Prioritize current to a specific device

[mthomason]

I'm not certain of the technical term for this, so I don't even know what to type into Google.

I have a circuit with a supply of ~2.1A of current at 5V. Sitting on the end of this is a Raspberry Pi 2, a LCD display, some sensors, and other miscellaneous pieces. At least one device hanging on here is happy to take the full 2.1A, but is equally happy with whatever it can get (it's being used to run a battery charger), the rest use far less but will be turned on and off intermittently. I'm unable to supply any more current.

I want to ensure the Pi gets all the current it wants, before giving any to the rest of the circuit. Is there any way to feasibly achieve this, short of giving it it's own dedicated power source? Is there some way to tell a circuit "prioritize the current supply down path A, and give path B whatever is left over if there isn't enough" ?

I've already put in a second power source to run motors, in order to minimize issues there, so I really don't want to add a third specifically for the Pi - there's not much scope left for any more weight on the chassis, and more batteries could tip the balance.

I'm more than happy to go read long texts on the subject to learn - I just don't know where to start looking for them

[hippy]

There are "current limiting circuits" which can, as the description suggests, limit current. You could perhaps measure the Pi's instantaneous current draw and limit the current available for something else from that to keep combined current draw consistent.

[mthomason]

Thanks for the quick reply!

Yeah, I was considering simply measuring the max draw of the Pi (with wifi and all USB devices running at max), putting that on "Path A", and then simply limiting "Path B" current to (2 amps - measured current for Pi), I was just hoping to somehow dynamically allow extra current to path B if path A isn't using it right now.

I suppose I could do something with a microcontroller to measure the current the Pi is using, then use that to control a digital pot in something like this: http://freecircuitdiagram.com/2008/08/2 ... r-circuit/ - I was just hoping to avoid the complexity if there was an easier way to just designate a path in a circuit as "priority path"

[mthomason]

Thinking about it a bit more... I could simplify it by having three or four preset current-limited paths available to "Path B", and then use MOSFETs to open and close them depending on how much excess power I have (measure current draw by Pi, measure current available at source, and open enough paths to use most of the difference, leaving say 0.2A as a buffer, and add capacitors to ensure the Pi doesn't suddenly find itself short), using an Arduino to do constant measurements and adjustments.

Now I just have to go build that in simulation, and prototype it if it looks like it'll work. Unless anyone has any better ideas, or reasons why this shouldn't work?