testing your psu with a multimeter















I've noticed there have been a lot of questions regarding Power Supplies lately. There have been questions about the voltage being up to par and should the PSU be replaced. The problem with some of this is that the software on the motherboard doesn't usually give the most accurate of results. In order for the software to read your PSU's rails, or lines of voltage required to run your computer properly, a chip on your motherboard needs to read these voltages. There is usually resistance between the PSU and the monitoring chip on the motherboard. I experienced this myself recently and according to my motherboard monitoring software, my 12-volt Rail was way under spec. You want your voltage to be within 4%. So you want your 12-volt rail to be within 11.52 to 12.48 during load. This means when you are running your cpu at 100% you do not want to drop below 11.52 or you may experience stability problems including but not limited to system restarts and Windows crashes. For the 5-volt rail, you want it to be within 4.8 to 5.2 to be within the 4% range. As for the 3.3 volt rail, you want it to be within 3.17 to 3.43

What are rails? Rails are the above mentioned voltage lines coming from your PSU to your motherboard and devices in your motherboard. The 3.3-volt and 5-volt generally power the devices like the motherboard, PCI slots, AGP slot, etc. The 12-volt is usually used for things with motors like the hard drives, CD-ROM, fans, etc.

My guide here today will show you how to test your PSU with a multimeter to ensure accurate results. If you are afraid of doing such a thing and you are not sure what any of this guide means, I encourage you to NOT do this. Electricity is dangerous and can kill you in a heartbeat. Remember this is only a guide and you can not hold myself or Driverheaven responsible. If you arent sure of what you doing, DONT DO IT.

You will of course need some equipment for the job.


Shown above is a Multimeter with a digital read out and 2 ties. It may seem odd to need a tie but as you will see later, it will help prevent a headache and potential disaster.

**If you have them available use Rubber Safety gloves typically to help prevent electrical shock.

The reason for using a Multimeter with a digital read-out is to ensure the most accurate of readings. A multimeter with needles to show the results will not be the most accurate in this testk. If you do not have a multimeter, a good one can be purchased at places like Radio Shack for around 20-30 U.S. Dollars. The one shown above is in the 30 dollar range.

The first thing you want to do is download Prime95. Download it here: http://www.mersenne.org/freesoft.htm. Prime95 is used to put your system at full load. You want your system at full load when checking your voltages to ensure your PSU is up to spec. The whole point is to make sure your PSU can handle what your system asks of it. This can only be done by stressing your system during the voltage test.

Now you want to shut down your system and then open the side so you have access to your 20-pin and a Molex plug. How to test with the Molex plug is discussed later in the guide. Lay your computer on it's side during this test or this will be more difficult than necessary.

This is your 20-pin Motherboard plug:


This is what a Molex looks like:


You have two different ways of doing this. You can test your rails on the Molex plug or from the 20-pin. The Molex is easier but you get more accurate results from the 20-pin since it is the 20-pin that the main power is being drawn from.

Before we get into doing the actual testing, it is important to know what you are testing. We will be testing the 3.3-volt, the 5-volt and 12-volt rails. Remember which is which is quite easy. Think alphabetical order. Orange wires are 3.3-volt, Red wires are 5-volt and Yellow wires are 12-volt.

***The computer should still be off during the initial start of the testing.

Now take the multimeter and put the black part or ground rod into a ground of the 20-pin. All of the blacks wires are ground.

Now what you want to do is make sure you push it hard enough to get it in there but not too far as to damage the computer. You want it to make contact with the actual wire.

Now take one of the ties or something like it, it is best that it be non conductive. Now wrap it around the ground of the multimeter to the cabling of the 20-pin. This will prevent the ground from falling out during the testing because you will need both of your hands for this.

Make sure you have your multimeter laying on something none conductive like a mouse pad along with the red or positive end of the multimeter.

Now what you want to do is turn on your PC, and then run Prime95.

While Prime95 is running, your system will be at full load, this is when you want to test the voltages and get a good idea if your PSU is handling the load and whether or not your software monitor is showing accurate readings.

I usually check them in alphabetical order. Orange, Red and then Yellow. 3.3, 5 and 12 volts respectively.

Make sure you have the multimeter on the correct setting if it is not auto-switching. I put mine on DC or Direct Current and on the “20” volt setting where you will get the most accurate results in this range.

First the 3.3 volt Rail. Be sure to leave it there for several seconds and see if the reading isn't jumping up and down too much.

Now check the 5-volt rail. Do the same here as with the 3.3 and keep it there 10 seconds or so to ensure consistent voltage.

Now check the 12-volt Rail.

Between each Rail testing you should have the appropriate voltage range on the multimeter. For example if you are testing the 12-volt rail (Yellow wire) then your voltage should be in the 12 volt range. Double check and make sure you have your multimeter.


I know getting metal objects close to your motherboard can invoke a little fear, so you also have the option of doing this same thing with the Molex. Just make sure you have a free one available, or remove one from your cd-rom or dvd-rom. Make sure your computer is still laying on it's side, grab a non conductive flat surface like a mouse pad and lay the Molex on that. You only have the 12-volt and 5-volt rails available for testing on the Molex. The two middle wires on the Molex are grounds, so you can use either and I recommend you separate each test with one of the grounds.


Now if you have one of these plugs, you can test the 3.3-volt on it. Otherwise you will have to use the 20-pin above.

That is it, you are done. Now you know whether or not your PSU is handling your system like it should or if you need to look for a better one.


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Click here to go to application and install page Click here to go to pcmark2004 page Click here to go to the results page Click here to go to the conclusion page