How to check if your Power Supply Unit is working

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How to check if your Power Supply Unit is working

Post #1 by huddy » Thu Jun 27, 2013 4:11 pm

This guide gives you a basic understanding of your PSU and how you check for basic faults.

Your expensive components rely on one common factor; power. Without a clean, reliable and sustainable power source your system will become unreliable, unstable or may not work at all. So it’s important that to know that when thing go wrong, 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!

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 has 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 on-board 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. I suggest twice as much. Not only does this give you upgrade potential but it makes the PSU run far more efficiently because the load is less. Running a PSU at near maximum load isn't a good idea.

PSU Calculators:

http://www.extreme.outervision.com/psuc ... orlite.jsp

PSUs conform to the ATX form factor and there 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.

Is there any advantage of a multiple rail PSU over a single rail?

The demands of high powered device puts extra burden on single rail PSU meaning the cables could potentially overheat or collapse under heavy load . As such, each rail has a specified load limit.. a bit like a circuit breaker. Multiple rails are an advantage because each line can guarantee a respectful current without such risk but generally this is much lower than a single rail. However, all multiple rails are connected to the same 12v source but are independent in terms of the cabling thus avoiding any potential overheating and damage.

Nvidia and ATI insist that each PCI-e has its own rail.. Hence PSUs badged with SLI or Crossfire certification but it doesn't make a real difference.
The “BeQuiet Pure Power L8 530W '80 Plus Bronze' Modular Power Supply” has two 12v mutil-rails. The PSU is this case has one 28amp rail (+12V1) and one 20amp rail (+12V2) which is more than adequate for most systems.

Always make sure your PSU is at least 80% efficient (the amount of power it draws from the mains to achieve its rating) and a trust brand. So as long as the PSU meets all your system power requirements with at least 20% capacity to spare then you shouldn’t run into too many issues.

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 = Ground

Please read my PSU cables and connectors section for further 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.

The check list

1. Before you start

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 fuse’s as well.

2. Visual Check.

Most PSUs come with fans to prevent them overheating. If the PSU has a fan that isn't spinning, then chances are that the PSU is dead. Bear in mind energy efficient PSUs are passively cooled until the power draw requires the circuits to be cooled. Do not attempt to replace a faulty PSU fan.

3. Check the motherboard Power LED.

Most modern motherboards have an LED that indicates that power is reaching the motherboard. If this is off, then it's a good indication that NO power is being received rather than inadequate power. Check your power connection as below

4. Check your Power Connections

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.

Firstly, check that you fitted the 20/24-pin ATX connector to the motherboard and it's securely in place.

Secondly, check if your motherboard requires a supplementary CPU power supply. Depending on the motherboard, this will either be a 4-pin ATX12V connection or an 8-pin EPS12V connection supplying 2x12v and 4x12v rails respectively.

Make sure your ATX12v or EPS12v is connected and secure. If your PSU doesn't have the right connector, you can purchase Molex to 4/8pin adaptors. It's also possible to connect a 4-pin ATX12v connector to pin one of an 8-pin EPS12v Socket (check motherboard manual). However, this doesn't guarantee stability especially when overclocking and not something I'd recommend..

In either case, if your main ATX and/or supplementary CPU power connectors are not fitted or not fitted correctly, your system will not boot.

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 for supplying the required power 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

The PSU can only normally be started when it receives a signal from the chassis power switch via the motherboard when you physically switch on the PC. However, we are going to switch on the PSU in isolation, without it being connected to the motherboard. We can do this by “hotwiring” or bridging the signal cable and any ground cable on the PSUs ATX connector.

By attaching a known working fan to the PSU we can soon tell if the motherboard or the front panel switch is the reason for your system not starting, not the PSU. We can also use this method to check for the PSUs voltage.

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 that has a green cable going. Thisis the signal cable and 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 PSUs Molex or fan 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 or faulty power switch 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 (10% for the -12v).
Here is a list of the ranges for each rail.

Minimum and maximum values

3.3v minimum of 3.14v, maximum of 3.47v
5v minimum of 4.75v, maximum of 5.25v
+12v minimum of 11.4v, maximum of 12.6v
-12v minimum of -10.8v, maximum of -13.2v

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

There are many ways to check the voltages:

My preferred method is to use a multi-meter which is a simple, effective and accurate method of checking the supply of power. I have described this in a separate 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 elsewhere. 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 you’re in the habit of building your own computers, it not such a bad idea to keep one floating around anyway. Very handy.

If you have any questions or wish to discuss then please join our forum - all are welcome

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