As a general word of warning and best practice, always disconnect from the power source when working inside of a device. Also note that some devices store a charge in capacitors and this charge can cause an electric shock. You should know how to discharge these capacitors before touching them.
When working with a power supply, do not wear an ESD wrist strap. You don’t want to ground yourself because you don’t want to make your body a path for electricity to take to ground. Wearing an antistatic wristband, standing on a grounding mat, or touching something already grounded will put you at risk.
Electricity is the flow of electrons (current) through a conductor such as a wire. Your computer or laptop uses DC (Direct Current) voltage or current. However, the power coming out of your wall is most likely AC (Alternating Current) voltage. In DC, the negative electrons flow in one direction around the closed loop of an electrical circuit. In AC, the flow of electrons around the electrical circuit reverses periodically and has alternating positive and negative values.
A volt (V) is the unit of measurement for the pressure of electrons on a wire. Computer power supplies convert 115V AC or 220V AC into either 3.3V DC, 5.5V DC, or 12V DC.
Electricity flowing through a wire is likened to water flowing through a hose. An ampere (A or amp) is the rate of electron flow past a point in one second. This is like the diameter of a hose through which the water is flowing through.
The voltage (V or volt) is the electrical “pressure” pushing the electrons through the wire. This is like how open the faucet is to allow the water to flow.
Watts (W) is the measurement of real power use. Watts is equal to volts multiplied by amps. (Volts * amps = watts). Going back to the water comparison, watts would be like the total amount of water flowing through the hose, which is equal to the combination of the pressure of the water (the volts) multiplied by the diameter of the hose (the amps).
The symbol for Alternating Current (AC) looks like ~. With AC, the direction of the current constantly reverses as it flows through the electrical lines. AC allows for efficient distribution of electricity over long distances.
Direct current (DC) is often portrayed with the symbol of a line with dashes underneath it. With DC, current moves in one direction with a constant voltage.
Input 115V vs. 220V
The frequency of the direction change of AC current is important and differs throughout the world. In the United States and Canada, AC power runs at 110 to 120 volts with a frequency of change is 60 hertz (Hz) or cycles per second. In Europe, AC power runs at 220-240V with a frequency of change of 50 Hz.
Your power supply may come with switch that allows you to manually switch between 110 V / 115 V and 220 V / 230 V of power input depending on what type of power you’re connecting to. If you’re not sure of what type of power you’re connecting to, you can connect a multimeter to the unknown wall outlets to see what the voltage is. Be sure to set the manual input voltage switch on the power supply before plugging the power supply into the power source.
Many power supplies are auto-switching meaning that they can automatically determine what the input voltage is and will adjust itself accordingly without having someone manually flip a switch to set the voltage input.
If you connect a power supply to a power input that doesn’t provide enough voltage, then usually no permanent damage will be caused. However, do not plug a 115V power supply into a 230 V power source. Doing so can seriously damage the power supply and other components in the computer.
Output 5V vs. 12V
The power supply commonly provides +/-3.3 V, +/- 5 V, and +/- 12 V of DC. There are different voltages depending on the needs of the different components in your computer. Voltages can be positive or negative. Voltage is a difference in potential. The electrical ground is a common reference point so a positive or negative voltage depends on where you are measuring from. For example, if you’re standing on the 1st floor of your house, the second floor would be + 10 feet, and the basement would be -10 feet.
+12 V is provided to larger components on the motherboard such as PCIe adapters, hard drive motors, cooling fans, and most modern components.
+5 V can also be provided to motherboard components. However these components are seeing a shift to using 3.3 V instead.
+3.3V is provided to components like M.2 slots, RAM slots, and motherboard logic circuits.
-12 V is not seen in modern motherboards. But older motherboards would use -12 V for an integrated LAN, older serial ports, and some PCI cards.
Modern power supplies don’t provide -5 V, but some older power supplies did. The -5V would be used for older ISA adapter cards. Since modern motherboards don’t have ISA slots, -5V is no longer needed.
24-pin motherboard adapter
Inside of your computer, the power supply will be providing different voltages to components on a motherboard. The primary motherboard connector is a 24 pin connector.
Older motherboards used a 20 pin connector which was the original ATX standard. An additional 4 pins were added for PCI Express power. You can connect a 24 pin connector to a 20 pin motherboard. Some power connectors even come with cables that allow you to disconnect the 4 pins to fit a 20 pin motherboard.
The individual pins on a motherboard connector are keyed. So some pins are squared and some are rounded so that there is only one possible way to plug in the power connector.
Most power connectors will include a locking mechanism. To remove the power connection you’ll have to push down on the locking mechanism to release the connector.
Wattage rating
Power supplies are rated by watts. The power supply will specify the overall watts and watts for individual voltages.
When selecting a power supply, you need to select one that matches the type of computer it’s going in. You don’t necessarily need to buy the biggest one because that will typically be the most expensive, and the extra power wont do anything to increase the performance of your computer.
Number of devices/types of devices to be powered
To select a power supply you want to calculate the watts required for all components. Look at the CPU, storage devices, video adapters, etc. to see how many watts of power they require, and total all the watts required. You’ll want a power supply that exceeds your needs so that you’ll have room to grow in the future.
Video adapters usually use the most amount of power. The video card manufacturer will most likely provide a list of recommended power supply wattage for that adapter.
When purchasing or replacing a power supply, you’ll also need to make note of what form factor the power supply needs to be in. Form factors include ATX, Flex ATX, etc. You’ll need the correct power supply form factor to fit the case.
Power supply connectors include the motherboard 24 pin as described earlier. You may also have SATA connectors and 6/8 pin PCI-Express connectors.
