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Author Topic: HD44780 on 3.3V Board  (Read 2317 times)
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Both the display and the backlight are not showing anything at all when I power the Arduino through the FTDI with 3.3V due to low voltage.
The 3.3V from the FTDI drop to 2.3V. That's not enough to power the display.
Well, no-one said it would, did they?  If the display (LCD) is designed to work at 5V, it works at 5V.

The backlight - according to that spec sheet - draws 150 mA - I think you should measure it (separately, via its own connection) to see if it is appropriate - in fact, it would be a good idea to measure all the various currents in the system.

The Vcc on the Arduino is at 3.3V but its not connected.
But this is the terminal to which the FTDI connects.

And by the way, the backlight is the reason for the voltage drop from 5 to 4.6V.
This is not happening when the backlight is off.
Yeah, should measure the current - perhaps assess the voltage/ current regulation of the Apple power supply using some sturdy resistors (or a piece of electric jug element) if you can.
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And by the way, the backlight is the reason for the voltage drop from 5 to 4.6V.
This is not happening when the backlight is off.
I don't see any reference to you using a current limiting resistor for your backlight.  The backlight should have a typical forward voltage of 4.2 volts when the current is 150 mA.  It is your responsibility to limit the current which you do not seem to be doing.  This is what is causing your power supply voltage to 'sag'.

I don't understand how you expect a 5 volt LCD module to operate properly at 3.3volts.  When operating with a typical 5 volt power supply the contrast voltage should typically be 4.5 volts.  This 4.5 volts is measured between VO and VDD which means it should be about 0.5 volts above GND.  We don't know what VO should be when VDD is 3.3 v but it very well could be below 0v (negative) with respect to GND.


Don
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@ floresta
Take a look at the HD47780 datasheet. It is specced to work at 2.5v or higher.
Regarding contrast, you have a point, but it has to be tested.
As far as I can understand from the datasheet of that module (which is actually a proper datasheet), is that there is a resistor built into the PCB, but J7 (I think it was) should be open. I think he closed it, so you are right, the backlight is probably chewing up more current than it should and causes the voltage to sag.

I suggested palavro to run the arduino and the LCD VDD from the 3.3v line of the TFDI chip while driving the LED backlight from the iphone charger, I don't think he tried to do that yet.
« Last Edit: August 29, 2013, 11:08:00 am by TheCoolest » Logged


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I don't really need the LCD to work at 3.3V, it just would have been handy so I don't have to switch the power supply after programming.

The Ampermeter part of my Multimeter is broken so I can't measure the current at the moment.
I did not bridge J7, the resistance between the J7 leads is 12 ohms.


I'll try powering the arduino from 3.3V FTDI and the display from 5V, will post the result in a bit.
Isn't that a short to frame though?
« Last Edit: August 29, 2013, 01:14:56 pm by palavro » Logged

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In my experience with this specific LCD: http://www.ebay.com/itm/400448319287
The backlight alone consumes about 33mA running off USB power, it has a 33ohm backlight resistor built in.
Simply run the backlight as a "separate" circuit, charger+ to the Anode and charger- to the cathode.
« Last Edit: August 29, 2013, 01:18:01 pm by TheCoolest » Logged


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Take a look at the HD47780 datasheet. It is specced to work at 2.5v or higher.
That's the LCD controller chip.  We are talking about his LCD board which is a different story.  It's minimum operating voltage appears to be 4.7 volts.

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I don't really need the LCD to work at 3.3V, it just would have been handy so I don't have to switch the power supply after programming.
Well, it clearly won't, but during development you can leave the Pro Mini connected to and powered by the FTDI only, and the LCD (both parts) powered by the 5V supply.  It would be nice to know the current details and the behaviour of the 5V supply.

The Ampermeter part of my Multimeter is broken so I can't measure the current at the moment.
I did not bridge J7, the resistance between the J7 leads is 12 ohms.
Checked that out.  Good.

Must be a newbie though - only one multimeter!  Cost about $5, need at least two (if only for simultaneous voltage & current readings) and clipleads.  Note: for currents in excess of a milliamp, the voltmeter should be connected on the component side of the ammeter as the ammeter has a significant voltage drop; for currents less than 1 mA, the reverse is likely to be the case.

I'll try powering the arduino from 3.3V FTDI and the display from 5V, will post the result in a bit.
Isn't that a short to frame though?
Not sure what you mean.  Don't make any sort of power connection between the Pro Mini and the display, but there must always be a common ground connection between all parts.
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