Drawing current from RPi USB power supply

[xiphocoleon]

Hello --
I am making a device that is composed of a RPi 3 and another circuit that draws max 20 mA.
I want to power both devices from a single wall supply; the RPi schematic shown below shows that PP1 is accessible before the F1 fuse. I am going to use PP1 as my 5V lead to my 20 mA circuit, and also connect this circuit back to RPi's ground. I am going to have a 5V/1A power supply; if my RPi draws no more than 700 mA (and probably doesn't get close to this as I'm not running USB peripherals).

Would anyone suggest it is not a good idea for me to draw 20 mA from PP1 as I've described above? Thanks.

[FTrevorGowen]

What voltage does your other circuit need when its drawing ~20mA. If it's not critical (ie. less stringent than the minimum of ~4.65V for USB and the Pi's low voltage warning threshold) then I'd simply use the "5V" pin on the GPIO connector and not worry about the polyfuse. (That's what I've done for all of my circuits needing ~5V**)
Trev.
** FWIW, see the links here for many examples:
http://www.cpmspectrepi.uk/raspberry_pi ... Boards_Etc.

[xiphocoleon]

My circuit needs 5V; currently it is receiving this from a 9V battery being voltage regulated by a 7805.

It was my thought that the GPIO pins could not source more than 16 mA. My 20 mA circuit actually seems to not draw more than 11 mA, but I am allowing it some headroom, thus considering it 20 mA. This is my main reason for not connecting my power source to this 5V pin. But now that I think about it, the 5V is regulated by the voltage regulation circuitry, not the microcontroller, so it seems that I could draw more than 16 mA limit of the GPIO pins. I would just need enough current to power the Pi, and drawing 20 mA in this case is relatively negligible, so it seems drawing right from the 5V pin is okay.

I have checked out your site -- is there any particular link you'd like to direct me to? Thanks.

[drgeoff]

It was my thought that the GPIO pins could not source more than 16 mA.

The power pins on the GPIO header are not GPIOs and are not subject to the 16 mA limit that the Broadcom chip imposes.

[FTrevorGowen]

My circuit needs 5V; currently it is receiving this from a 9V battery being voltage regulated by a 7805.
It was my thought that the GPIO pins could not source more than 16 mA.
...
I have checked out your site -- is there any particular link you'd like to direct me to? Thanks.

That clarifies your concerns and @drgeoff has already pointed out that the limit for the SoC GPIO's does not apply to the 5V power pins on the GPIO connector, but there is a limit to the current that can be drawn from the Pi's 3.3V GPIO connector pins (~50mA IIRC). To get around the latter some of my circuits use level-shifter modules that incorporate a 3.3V regulator (from the 5V supply) to power (external) 3.3V circuitry. Of the examples linked to previously this provides links to that approach (which may be of interest):
http://www.cpmspectrepi.uk/raspberry_pi ... dules.html
Trev.