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I have a project where I am using raspberry pi 4 and 5inch touchscreen. This screen is from waveshare but I have not enabled touch in it. I am using a USB splitter cable which on one end has usb male type and other end has two parts usb c type and micro usb. This C type usb is to power the pi and micro usb is used to power the screen. I am using 5v 2.1 amp power adapter to power pi and screen.

When I boot the raspberry pi, it starts to display the under voltage and also in dmesg I can under voltage warning. In order to resolve this, should I buy some adapter which has higher amps rating like 3.1amps or I should buy an adapter which has two usb ports so that the ports can supply 5v each to pi and screen. I am confused between volts and amps. Can anyone please help me. Thanks

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    Please edit your question to state the current requirement for your touchscreen. No one can answer definitively without that answer. Alternatively, buy a larger power supply as you're current supply is below spec for the RPi 4B
    – Seamus
    Jul 11, 2020 at 5:29

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If the undervoltage warning is displayed the Pi is not getting the correct voltage. In fact it only shows when the voltage is well below the USB minimum.

The current rating of the supply is largely irrelevant, and the many posts on this site are uninformed. Only poor supplies are incapable of delivering the rated current.

Incidentally the Foundation DOES NOT require 3A supply, although this is recommended.

The most likely cause of your problem is voltage drop in the cable. Virtually ALL available cables use woefully inadequate wires. Fancy splitter cables just make it worse. These MAY work for their designed purposes (which IS NOT to supply 5V power).

Buy an OFFICIAL Pi power supply. You should be able to power your screen from the Pi 5V via USB or the 5V pins.

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  • I am getting occasional undervolt warings from my RPi4 whilst it is essentially sitting there idle, despite using the official black 3 amp PSU.
    – Ed Randall
    Feb 13 at 16:31
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Raspberry Pi 4 specs indicate that a 3 amp power supply is required, or possibly 2.5 amps if all connected usb downstream devices require less than 500 mA.

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  • based on that table, by my calculations, pi4 needs 3.0A - 1.2A = 1.8A - so, with a 2.1A power supply you could allow up to 300mA USB device(s) :p
    – Jaromanda X
    Jul 11, 2020 at 5:41

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