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LCD Power supply notes

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I want to use I2C 20×4 LCD with my Raspberry Pi. It works when powered from 5V line of GPIO, but works also when powered from 3.3V line.

I know that this display in fact is designed to work powered with 5V. It works with 3.3V only because I adjusted contrast to maximum value.

I know that using 5V is not save for GPIO signal pins as they are not designed to get 5V signals.

My question is: Is that safe to use 3.3V line to power that display? I guess GPIO pins are OK with that solution, but won’t it damage 3.3V line of Raspberry ?

Do I really need to use I2C logic level converter/shifter?

Added: I connected my display (in 3.3V connection variant) as in here. This guide is for OLED I2C display that is designed to work on 3.3V, but my I2C display is designed to work with 5V. I don’t want to use 5V because lots of people says its not safe, as 5V display is returning that voltage to signal pins of Raspberry’s GPIO that shouldn’t be treated with 5V. Most of people says that to use 5V display I should use logic level converter that will trim returning voltage to 3.3V. But as I discovered my display works with 3.3V there is nothing to trim. However using 3.3V line is confusing as I don’t know if now 3.3V line is not in danger. My Raspberry is RPi4 ver B.

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Although your question does not make it clear how the display is connected I guess I²C. Like most devices on Amazon there is no meaningful documentation.

It is quite safe to connect to a 5V powered I²C device provided it has no pullups to 5V. The Pi has on-board I²C pullups to 3.3V.

  • 1
    Yes, I understand that if there is no pullups to 5V the GPIO logic pins are safe, but my question is about 3.3V pin: Is that connection safe for 3.3V power line? If I connect 5V display to 3.3V pin won’t damage 3.3V converter inside Raspberry? BTW. I added missing information to my post. – A J 16 hours ago
  • 1
    @AJ I don’t see how you would expect connecting an unpowered device to the Pi 3.3V power pin could possibly damage anything (provided it doesn’t draw excessive current – which given the nature of the device seems improbable). It probably won’t work. NOTE the 3.3V and 5V power pins are NOT GPIO – Milliways 5 hours ago
  • 1
    “because lots of people says its not safe” lots of people talk a lot of unmitigated rubbish about things they don’t understand – of which electricity seems to be the most common. – Milliways 5 hours ago

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