Power Supply The basic function of power supply is to convert the AC supplies available at wall socket, that which is unusable by the computer. The SMPS in a conventional desktop system is designed to convert the 220V AC, 50 HZ into +5V, -5V, +12V, -12V and +3.3 V DC. Types of Power Supply Linear Power Supply 2. SMPS Power Supply Linear Power Supply   In Linear power supply the input AC voltage is first stepped down through step down transformer, approximately to a required voltage and then applied to a bridge rectifier diodes. The regulation is achieved by sending the DC through series pass Transistor and changing the current flowing through it, by an error voltage obtained from a comparator (sensing) circuit. Or a regulator IC output is applied to the base of series pass transistor that determines the amount of current that has to flow through it to keep the output voltage constant. The excess current flowing the series pass transistor is converted to heat to achieve regulation. A lot of power is wasted in the form of heat.

Power Supply Power Supply In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan. This is a power supply removed from its PC case. The small, red switch at right, above the power-cord connector, is for changing line voltages in various countries.

COMPARISON OF LINEAR AND SMPS POWER

Block Diagram of SMPS

Power Supply In this photo you can see three small transformers (yellow) in the center. To the left are two cylindrical capacitors. The large finned pieces of aluminum are heat sinks. The left heat sink has transistors attached to it. These are the transistors in charge of doing the switching -- they provide high-frequency power to the transformers. Attached to the right heat sink are diodes that rectify AC signals and turn them into DC signals.

The interior of a power supply

Power Good Signal Power Good Signal   The power supply sends Power Good signal to the motherboard. If this signal is not present, the Computer does not run. The effect of this setup is that when the AC voltage dips and power supply becomes over-stressed or overloaded, the Power Good Signal goes down and forces a system reset or complete shutdown.

AT Motherboard connector details

Power Supply A PC power supply removed from its PC case. Cables and connectors at right supply DC voltages

Power Supply Personal computer power supply label. VSB is the standby voltage provided to the power switch.

Power Supply