How To Replace Power Supply Unit On A Desktop Computer

If your PSU has failed and does not switch on anymore or you have decided to make your desktop a gaming rig, you will need to replace the power supply unit.

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The computer power supply unit is all about power. Desktop computers that are used at home or in the office frequently have 300W PSUs.
Gaming desktops may have power supplies that can range from 500W to 1500W depending on a gaming card installed. In a case when the computer has two graphics cards bridged then it should have a power supply of over 1000W.

How to buy the right power supply. Today, modern power supplies are almost all the same with some exceptions. The difference will be in the cable that goes to the motherboard (it usually has 24 pins).

You need to check how many pins it has.
Power wires that go to the hard drive and DVD drive.

You need to have the same type in your new power supply. The more of those power wires the PSU has, the better.

Also, you need to check one wire that sends additional power to the motherboard. Usually it has 4 pins, but sometimes it can have 6 pins.

This additional wire is usually present in ATX power supplies (4 pin ATX cable). In older AT power supplies this power cable has 2 pins or is sometimes missing and the big flat connector has not 24 pins but 20.

Taking the power supply out. We need to disconnect it from literally everything. Some wires will go to the hard drive and DVD drive. Some wires will go to the motherboard (usually one long flat and another with 4-6 pins). If you have a gaming graphics card one wire will go to it and will have 6 or 8 pins. On ancient computers one power wire will go to a floppy disk.

After disconnecting all the wires we will need to unscrew a few screws with a Philips screwdriver (or with a flat screwdriver depending on what is there) that are located at the back.

Fitting in the new power supply. The fitting is basically the same as the previous steps when we disconnected it. Though, sometimes you will find out that the new PSU does not have Molex or required PCI-e power connectors with 8 pins. In this case you will need to use adapters.

Connectors on a modern PSU:

1x 20+4 Pin Connector. This is the connector that supplies the motherboard with all the required voltages.
1 x 6 Pin PCI-e Connector. A connector to power a gaming graphics card.
1 x 6+2 Pin PCI-e Connector. Additional power for a second graphics card.
1x 8 Pin ATX (4+4) Connector. This provides additional power for the motherboard. Usually this goes to the processor’s voltage regulator.
3 x SATA Connectors. Modern power connectors for devices like hard drive or DVD.
4 x Molex Connectors. Old-type connectors for devices like hard drive and DVD.

PSU Specifications

AC INPUT: 230 VAC 10A 50-60Hz
DC +3.3V +5V +12V1 -12V +5VSB. This is a standard for all ATX PSU.
Output  24A 30A 32A 0.5A 3.0A. This is also standard but more powerful power supplies have these higher numbers.
Max 750W. Maximum power that it can produce. Although peak power or momentum power can be higher, as constant power output it can produce 750W.

Detailed Power Supply Unit Specifications

Dimensions: 150mm x 85mm x 140mm (W x H x D). This is a standard for all ATX PSU.
Input Voltage: 230 VAC. For Europe it is 230V, for America it is 110V.
Input Current: 10A
Input Frequency Range: 50-60Hz
PFC: Yes - Power Factor Control
Power Good Signal: 150-450 ms. This tells the motherboard that the power output is good.
Hold Up Time: 1ms-40ms
Efficiency: 75%. The bigger this number is, the better. 75% is acceptable.
MTBF: 100,000 Hours. This is the minimum time before a failure.
Protection: OVP / UVP/ OPP. Over voltage protection, under voltage protection, over power protection.

OCP – over current protection.
OTP – over temperature protection.
SCP – short circuit protection. This prevents the power supply from starting if there is something that gives a short circuit. This can be a processor that failed and has impedance of 0.2 or less ohm and the power supply will sense this and will not start. You may hear a clicking noise, a fan will spin a little but nothing will happen further.

Power: 750W
Operation Temperature: 45°C. Maximum temperature of the environment.
Safety: CE / ROHS. This basically tells us that it does not have any harmful substances.
Cooling: 12cm Blue Fan
Noise: < 17 dBA. The lower this number is, the better. It is more pleasant to work next to a quiet computer.

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