A PC power supply is the internal component that converts 110 or 220 V wall voltage to the DC voltages used by the PCs other componenets. A PS typically has a fan to cool it and (in some cases) help cool other components, too. It provides some "conditioning functions" to smooth out DC supplies during very brief spikes and drops from the supply voltage.
You often need to adjust the PS to run on either 110 or 220, and there is normally a small switch on the back for this adjustment, next to the elec cord port. (Be sure to turn the PC off before you do this).
PSs are rated according to the watts of DC power they output to the rest of the components. Modern PSs usually offer 300 watts; older ones typically offered 200 or less.
Note that the PS rating doesn't indicate what it will draw from the outlet. They only draw what is needed to supply the internal comps. If your system needs less than the full output, the PS will only draw enough elec to meet those reqmts.
The total wattage drawn by the comps should be less than what the PS offers:
- Mobo: 30 W (excluding CPU and memory)
- Memory: 10 W per 128 MB
- CPU: 65 W (Pentium 4 and Athlon) or 50W (older)
- Hard Drive: 5-15 W
- CD-ROM: 10-20 W
- FDD: 5-10 W
- Adapter card: 5-30 W
