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pc power supply


The power supply converts the alternating current (AC) from your mains (110V input or 220V input) to the direct current (DC) needed by the computer 2. In a 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. Power supplies – often referred to as switching power supplies, use switcher technology to convert the AC input to lower DC voltages. The typical voltages produced are:

  • 3.3 volts
  • 5 volts
  • 12 volts

pc power supplypc power supply 2

Figures: left: A Typical ATX 1.3 power supply. From left to right, the connectors are 20-pin motherboard 3, 4-pin “P4 connector”, fan RPM monitor (note the lack of a power wire), SATA power connector (black), “Molex connector”, and floppy connector; middle and right: shows the power supply with 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.

pc power supplyFigure: A power supply schematic showing the AC input, fan, EMI filter (yellow), transformers, cylindrical capacitors, filters, aluminum heat sink, rectifying diodes, and 3 outgoing DC voltage lines. (White, 2008)

The 3.3-volts and 5-volts are typically used by digital circuits, while the 12-volt is used to power fans and motors in disk drives. The main specification of a power supply is in watts. A watt is the product of the voltage in volts and the current in amperes or amps.

The form factor of the power supply refers to its general shape and dimensions. The form factor of the power supply must match that of the case that it is supposed to go into, and the motherboard it is to power.

Power Supply Wattage:

A 400-watt switching power supply will not necessarily use more power than a 250 watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices since the supply should not be loaded to 100 percent of its capacity.

According to PC Power & Cooling, Inc., some power consumption values (in watts) for common items in a personal computer are:

Component Requirement
AGP Video Card 30W – 50 W
PCI Express Video 100W – 250W
Average PCI Card 5W – 10W
DVD/CD 20W – 30W
Hard Drive 15W – 30W
Case/CPU Fans 3W (ea.)
Motherboard (w/o CPU or RAM) 50W – 150W
RAM 15W per 1GB
Processor 80W – 140W

If you use a PSU that does not supply enough power for the system, any of the following symptoms might occur:

  1. The system does not boot,
  2. System randomly shuts down,
  3. Add-in devices do not work properly. Intel (2014a)

For overall power supply wattage, add the requirement for each device in your system, then multiply by 1.5. The multiplier takes into account that today’s systems draw disproportionally on the +12V output. Furthermore, power supplies are more efficient and reliable when loaded to 30% – 70% of maximum capacity.

HEAT SINKS, FANS Motherboard

This post contains the content of book Computer Hardware_ Hardware Components and Internal PC Connection


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