Power consumption versus contrast?

[Dave Shaffer (Arizona)]

I was playing with the 10BII that I got as a door prize at HHC2007, and cranked up the contrast of the display to make it more readable on the airplane. i.e. the displayed numbers seemed much darker with respect to the background.

Then, I wondered if I had changed, for the better or worse, the power being used from the battery.

Does anybody know if or how the contrast affects power use by the display? I am under the impression that LCD characters are basically a static response to an electric field, so I'd guess that there is not much affect on power consumption. But, how do they actually change the contrast - a +/- voltage effect, or a phase effect (due to voltage?) in (one of) the polarizer(s)??

[Randy]

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Does anybody know if or how the contrast affects power use by the display?

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For all practical purposes, there is no difference in power consumption at various contrast settings.

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But, how do they actually change the contrast - a +/- voltage effect, or a phase effect (due to voltage?) in (one of) the polarizer(s)??

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The displays are multiplexed so the voltage applied to the elements is pulsed. Varying the pulse width changes the RMS voltage level which in turn affects the contrast. Not all LCD's are the same but this is the general method used in most HP's that I have seen (passive-matrix addressed).

Wikipedia LCD article

Edited: 23 Oct 2007, 8:59 a.m.

[Dave Shaffer (Arizona)]

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Varying the pulse width changes the RMS voltage level which in turn affects the contrast

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Makes sense.

Thanks, Dave

[Garth Wilson]

The off-the-shelf LCDs I've worked with have a backplane voltage pin that takes an extremely small current. Although it's called "contrast," what it's really adjusting is the viewing angle that gives the best contrast.