on the 3.3V rail is regulated and very stable, so the output 5V will be stable, as well, even as your input battery power falls below 5V. Assuming the ADC requires minimal current from the reference voltage connection, the 3.3V rail & converter should be able to supply it without any problem. This is a creative solution (offered in the spirit of brainstorming) that attempts to work around the constraints and deficiencies in the power architecture of the Pi. You would have to test it as a POC to assess if it works for your configuration.

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No, they are not regulated in any way. Between the micro USB connector and the 5V GPIO pins are:

  1. A 2.5A resettable fuse (MF-MSMF250/X)
  2. A p channel mosfet (DMG2305UX) basically used as a diode

Source: https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/Raspberry-Pi-3B-V1.2-Schematics.pdf

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No the 5V is unregulated. See Raspberry Pi Power Limitations for an explanation of the Pi power circuitry.

The voltage on the 5V pins is slightly lower than the μUSB connector as it goes through a polyfuse and an ideal diode.

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No, it does not and it cannot. None of the available schematics for any model of the Pi (here, especially the Pi-3) show a voltage regulator for the 5V rail. Given the somewhat minimized price of the Pi (and thus the cost to make them) and the targeted audience additional regulation and step-up conversion of the 5V input/output has been skipped - and rightly so.

You need to provide apropriately regulated 5V to the ADC. Depending on the required accuracy of the ADC it might be advisable to properly decouple its analog supply voltage from the digital circuit of the Pi. While it is in this particular case related to the Pi, I guess that you'll find better hits and help at https://electronics.stackexchange.com/

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