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:

> 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@sudoroom.org> wrote:
>
>> 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:
>>
>>> 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@spaz.org> wrote:
>>>
>>>> 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-consumption
>>>>
>>>> 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:
>>>>
>>>>> 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>
>>>>>
>>>>
>>>
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