OT: LCD contrast adjustment for temp

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That generator info display project will have a box sitting outside in a shed - one that can get bloody cold during winter. LCDs are very temp-variable, and need their contrast adjusted to compensate.

I've done some digging around, and there are a couple of ways to do this. I've included a simple one below, modelled in SwitcherCAD spice. This is using a typical 10k thermistor, which ranges from around 70k at -20C to around 6k at +38C. There's a pot in there to allow you to adjust things a bit. The -ve voltage V1 is generated with an ICL7660. I have 12V available on the pcb, so V2 is at 12V. Using 5V just narrows the min Vee from -5.4V to -7.2V.

The green trace is with the pot at 470R, and the bottom dark blue one is the pot at 47k. One would adjust the pot at room temp (25C/10k) for good contrast, and as it gets colder or hotter, it *should* adjust. Getting it adjust correctly, ah, therein lies the art.

The articles I've seen talk about using a bias resistor (R3 here at 470k) equal to the thermistor resistance at room temp, in this case 10k for a 10k thermistor. But doing that really reduces the range of -ve voltage available to the LCD Vee. Hrm.

So, anyone done this before ? (Surely someone has). Does this look reasonable ?

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Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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I can't remember--what "LCD" are you using?

If standard character LCDs, or graphics LCDs, keep the backlight on. That keeps them un-cold nicely.

Or spend the extra $10 or $20 and get a Varitronix VFD. It is much easier to read anyway.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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It's a Varitronix MDLS40466, the extended temp 4x40. The plan is to keep the backlight on except during a power outage - it needs a good 200mA just for mediocre brightness, and looks nice and bright at 300mA+.

If the LCD is in the main box, there is as 12V LDO regulator and a 5V following that. I'm debating replacing the 5V linear with a switching to reduce the power losses, but together they would generate some heat. But the plan is to allow the LCD to be remotely mounted in the outer skin of the door. In which case, that box will only have the LCD and rotary encoder, so no regulator heat.

You know, come to think of it, 200-300mA at xV really isn't all that much compared to the rest of the power loads the inverter is feeding. I'm so used to thinking OMG! 200mA! GOTTA GET IT DOWN TO 30mA!. Maybe I will just keep it on all the time. How much heat dissipation is typical in these backlights ? I don't see anything in the datasheets.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Quote:
How much heat dissipation is typical in these backlights ?

LEDs are not particularly efficient at making light, so power dissipation
is pretty much going to be E*I.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Almost forgot, need a pot to actually adjust where the Vee voltage range will lie. So below, U1 changes what the thermistor actually controls - it sets the voltage range limits. U2 is the usual LCD contrast pot, in this case it sets the offset of the range that U2 is producing.

The top graph shows various settings for U1 - the steps are 10% from 0-100% of a 47k pot. The bottom one shows what is available when U1 is set to 20% (around 9.4k), also in 10% steps using a 10k pot.

The scale across the bottom is the range for a typical 10k thermistor.

I've read that the bias resistor (R3 here) is supposed to be selected to be equal to the resistance of the thermistor at room temp, (25C), 10k. However, if I do that, it seems to narrow the range that U1 can produce. The dropoff seems to start at around 100k - above that the range is widest.

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Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)