I'm trying to power up my Arduino with PC power supply.
I made a test scetch where I blink two LEDs and write something on the LCD display.
Everything works when Arduino is powered thru USB, but only the leds work if I power it with PC-power.
Voltage meter show 5,32V from PC-power. Should it be regulated more accurately to 5v or something like that?
Maybe the LCD display is connected to Vin pin, wich gives out the voltage used to power the board, and the PC provides 12 V rather than 5? It's fine for the board, but this would damage the display...
#1 ATX Power supplies require a minimum load. The one I have sitting next to me requires a total of 20Watts to stabilize its outputs. Make sure you are properly loading your supply. 5.3V sounds high and is probably an indication that you are not loading the 5V supply enough.
(Sometimes connecting a couple of hard drives will provide enough load.)
#2 If you are connecting 5V to your Arduino, you need to connect it to the 5V pin and not the Barrel Jack. The Barrel Jack is connected to the on-board voltage regulator and requires at least 7V to operate. (You should not keep both USB connected and 5V supplied to the 5V pin at the same time.)
So I need to supply atleast 7V:
Can I supply 12V from ATX to Arduino's Barrel Jack?
Technically, yes. I think it can go to 20v. I have used 12v and the VR gets pretty hot. Within spec and it is probably fine but I wouldn't do it as the standard long term power source. Less wasted energy with 7.2v NiMH or 7.4 LiPo.
The recommended input is between 7 and 12v. Keep in mind that if anything hooked up to the arduino draws more than 200mA, the voltage regulator is going to get hot. As arbarnhart says, you can technically use up to 20V, but the heat dissipation above 12V becomes an issue. (So you'll want to hook up your LEDs directly to the power supply and not through the Arduino board.)
Don't forget to put a proper load on your ATX supply. Each supply probably expects to have a load of at least 500mA if not 1A.