5110 Display - Backlight power requirements

First Post! (Guess I have to start somewhere!!)

I am halfway through my first Arduino project - and I must admit, I haven't had so much fun in years!!! I have a rudimentary knowledge of C++ (programmed with it 14 years ago) and zero knowledge of electronics (I used a soldering iron for the first time ever yesterday!!!)

I have successfully connected a 5110 lcd to my Arduino Nano following these guidelines: http://www.ebay.com/itm/131082227580?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 (scroll a bit down to see the connections)

Summary: The Vcc and BL of the LCD is connected to the 3.3V output pin of the Arduino.

My problem: I read that the Arduino pins are only capable of 40mA output and that the backlight of the 5110 needs more than that. So in time I will burn out the pin. I agree with this statement in part because the Arduino Nano specs state that I/O pins are max 40mA. However, my question is this: Does the 40mA apply to the 3.3V pin as well or only I/O pins like D1-12 and A0-7?

Second question (and bear with me, I am completely new to this hobby): How exactly do I measure the amperage draw of the backlight?

Many thanks in advance! CX

I read that the Arduino pins are only capable of 40mA output...

It's a lot more complicated than that. The 40mA value is the ABSOLUTE MAXIMUM that should be drawn from any one pin. It is implied that the pins should typically be limited to 20 mA since that is the value that all the tests are done at. I believe there's also a limit on the total current drawn by certain groups of I/O pins and by all of them together.

Does the 40mA apply to the 3.3V pin as well or only I/O pins like D1-12 and A0-7?

This would be determined by the rating of the voltage regulator on the board, not by the microprocessor. It would also depend on the heat sinking (or lack thereof) and the ambient temperature.

How exactly do I measure the amperage draw of the backlight?

The most straightforward way is to use a milliammeter in series with the backlight. Otherwise since there must be a current limiting resistor in series with the backlight you can measure the voltage across the resistor and divide by the resistance.


Thanks for the reply Dan

Yes, I also understood 40mA to be an upper limit.

I measured the Amps in series between the 3.3V Pin and the Backlight Input on the display. My multimeter indicates 0.004 A. So that would be 4mA. Does that sound right?

The backlight is 4 very tiny blue LEDs.

I also measured the total current from the battery between the +terminal of the battery and Arduino Vin pin. That comes to 54mA without backlight and 58mA with backlight on.


The reason I asked the question was that I read in a thread somewhere that the backlight of the 5110 display will burn out your Arduino pin because of it's current draw. So basically I'm not too worried about exact values, I just want to know if it's OK to power the 5110 display (with backlight) from my Arduino 3.3V pin.

I have no heatsinks, the unit will operate in fairly high temperatures (94deg F / 34deg C). Unit will operate for no more than 1 hour at a time though.

I'm (my stydents) are using a lot of theese displays.. always connected backlight via a 150..220 Ohm to a PWM-pin. Never experienced problems..

CX15: The backlight is 4 very tiny blue LEDs.

Some 5110s are wired differently to others. On mine the LEDs are switched to ground. If yours is the same and you have a meter handy, it won't be hard to measure the current. I have heard that there is a limiting resistor built in. With the displays with white lights, I use a 220 ohm resistor and it is fine. Since you have blue backlights, I suggest you try using it with them disconnected. This does wonders for your power requirement problems and is likely to give a more readable result.

The is nothing kool about blue backlights. Indeed, I understand they consume more power than proper backlights, though I wouldn't think the difference is substantial.