I want to connect the 5v and 12v rail on a PC power supply in parallel to get higher current. I need a 255 watt 12 volt power supply, so I tried connecting the 5v and 12v rails together but, the power supply turns off when I do. The 12v rail can supply 15 Amps and the 5v can supply 20 Amps. I hope that I will solve this problem. Thanks in advance.
Current will flow from the 12v to the 5v volt rail, causing a short and the PSU to shut down.
You know that if you short out i.e. 5v and ground then high current will flow and the power supply turns off to a) don't overheat and b) don't burn you wires (or something along those lines). Voltage is relative, so 5v - 0v = 5v difference; 12v - 5v = 7v, that's the same as shorting 7v and ground.
First, it turn off because there is a short, (12v-5v=7v, 7v to ground=short!!!) and the protection circuit came online. it's a good thing for you. Second you can't get any current from a 5v source if your device requires 12v. for instance, powering a 12v led with 5v, the led will not even turn on. Third, make sure your psu can handle 255 watts. Even it says it can output 12v @ 15A=180w and 5v @ 20A=100w which is 280w in total, It is not necessary meant that your psu can handle 280w. Finally, to address your problem, you need to step up your 5v to 12v using a 75w dc-dc circuit, then you can combine this stepped up 12v to your psu's 12v to get more current. 255w-180w=75w. (make sure your psu can handle up to 300w because power conversion is not 100% efficiency, thus you need more power to get to your 255w requirement.) GOod luck