How to check Your PSU is working 

Your expensive components are reliable on one common source, the power they receive. So it’s important that your power supply is working and functioning as expected.   This guide gives you a basic understanding of your PSu and how you check for basic faults.

Safety first!

What does the PSU actually do?

Cables and connectors

How do you know if it's a failed PSU?

Things to check

Conclusion

Safety first!

Firstly, if you suspect or find your PSU is faulty then don't try and attempt to fix it yourself. You need specialised equipment to ensure that the PSU ahs discharged all stored electricity before handling the inside. It's safer, easier and cheap enough to buy a new one. If your PSU is still under warrantee, then chances are that you'll get a free replacement.

What does the PSU actually do?

In basic terms, a PSU regulates and delivers power to the system and its components. It’s responsible for converting incoming AC (Alternating Current) in to three DC (Direct Current) voltage supply lines. These three lines are better known as “rails”.

The three rails supply 12volt, 5volt and 3.3volt to the system and are responsible for providing suitable power requirements for specific components. The 12volt rail, for example, supplies power to the motherboard, graphics card, CD and HDD drives etc. whereas the 5volt and 3.3volt rails supply power to other onboard devices.

Power supplies are often rated by the amount of power output in Watts and these range anything from 250w to a whopping 1100watts (even more for servers).  The wattage is calculated by multiplying the supplying voltage by the amperage of the component. So, you have to know what each component draws in amps and the voltage of the line it’s on. For example, a motherboard that takes 5AMPS on a 5Volt line requires 25W. By adding all the components up, you’ll get any idea of how much power you’ll need. So basically, the more components you have in your system the bigger PSU in Watts you need.

Thankfully, you don’t need to be a mathematician to work this out. There are plenty of PSU calculators around to help determine what PSU is required to power your equipment or if your existing PSU is up to the job it's required (links below maybe useful). However, the general rule of thumb is to always have more than you need. This is far more efficient as the PSU won’t be running at full load. This not only prolongs its life but is its quite handy having the extra headroom for any future upgrades too. 

PSU Calculators

http://www.extreme.outervision.com/psucalculatorlite.jsp

http://web.aanet.com.au/SnooP/psucalc2.php

PSUs conform to the ATX form factor and their have been many variants of throughout which is something to bear in mind when upgrading components, for example.   

The biggest change that was introduced with the ATX PSU was the ability for the computer to be turned on and off via software. This is to say, that the PSU is not directly connected to the case power button but via the motherboard instead. Therefore, power is constantly being provided to the motherboard and most motherboards will show a light to indicate this.

Cables and connectors

There are lots of cables and connectors on the PSU which you will need to familiarise yourself with before attempting any PSU diagnostic checks.

However, for the purpose of this section, you will need to know they following colour coding:

Yellow = +12v

Red = 5v

Black is ground.

Please read my PSU cables and connectors section for futher details.

So how do you know if it’s a failed PSU?

 Not all problems are related to a failed or faulty PSU, but it’s best to eliminate the PSU when making your diagnosis.

Symptoms of a failed PSU include:

- No power to the System

- Periodically turns off

- Random re-boots meaning that the PSU is not providing enough power to keep it going.

- Noisy fan or failure. Some motherboards have a PSU fan detector and if the PSU is connected to this, the system will detect a PSU fan failure.

Things to check

1. Obvious checks...

It sounds obvious but the first check is to make sure that the PSU is plugged in at the wall and switched on. Don’t laugh, it does happen. Make sure the switch on the back of the PSU is also on.  You might want to check any fusses as well.

2. Check the PSU fan...

Most PSU fans come with fans to prevent them over heating. If the PSU has a fan that isn't spinning, then chances are that it's burnt out.  Do not attempt to replace a faulty PSU fan.

3. Check the motherboard...

Most modern motherboards have an LED that indicates that power is reaching the motherboard. Check that this is on.  If it’s not on, then check that the 20/24 pin connector is securely in its place on the motherboard. If you have an 8pin EPS12v connector on your motherboard, make sure the 8pin cable is also securely connected.

4. Fasten everything

Your next step is to check that all your power leads are securely fastened and are where they should be. Check the physical condition of the cables for anything obvious like a broken cable.

5. Can it cut it?

If you’ve been upgrading your system and neglecting the PSU, it could be that the existing PSU isn’t up for the job. Check that PSU is suitable and supplying the required wattage as described earlier. Use the PSU calculator (links above) to check this. If they are not up to the job then you might have to invest in a more powerful PSU.

