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I'm driving an LCD HD44780 display using my Raspberry Pi B+.

Using a GPIO pin I'm managing the contrast adjustment (pin n. 3) on my LCD display with PWM.

In the display datasheet, I tried to search something that talks about minimium resistor between the GPIO out pin, and the contrast adjustment pin on the display, unsuccessfully.

This is the HD44780 datasheet.

So I connect the display and I measured the current flow first without any resistor and then with a 1kOhm one. Here my result.

  Resistor  |   Current
------------|------------
    0ohm    |   20mA
------------|------------
   1000ohm  |   12mA

My questions are:

  • Can my display/raspberry be hurt using the contrast adjustment without any resistor?
  • Is 20mA a dangerous current for my display/raspberry?
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First off, the HD44780 is the controller chip, not the LCD itself. The LCD panel has its own specs, and an LCD module (controller + panel) should have its own datasheet. Sadly, datasheets for displays are often pretty terrible.

20mA seems like a lot for a digital IO. This page suggests never sourcing or sinking more than 16mA from a single pin. Your Raspberry Pi datasheet should have more information about output current limits. Look for an absolute maximum rating.

I'm not sure why you want to adjust the contrast at run-time. Contrast is usually something you set once with a resistor or trimpot.

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