I just totally renewed an old PC from '09 and now I'm stuck with a blackscreen and shutdown 20-30 seconds after i start it up, I can hear and see the fans turn, even though they work they work at max speed...
AMD Athlon x2 3ghz
ATI Radeon HD5450
4GB kingston 1333mhz RAM
And a 300 watt PSU, this might be the problem but i have no idea.
(And yes, I tried with the onboard GPU and another CPU. No luck.)
300 watt PSU would only deliver about 220 watts on the 12v rail so it's probably a power shortfall happening once you get to desktop or start a game or application. On boot up, the maximum amout of power is determined by all of your hardware and components then the PSU fills the capacitors with thamt maximum. If there is more power needed than your PSU can supply, it might suffer a stroke and the system goes into shock causing the shut down. The power check only lasts for a second (or two) but that is all that's need.
There are not many 300 watt PSU's with a 24 pin power connector nor even ATX models, are you using a 20 pin to 24 pin adapter or is the PSU actually got its own 24 pin power connector?
Even though the CPU is only anbout 45 to 65 watts, the graphics card requires a higher watt on the 12v rail that your 300 watt PSU can deliver, a 400 watt PSU would deliver about 340 watts, about 40 watts higher than your total output but a shortfall on the 12v rail of approximately 120 watts.