6. "Hotwire" Test

Don't panic, this isn't anything illegal. When your PSU is attached to the motherboard by the 20/24 pin ATX cable, the PSU will only come on when it receives a signal to do so from the power on switch on the case control panel. However, this can be easily overridden using the hotwire trick. This basically tricks the PSU in thinking that the power switch has been pressed.

The test here is to start up the PSU without the motherboard or power switch so that power is distributed along the rails as soon as the PSU is switched on. By attaching a known working fan to one of the Molex connectors we can soon tell if the PSU is working or it's a problem with the motherboard. Here's how:

- Firstly, make sure the PSU is switched off and mains disconnected.

- Disconnect all power leads to all devices including the motherboard ATX connectors.

- Now take a small piece of old cable and strip the ends.

- Take the 20/24pin ATX connector and look for the pin the has a green cable going. There's only one.

- Place one end of the cable into the green pin and the other end into any black pin, as shown below. Make sure they are securely in place.

- Attach a known working fan to one of the Molex connectors

- Plug the PSU back in and switch on. The PSU should start and the fan should start spinning immediately.

If the fan doesn't spin then it would appear to be a faulty PSU. If it does work however, then it could be a faulty motherboard but don't discount the PSU yet. Switch off the PSU and leave the PSU "Hotwired" for the time being and read on for further checks..

7. Checking the voltages

The next thing is to check that the PSU is supplying the correct voltages. Of course, it’s unrealistic to expect your PSU to be supplying spot on voltages as described earlier. They key is to have stable voltages within an allowable tolerance. So, you will have to allow for some deviation of around 5% either side of the expect rails. So for 5volts you need to allow between +4.8volts and +5.3volts and for 12volts between 11.4volts and +12.6volts.

Anything outside this range is certain to be a problem with the power supply.

There are many ways to check the rails:

My preferred method is to use a multi-meter. In some cases, you may not have a choice but generally using a multi-meter is simple , effective and accurate method of supplying you the information you need.  I have described this in a seperate section - Using a multi meter to test your PSU. Give this a read.

If you are able to power on the system but having stability problems, then one option is to use monitoring tools which show the voltages being supplied as other information including system temperatures.

The easiest method to access this information is through the system BIOS. Depending on the BIOS installed, there will be a menu displaying the voltages. It’s pretty basic and there are no legends of history, so you’ll have to keep a sharp eye on what going on.

Alternatively, there are plenty of software utilities which can read the same sensor information but instead can process the information in a presentable GUI. Most motherboards, such as the ASUS, come with their own utilities (Asus Probe) for diagnosing system stability and are excellent for showing the history of the rails across a period of time. You’ll soon be able to tell if the PSU is supplying incorrect voltages.

If however, your motherboard manufacturer does not provide such a tool, there are plenty of third party solutions such as Motherboard monitor, HW Monitor, Everest, NestSensor etc to name a few. These are all capable of showing the stability of your PSU.

8. Strip down the bare essentials

If you’re down to this step, then everything is working as expected; you’ve checked the cables, the connections, the voltages etc and still the problem hasn’t been determined. It’s looking more likely that it isn’t a PSU problem but you need to completely eliminate it first before looking else where. It’s time to take some dramatic steps.

Firstly, you can take out non critical devices such as sound cards, TV cards, NICs, CD/drives, floppy drive, USB devices, non essential fans etc. If you have more than one physical HDD, then detach these leaving only the boot HDD. Everything you can get away with basically. Then test.

If everything works fine, and works fine for a while under load, then it looks like one of those devices you’ve just removed is the route of your problem.

Add the items back into the system one at a time and test until all items are back. If it becomes unstable on re-instating one particular component, then you may have found your culprit or the PSU isn’t supplying the required wattage but you checked this earlier, didn’t you?  

9. Swap out

The very last thing you can try to completely eliminate your PSU from the equation, is to swap out the PSU with a known working one. Of course this means having a spare PSU hanging around which is a luxury for most, but if your in the habit of building your own computers, it not such a bad idea to keep one floating around anyway. Very handy.

Conclusion

When a PC just dies and there is no power to the machine, then the obvious culprit is the PSU. However, less obvious are faulty PSUs which cause all sorts of instability issues and the PSU is quite often overlooked as the source of problem. In time the efficiency of a PSU degrades until one day, you just get all sorts of problems. However, if you have followed my guide above then you should have a good idea if your PSU is faulty and the root of your problems.