People seem to get confused as to why there is a power switch on the computer (which they always use) and sometimes another on the power supply itself (which they never use).
The power switch directly on the power supply itself provides the ability to cut electrical output to all physically attached components. This is useful, for example, if the COMPUTER power switch (typically on the front of the computer) has been damaged or is otherwise not functioning. It is a directly connected mechanical switch. It is also used during the replacement and building of computers.
DO NOT simply turn off the computer and then attempt to replace the power supply. The proper way to do this is:
1. Use switch on the current power supply itself and put it in the “off” position.
2. Unplug the current power supply from the wall socket.
3. Press the power switch on the front of the computer twice (as a safety precaution).
4. Take note of all electrical connections, locations, and plug types.
5. Remove the current power supply from all electrical connections and physically remove from computer.
6. Prepare your new power supply for installation by placing the switch in “off” position.
7. Install power supply and connect to all electrical connections as noted in step 4.
8. Plug new power supply to wall socket.
9. Change switch position of new power supply to “on”.
10. Use the switch on the front of the computer to power on.