Hi I am in the process of building a P3steel Prusa version4. 300x200.
I am using an Arduino mega 2560 and a Chinese Ramps 1.4 with Chinese DRV8825 drivers set at 0.5vdc
using a LCD12864 display diodes 1 and 2 are in.the ramps.
Nothing is connected to the unit. (steppers,endstops, thermisters heatbed.)
If I use only the 12 v power supply zyltech 360 set at exactly 12vdc output, The display is very bright and I have to turn the trim pot down to adjust the contrast. sometimes it might display the Marlin logo, other times nothing.
If I boot only with a USB cable it boots up into the menu and I can scroll through and select available options. I have to turn up the brightness of the display.
Now I connect the 12v PSU and it works for a short time then freezes. Have to turn down the display as it is too bright. Some times the #1 LED flashes red quickly then goes off and maybe flashes again. If I disconnect PSU or try the reset button on the ramps there is no response (frozen)
Nothing gets warm on the Ramps. (no loads)
The 5V regulator on the Mega is to weak for most graphic LCD's.
You'll have to remove D1 on Ramps and feed 5V through a separate source ( PC PSU, active USB Hub or RC-UBEC )
Or get a 20x4 character LCD, they are better in many terms ( less CPU load, too )
When it runs off USB power it seems fine.
When runs 12v through ramps it plays up.
So the way I think you mean is that when the USB is disconnected the 12vdc going through D1 to mega transforms 12dc to 5v but not enough current to run lcd.
So cut D1 and connect a 12vdc supply to the mega power input. Should I put a small buck in between and reduce the input voltage say to around 7vdc?
So I cut D1 and powered up the 12vdc to the Ramps PWB. No voltages applied to USB or DC input on Arduino.
The Arduino still boots, goes to the menu and after awhile freezes. If I reboot nothing happens. If I remove 12vdc, reconnect after 10 seconds, it freezes at the Marlin screen.
Does the LCD display get all its, data, voltage's directly from the Mega board?
Think I need to disconnect (cut) something else.