voltage sensitive devices powered in parallel - Page 1
Forum section: electronics / beginners

author: mouserat
author_rank: Newbie
timestamp: June 13, 2021, 05:01:14 pm
content: Sorry for being an idiot, but I'm stumped. Raspberry Pis are very voltage sensitive - anything less than around 5.05V and they start complaining, anything below 5.0V and they're definitely complaining.I was trying to power 4 Pis from a bus bar (1 Pi4, the rest 3s and 2s), so I wired them to the bus in parallel. Despite trying 3 different power supplies 5V 5A, 5V 15A, and a variable bench supply, the pis all went low voltage (~4.8-4.9) when all four were wired up. The max current draw was somewhere around 1.5 as measured by one of those digital inline USB power meters.I had to run the bench supply close to 6V to eliminate the pis' low voltage warnings. If I remove the Pi4 (the biggest draw) from the bus, the other Pis seem fine.I think I don't understand the voltage drop in a parallel circuit when the power supply should be far more than capable. IIRC if I wire everything directly to the 5A supply (4 wires directly to the + post), it all works fine. The tracks on the bus visually look like they're more than wide enough, so I feel like I'm doing something dumb here. Any suggestions would be appreciated.https://www.amazon.com/dp/B07DW2C4ZB?psc=1&ref=ppx_yo2_dt_b_product_details

author: RES
author_rank: Regular Contributor
timestamp: June 13, 2021, 05:17:45 pm
content: What you could try is near each board an alu cap of lets say 100uF (anti supply ripple/glitches, bçause the brown out detectors react lightning fast) Or use a buck converter to supply each board separately.

author: mouserat
author_rank: Newbie
timestamp: June 13, 2021, 09:43:08 pm
content: Thanks for the feedback. I forgot that I tried putting a capacitor across the bus, ie across the pi4 power leads, eventually going to the largest cap I had on hand. It didn’t seem to help, I think because the cap would only be able to cover a dip for a limited time?For whatever reason this doesn’t appear to be a momentary voltage dip, but rather dragging the supply voltage down for the whole bus at a more or less constant value. If boost converters work like I think they do - exchanging a higher current draw to pull the line up to a specific voltage - that could be an option. I guess I’d really like to understand more about why a 15A supply isn’t itself enough, and why the voltage is getting dropped.While this particular project was a temporary (“art” for lack of a better term) installation, I had planned to rewire my home network closet, replacing more than a dozen AC transformers and SMPS @ 5-18VDC, with one or two higher power (ie 15A) AC->DC psus. It’s the exact same principle as this project, but now it’s a giant question mark.

author: Manul
author_rank: Super Contributor
timestamp: June 13, 2021, 10:07:36 pm
content: Everything is good with your parallel connection and thinking, just that you hugely underestimate the drop caused by all the resistance. These traces are not very wide, screw terminals and all the wires, PI header pins and tracks are also not zero Ohms. For example a cumulative resistance 0.1 Ohms at 1A of current will drop 0.1V, so it is already not 5.0V, but 4.9V. And your setup is clearly worse then my example (more current and more cumulative resistance).How you power PI? Through USB or header? If header, are you using all the available 5V and GND pins in parallel? If USB, how long is wire? USB connector also adds drop... Because of these drops the PI probably sees even less internally, then you measure. This is why it complains at 4.9V (it should not, as I know).Moral of the story: positive power rail has a drop and also ground has a drop (current returns through it), so it adds up and this is what you see. A few Amps is not a small current.