Power Supply Types | CompTIA A+ 220-1001 | 3.7

In this video you will learn about power supplies & their features.

CAUTION: Before you begin working on any type of electronic device, always disconnect the device from its power source. The next thing you need to do is discharge any capacitors before working on the device because capacitors can store electric charges that could shock or electrocute you. Also, be sure to properly ground your body by way of a ESD (electrostatic discharge) strap before messing around with any type of electronic device.

Power Supply

A power supply unit (PSU) converts mains alternating current (AC) to low-voltage regulated direct current (DC) power for the internal components of a computer.  AC power comes out of the walls and is then converted into DC to be supply power to desktops and laptops. Modern personal computers universally use switched-mode power supplies.  Some power supplies have a manual switch for selecting input voltage, while others automatically adapt to the mains voltage.

Power Supply

115V vs. 220V

The majority of power supplies are dual voltage power supplies that deal with two ranges of voltages:

  • 115-120V/60Hz (North American standard)
  • 220-240V/50Hz (European & Asian standard)

Power Supply Ratings

Modern tower case (upright case) computers typically have power supplies of 500 watts or more which enables these computers to support a greater number of drives and cards that can be installed. Smaller desktop computers have ratings of around 220 to 330 watts. The power supply rating on a power supply can be found on the power supply outlining information pertaining to safety and amperage levels produced by the power supply’s different DC outputs.

Power Supply Rating

You may come across older power supplies that have a voltage switch indicating as to whether your are using 115V (North American 110-120V/60Hz AC) or 230V (European & Asian 220-240V/50Hz AC). If a power supply is set to the wrong input voltage, the system will not work or at worse, it could fry your system and render it useless. Most modern power supplies can automatically determine voltage levels & cycle rates without the need for manual switching. This is referred to as autoswitching.

Power Supply Voltage Switch

24-Pin Motherboard Adapter

Motherboard form factors ATX, microATX, & Mini-ATX utilize the 24-pin power supply connector, although some legacy 20-pin connectors are still in existence used by other older ATX-family motherboards.

24-Pin Connector

Standard power supply wires are color-coded thus:

  • Red:  +5V
  • Yellow:  +12V
  • Orange:  +3.3V
  • Black:  Ground (earth)
  • Purple:  +5V (standby)
  • Green:  PS-On
  • Gray:  Power good
  • White:  No connection (24-pin); -5V (20-pin)
  • Blue:  -12V

Wattage Rating

By definition, a watt is the standard unit of power in the International System of Units (SI), equivalent to one joule per second and equal to the power in a circuit in which a current of one ampere (amp) flows across a potential difference of one volt. In layman’s terms: in electrical systems, a watt is a power measure of the rate at which energy flows.

Another way to visualize this is to think of water flowing through a pipe:

  • An ampere (amp, A) is how many electrons are flowing down a wire (how much water is flowing through a pipe, aka the current).
  • A volt (V) is the pressure being exerted on the flow of electrons as they move down the wire (How wide open is the faucet that is allowing water to flow? Is it allowing only a drip or is the water rushing out?)

The formula for calculating watts is: watt = amp * volt (amp multiplied by volt).

Current-Flow Visual

To calculate the wattage rating you would need for a replacement power supply, simply add up all of the wattage ratings for everything you need to have connected to your power supply (motherboard, memory, CPU, cards, bus-powered USB devices, drivers, & any external devices you may use). If the total wattage you will need exceeds 70% of your current wattage rating for your old power supply, then upgrade to a larger power supply; be sure to check the vendor specification sheets for wattage ratings. If you have amperage ratings instead of wattage ratings, multiply the amperage by the volts to determine wattage and then start adding. If you have a device that uses 2 or more different voltage levels, calculate each voltage level as well & add up the numbers to figure out the appropriate wattage requirements.

Number of Devices/Types of Devices to Be Powered

The power supply also powers various peripherals, such as:

  • Hard disks and optical drives
  • Case fans that do not plug into the motherboard that use 4-pin Molex power connectors
  • SATA L-shaped 15-pin power connectors for hard disks
  • 6/8-pin PCIe power cable for high-performance PCIe x16 video cards that utilize 12V power
Power Supply Connectors