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>
>