Why is a power supply dangerous even after the power is disconnected?

Answer 1

Power supplies have large capacitors in them. These capacitors store electricity in them even when mains power is shut off. If you were to touch the connections on the bottom of the capacitors, you could receive a potentially lethal shock. For this reason, you should never disassemble a power supply unless you have received training in how to properly discharge the capacitors.

More answers

Because of the big electric charge held within the capacitors inside it.

Computer power supplies have very large capacitors (elements that store charge) that could give you a very powerful shock if you managed to discharge them. They're like batteries that let go of all their charge at once.

Capacitors store energy (power) for an indefinite time. Touching both leads of any charged capacitor can give you a real good jolt.

For example, the tube of an old-fashioned television set is a giant capacitor in electronic terms: even though a TV may have just been switched off and unplugged, there may still be about 40,000 volts stored up in its cathode ray tube (CRT) that will cause great harm unless proper precautions are taken to discharge it safely.

Answer 2

Because power supplies have several large capacitors. A capacitor's job is to store energy. So, even when the power supply is unplugged, the capacitors carry a stored charge that could shock you.