Most desktop personal computer power supplies are a square metal box, and have a large bundle of wires emerging from one end. Opposite the wire bundle is the back face of the power supply, with an air vent and an IEC 60320 C14 connector to supply AC power. There may be a power switch and/or a voltage selector switch. Historically they were mounted on the upper part of the computer case, and had two fans: one, inside the case, pulling air towards the power supply, and another, extracting air from the power supply to the outside. Many power supplies have a single large fan inside the case, and are mounted on the bottom part of the case. The fan may be always on, or turn on and vary its speed depending on the load. Some have no fans, and so are cooled completely passively.