RepRapDiscount Full Graphic LCD low quality with PSU on

Hi

I have the RepRapDiscount Full Graphic Smart Controller, both it and the printer are working fine, however when running with the ATX PSU turned on the LCD background brightens up and appears to have a grid of various brightness lines across it, the display is a little more "shadowy", and the highlight as I move between menu items seems to take a while to fade, leaving a trail of highlighted items behind it. Basically its slightly less readable/usable, but still fully functional.



When running on USB power, the LCD acts as expected; clean looking background the same color as the borders, crisp pixels, and responsive highlighting.

I thought this could be an over-voltage issue, I have measured the output from the PSU (11.95V) and the supply to the LCD (5.0V) while on PSU. While on USB power the supply to the LCD is 4.3V so there is a bit of a difference, but I would expect the better display quality to be while on 5V! It looks perfect at 4.3V!

Is there something I can adjust? Or something else to test? I have seen contrast pots on LCD's before, but I cant see one on here. Adjusting the contrast in the menus doesn’t seem to have any affect.

Thanks

Edited 1 time(s). Last edit at 01/13/2015 05:34AM by bobemoe.

Just found this thread [forums.reprap.org] which is similar but not exact.

The solution of playing with the trim pot or adding resistors seems tempting but it looks like a have a much newer board and the pins aren't quite the same

SOLVED from advice on the thread linked to above.

A 2k2 resistor between GND and Contrast pins as labeled below:



Now, of course, the contrast is too low when running on USB power, but I'd prefer it that way round!

What's the value of the backlight LED resistor (RBL)? Mine is 101 (100 Ohm). If it's much lower (can't read it on your photo), maybe your backlight is drawing too much current and that might be the reason for the USB voltage drop...

Hmm, my RBL says 0, thats just a jumper isn't it?

My 5V rail goes up from 4.45V to 4.70V when I disconnect the smart controller, so it is pulling the voltage down a fair bit, although its not starting from a great place to begin with.

The USB port with nothing connected at all (apart from an Arduino Duemilonove) measures in at 5.05V With a 10ohm resistor across it (that should draw the ports max output of 500mA) it drops to 4.3V. Would you expect this much drop when a port is at max output? Maybe its not the best motherboard.

Just been doing some interesting reading on usb voltages:
[answers.yahoo.com]
[plugable.com]

Guessing my motherboard/usb controller isn't doing too well, I'll test on some other supplies soon.

I gave up on using my USB jack (on my laptop) for voltage a long time ago. I only use mine to communicate with a powered hub, using the hub to connect to peripherals and supply power to cell chargers, Arduino, etc.

I stopped using the USB jack for direct connection to things when a shorted (cheap promo) thumb drive killed the jack (motherboard!) on a previous laptop. Never, ever again...

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-David

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