The power supply unit (PSU) converts alternating current (AC) from the wall outlet into the low-voltage direct current (DC) that computer components use. A weak or failing PSU causes random shutdowns, reboots and failures to start, and those symptoms are easy to blame on other parts, so choosing and checking the right PSU is an important skill. Never open a PSU: its capacitors can hold a dangerous charge even when unplugged. Failed units are replaced, not repaired.
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