LCD Contrast Pin Current

neiltechspec

Senior Member
Voltage seems to be between 0v & 1v approx, depending on display.
As per the title, does anybody have an idea of how much current the LCD contrast pin draws.

I'm looking to drive it off the DAC pin, should be possible if very low current required I would have thought.

Neil.
 

Jeremy Harris

Senior Member
I've not measured it, but the majority of the current (maybe 10x the actual contrast pin current or more) flows through the adjustment pot. Sometimes you can get away with just connecting the contrast pin to 0V, if you're lucky, which saves the 500µA flowing through a 10k pot. (assuming a 5V supply). Should be no problem at all with driving it of the DAC output, as it can't possibly draw more than a few µA at most.
 

neiltechspec

Senior Member
That's more or less what I thought.

Haven't really got room for a pot & having one would mean having to dismantle the casing I'm fitting the LCD into.

Think I'll give it a try.

Neil.

This is the LCD I'll be using.
DIPS082.jpg
 
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neiltechspec

Senior Member
Well, I tried it.

It would seem that the DAC output cannot sink enough current, even with daclevel 0, still showed 2v on the contrast pin. Must
be some kinda internal pullup.

So, ended up with compromise fixed resistors, 510R to Gnd & 7.5K to +ve, gives a readable display from 4.8v upto 5.2v supply,
5v being best.

The application runs off a 78M05, so should be ok.

(had room for 2 resistors, but not a pot - before anyone asks).

Neil.
 

Jeremy Harris

Senior Member
That's worth knowing, as it implies that a single resistor to ground could be used for contrast adjustment, which would take up less board real estate than a pot. You could probably bin the 7.5k to +ve and just find a value of resistor to 0V that would give the right voltage at the pin, as there seems to be an internal pull-up.

I wonder how repeatable the value of any internal pull-up might be? Could be useful to know. I might try and experiment, as I think I have a few spare LCDs around somewhere.
 

Circuit

Senior Member
Well, I tried it.

It would seem that the DAC output cannot sink enough current...

Neil.
My understanding is the DAC is a current source, not a sink - and basically only a voltage reference, there the recommendation is to buffer the DAC through an Op. Amp.
 

neiltechspec

Senior Member
I could have picked a nearer resistor to ground, but 510R (as was the 7.5k) was handy & just a starting point.

Just played with another LCD, disconnected the +ve of the 10k pot, the pot is now set about half way for a good contrast.
So I would guess around 4.7K to 0v would be a good starting point.


The DAC could well be like that. The fixed resistors I have used will be fine for my application.

Neil.
 

AllyCat

Senior Member
Hi,

My understanding is the DAC is a current source, not a sink - and basically only a voltage reference, there the recommendation is to buffer the DAC through an Op. Amp.
The DAC is a string of 32 x 5kohm (nominal) resistors, so it's effectively a 160k potentiometer between the "Reference" (the PICaxe supply rail or FVR, etc.) and ground.

Basically, a 160k pot at the centre of its track has a source impedance of 40k (i.e. two 80k resistors in parallel*) and the impedance falls towards zero ohms at either end of the track (but it's still around 30k at the 25% and 75% positions). In practice, the top value (31/32) has a source impedance of 5k and the lowest (0/32) still appears to have an (undocumented) impedance of around 1k.

Generally, it seems the LCD requires a control range of about 0 - 1 volt, so I'd try adding a pull-down resistor of around 1k - 2k and see if the DAC can then trim the contrast sufficiently. It might be only a few of the DAC "end" values (0,1,2, and 30,31) that give a useful change.

* IMHO Thevenin's Theorem is perhaps the second most useful concept in electronics design, after Ohm Law. ;)

Cheers. Alan.
 

neiltechspec

Senior Member
The following seems to work quite well.

Code:
	fvrsetup 1024
	low c.2
	dacsetup %10101000
	daclevel 5
A 6.8K to 0v and daclevel 0 to 10 give a good contrast range.

Neil.
 
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