Hello, i have arduino mega 2560 working with 3.2" LCD connected on the TFT shield and 1 LM35DZ temperature sensor on analog pin . Everything is great when this setup is connected via PC usb. Readings is stable with oscillations 0.1 degrees C if - middle value of 120+ readings. When i connect it to the power supply 12VDC 3A (http://www.ebay.com/itm/AC-110V-220V-TO-DC-5V-12V-24V-Switch-Power-Supply-Driver-Adapter-LED-Strip-Light-/152134115704?var=&hash=item236be66178:m:mnohYz4NZlT_Dq5l5lWNUMg) it give oscillations of +/- 2 degrees C with middle value of 120+ readings! Than if i feed it to the same power supply brand- but weaker - of 12VDC 2A the oscillations raises more. This became problem after a while (few months of usage), the other weaker power supply is brand new. Again, on pc usb powering mode it is perfect. I tried to power it from different safety fuse (is that the word?, wall jack on different fuse), tried to solder new LM35DZ- some very very little improvements.
It would be ideal if i can get hands on a power supply that is same as usb on pc. This was powered with laptops usb so maybe to use laptop adapter and its battery but a cheaper version? also there is maybe more than only this 2 things in the whole picture? (some filters on motherboard, regulations or something else?) Or maybe ATX power supply with rectifier diodes but little smaller? Or even 2 mobile phone batteries in series for 2x4V and than those 2 batteries always connected to the battery charger?
I can provide pictures and more info if needed.