How to replace a desktop PC power supply unit (PSU) – tutorial with pictures
By John Pospisil
Want to replace your desktop PC computer’s power supply unit (PSU)? In this step-by-step guide we show you what’s involved.
Not so long ago, when a PC failed, most people just went out and bought a new one. But in these austere times, people are thinking twice before throwing out an older computer that might be easy to fix.
Replacing your PC’s power supply unit is one of the easiest and cheapest repairs that you can do. As long as you’re handy with a screw driver and you don’t mind opening your computer, you should be able to carry out this repair.
Safety warning
As with any electrical device, you need be very careful when opening up your PC. You are dealing with electricity which kills. The number of people who get a nasty shock because the computer is still plugged in is legion.
Make sure you disconnect your PC from the mains electricity supply before you start, and as an added precaution, stop and check that the electrical socket where the computer had been plugged in is empty. Please, please, please make absolutely sure that your computer is not plugged into the mains electricity.
And as another safety tip, use a plastic handled screw driver.
As an added precaution, you might also consider using a grounding strap, also sometimes called an anti-static wrist strap. A grounding strap can help to prevent damage to your computer due to electrostatic discharge (ESD) by safely grounding you while you work on your computer.
How do you know whether your PC power needs to be replaced
I’m assuming that your knowledge of electronics is fairly basic, so that the diagnostic options available to you are fairly limited. Here are some indications that your power supply has failed, or may be on its way to failing.
- There is no power going to your computer or to any of its internal peripherals
- There is a burning smell coming from the power supply
- It’s difficult to switch on your computer, by which I mean it takes several pushes of the on/off button for the computer to boot.
Of course, if you’re more electronically minded, you could use a multimeter to test the current coming from the power supply unit (but please don’t try to do this unless you know what you’re doing, and can do so safely).
Before you go down the path of replacing your PC’s power supply, it’s also worth making sure that the wall socket is working by plugging something into it and turning that something on. You should make sure the power strip (power board) is working the same way (if one is in use). You should also try using a different power cord. These are pretty common. If you don’t have a spare power cord to hand, try the one for the monitor, which is probably the same.
If you don’t know for sure that your power supply has failed, you may find that you replace your power supply, and your PC still doesn’t work. In this case, you need to consider whether it’s worth the risk of spending the US$30 or so it will cost to buy a new power supply and possibly find that there’s another problem with your computer.
Before you start
Make sure you’ve got the right replacement power supply. These can be readily purchased from computer parts shops.
To check what kind of power supply you need to get, look at the sticker on the existing power supply (you’ll need to open your computer to do this). In my case it read “A350w ATX P4 power supply” — this meant I needed an ATX power supply unit.
There are three types of power supply in common use:
- AT – used in older PCs
- ATX – used in more recent PCs
- ATX V2 (also known as ATX12V) – a newer type of power supply designed for the power requirements of modern PCs. Generally speaking, you should be able use an ATX V2 PSU with an older computer that had an ATX PSU. ATX V2 has a 24 pin connector going to the motherboard, while ATX has a 20 pin connector, but as discussed below, you should be able to work around this.
You’ll find a very useful article about ATX and ATX V2 power supplies here.
If there’s no sticker visible on the power supply, find out what kind of motherboard you’ve got, and use Google to find out what kind of power supply it works with.
Alternatively, remove the power supply from the machine (following the instructions below) and take it with you when you go to buy a replacement power supply.
Make sure that the new power supply has at least the same number of watts as the old power supply.
How to replace a PC power supply
- Turn off your computer (if the PSU hasn’t failed completely, and it happens to be powered up) and unplug the power lead from the mains.
- Remove the cover or access panel – this will depend on your computer case. With my PC, I was able to remove a side access panel, but you may need to remove screws and take off the whole cover. You might find it easier to work on your computer if it’s laying on its side.
- The power supply is usually located at the top, back of the case. It’ll usually be secured by four or so screws. The mains power lead plugs into back of the power supply (the side that backs onto the rear of the computer case). On the front of the power supply (the side that faces the inside of the computer) you’ll see a number of cables leading to the motherboard and DVD drives, hard disks etc.
Gently pull out these leads from their sockets. You might need to use your thumb to release the catch that secures the connectors to the motherboard. Take note of which lead goes where. While it’s hard to plug connections into the wrong sockets, you’ll find it easier to put your computer back together if you note what goes where.
One colleague suggested taking a digital photo of where the cables go (while everything is still in place), while another uses sticky labels on both the sockets and cables to help him remember what went where. He puts one label on the socket from which he is removing a cable and marks it ‘A’, and then wraps a label around the cable he is removing and marks that ‘A’ as well.
And another colleague suggested drawing a diagram of which lead goes where. Obvious, use whatever system will work best for you.
- Now we need to remove the power supply. In my case this was down by removing the four screws that secured the power supply to the chassis of the computer, and then sliding it out of its position.
- Take the new power supply, put it in place, and tighten the screws that keep it in place.
- Attach the two motherboard cables to the motherboard. If you have an older computer, and you’ve bought an ATX 2 power supply, you may find that the main motherboard connector has four extra pins (see picture, above). You should be able to detach these extra pins so that the plug fits, or if this is not the case, hope that there is enough clearance to right of the socket on the motherboard to accommodate the extra pins.
- Reattach the power leads for the hard disks and optical drives, and any other component that draws power.
- Replace the cover.
- Plug in the power lead to the back of the machine, and switch on your computer.
All going well, your PC should come to life. if it doesn’t, disconnect the power lead from the mains, and open up your computer. Check all the connections, especially the leads running to the motherboard, and try again.
If you’re confident that all the connections are correct, it may be that there is another fault that is stopping your PC from working.
Disclaimer: While due care has been used in preparing this tutorial, BLORGE disclaims all responsibility for any harm or loss arising from use or otherwise of the information provided. You should not rely nor act upon any of the information contained on this page without obtaining professional advice.
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