18 Nov
2022
18 Nov
'22
11:07 p.m.
I need to power a device with an LCD screen running on a Raspberry Pi.
Both need 5 V. The Pi needs about 3 Amps. The screen needs between 0.5 and
1 A, depending on brightness. So I figure I can power both off of a shared
5 V power supply as long as it provides at least 4 A. But I can't find a 5V
4A power supply. So I'm wondering... could I just wire two 5V 2.5A power
supplies in parallel? Would this work? I feels wrong but I can't think of
why.
[image: image.png]
*Andrew R Gross, (he/him)*
412.657.5332 - shrad.org <http://www.shrad.org>
19 Nov
19 Nov
12:22 a.m.
the Pi does not need 3 amps. That would be 15 watts. Since the Pi has USB sockets, its
theoretical demand could include those sockets, but the Pi itself probably tops out at
less than 1.5 amps:
"...the RPi 4B power dissipation tops out at approximately 6.4 Watts..." which
is 1.28 amps.
https://raspberrypi.stackexchange.com/questions/114239/pi-4-maximum-power-c…
to answer your other question about power supplies, first I can say that there are
definitely 4-amp 5V supplies and I can probably find one, but it won't be necessary, I
think 2 or 3 will be enough for what you're doing.
As for why you can't put regulated power supplies in parallel, think of it this way: a
regulated power supply (like all the ones you looked at) has a chip which is adjusting the
power production in order to achieve the correct voltage at the output terminals, which is
in this case 5.0 volts.
If you connect two power supplies together, their output terminals see the voltage that
they're both connected to, so if one does the job of making 5 volts, the other says
"well I guess that's taken care of, so i don't have to do anything" or
worse, they're exactly equal, and they both simultaneously try to do the job of making
5v, but end up tripping over each other and bumping into each other, causing chaos. In
most situations it will probably work OK, but only because switching power supplies can
usually do more power than they're rated for so you're getting lucky.
When people do "power injection" with long addressible LED tapes, the resistance
of the LED tape's power wires ends up helping them by creating electrical distance
between the power supplies.
-jake
On Fri, 18 Nov 2022, Andrew R Gross via sudo-discuss wrote:
11:13 p.m.
That explanation is excellent, thank you!
I do believe I'll need your help acquiring a 4A power supply, though, since
the current prototype is experiencing power loss on a 3A wall wort. Part of
this (much of it) might just be resistance along its wire. It's got like a
6 foot UBS cable.
Can you recommend a part? I genuinely wouldn't start contemplating
Frankenplug up there if I wasn't reaching the bottom of the idea barrel.
*Andrew R Gross, (he/him)*
412.657.5332 - shrad.org <http://www.shrad.org>
On Sat, Nov 19, 2022 at 12:22 AM Jake <jake(a)spaz.org> wrote:
11:56 p.m.
Oh! Also it uses one of those Jabra video conference microphone and speaker
pucks. I'm not sure what that uses, but I'm pretty sure it's non-trivial.
I'll check later.
*Andrew R Gross, (he/him)*
412.657.5332 - shrad.org <http://www.shrad.org>
On Sat, Nov 19, 2022 at 11:13 PM Andrew R Gross <arg5029(a)gmail.com> wrote:
20 Nov
20 Nov
8:28 a.m.
3A wall warts are pretty easy to find. In case you decide you want 4A,
heres a few available at Jameco in San Mateo
https://www.jameco.com/c/Power-Supplies-Wall-Adapters.html#/filter:ss_attr_…
-steve
On Sat, Nov 19, 2022 at 11:56 PM Andrew R Gross via sudo-discuss <
sudo-discuss(a)sudoroom.org> wrote:
--
-steve
12:55 p.m.
I'm saying a 4A supply is not going to change anything, i'm sure your power supply
is not the limit. For one thing, a regulated supply like that doesn't lower its
voltage if you overdraw its current limit - it overheats or fails.
If your issue is resistance over the wires you're using, which I think is likely, it
wouldn't matter if your power supply were 1000 amps, because the resistance would
lower the voltage according to ohm's law.
If you want to meet up and look at everything together, I can show you how to see exactly
what's going on (with a voltmeter) and we can find an appropriate power supply and
wire it up so it works!
-jake
On Sat, 19 Nov 2022, Andrew R Gross wrote:
1:41 p.m.
Another possible option would be to use the two power supplies separately
(one for the Pi, one for everything else) and tie the grounds together
(with a wire connecting all the GND pins). This has the added benefit of
helping you isolate which part of your system isn't getting enough power!
On Sun, Nov 20, 2022 at 12:56 PM Jake via sudo-discuss <
sudo-discuss(a)sudoroom.org> wrote:
1:55 p.m.
USB carries ground and 5v, so the easiest thing to do is power the display from its own
supply and everything else from another. Unless the display needs USB to connect the
touchscreen, and maybe that USB is also how it gets power?
you can cut the 5v line on a USB connection and feed 5v another way, but sometimes that
interferes with USB enumeration (although it shouldn't)
-jake
On Sun, 20 Nov 2022, J.D. Zamfirescu wrote:
9:50 p.m.
@ JD: It's a very simplified diagram, but the display takes 5 V
independently over a dedicated USB plug just for power, and receives data
over HDMI. The display has no touchscreen.
@ Jake, can you rephrase what you're recommending? The wire appears to be
about 1.5 ohms. I think we're miscommunicating somewhere, because I've
definitely encountered situations like this where I was trying to run a
Raspberry Pi on a 2A or 2.5A power supply and it was giving undercurrent
errors, and the pi would be starting and losing power and rebooting over
and over, or the display wouldn't turn on, and the issue would go away once
I used a high-quality USB power supply rated for 3A.
I know that I can power the system by running two separate wires and
plugging each in separately, since that's how I'm currently powering the
prototype. But I really don't want to do this, because it'll look messy as
hell when this is attached to our front door. Is it possible to supply the
entire system from one 5 V power supply of any kind? Should I deliver power
to the whole thing with a 9 or 12 V power supply and then regulate it down
to 5V inside the enclosure?
*Andrew R Gross, (he/him)*
412.657.5332 - shrad.org <http://www.shrad.org>
On Sun, Nov 20, 2022 at 3:53 PM J.D. Zamfirescu <zamfire(a)gmail.com> wrote:
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