Using a multi-meter to check your PSU

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Using a multi-meter to check your PSU

Post #1 by huddy » Tue Jul 02, 2013 12:28 pm


If you are having problems with your PSU or suspect your PSU isn’t supplying a stable or sustained voltage, then a multi-meter is an excellent means of accurately checking that your power supply is working correctly. This guide explains the basic method of using a multi-meter with your PSU.
Multi-meters are a good way of checking:

 AC voltage (Alternating Current - Mains Voltage)
 DC Voltage (Direct Current)
 Continuity or current
 Resistance

A good Multi-meter will typically consist of:

 Two probes. 1 positive (red) and one negative (black)
 Either an Analogue or digital display
 A switch for type of power test

Before we start, make sure you are familiar with the cables on your PSU. Each cabled is colour coded to make it easier to identifier the supply they are carrying:

 Yellow cable supply the +12v rails
 Red cables supply the +5v
 Orange cables supply the 3.3v rails
 Black cables are ground.

AC and DC voltages

AC voltage (Alternating) is the power straight from the wall. This is typically -240v to +240v. This is the Power coming into your PSU.
AC voltage can be transformed in to a DC current to a manageable form you’re your delicate components. Your PSU in this case is the transformer. When you hear about the efficiency of a PSU, it refers to the capability of converting AC power to a DC current.
AC and DC are very different so be sure to select the correct mode when testing for voltages.
Measuring Voltage

When using a multi-meter, ensure that positive side of the multi-meter connects to the positive side of the target device and the negative side (ground) to the negative. You also need to bear in mind, that when testing voltages, that the two points must be powered from its source. Therefore, your PSU will need to be “on” whilst performing your tests.

Depending on the voltage test set your multi-meter to either AC or DC voltage. When the two points are connected you get a reading.
When testing the rail output of your PSU, make sure to select DC. You need to allow a deviation of 5% of either side of your reading (10% for the -12v rail). So for example, a 5v rails should show a reading between +4.8volts and +5.3volts. Anything outside this range is certain to be a problem with the power supply. I've listed the ranges for each rail below:

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

Here’s how to test the PSU rails:

Power test for Power Output 5v

1. Turn PC off but leave plug connected.
2. Take a Molex power connector and insert black probe into black wire connector.
3. Insert red probe into red wire connector.
4. Set multi-meter to 20volts and turn on PC.
5. Read out should between 4.75v and 5.25v


Power test for Power Output 12v

1. Turn PC off but leave plug connected.
2. Take a Molex power connector and insert black probe into black wire connector.
3. Insert red probe into Yellow wire connector.
4. Set multi-meter to 20volts and turn on PC.
5. Read out should be between 11.4v, maximum of 12.6v.


Power test for Power Output 3.3v

Reading the 3.3v is a little more difficult than the 12v and 5v rails as the Orange cables aren’t as accessible.
The Motherboard normally receives a 3.3v connection via the ATX connector and possibly an AUX connector, both of which are quite awkward to get a probe near.

Option1 – Switch off PSU and unplug the ATX connector from the motherboard

Insert a spare piece of cable into the Green wire connector on the ATX block and the other end to any black wire connector. This creates a bridge which forces the PSU to start when switched on (see Section 6 of "How to Check your PSU" for further details)
Turn on PSU and set multi-meter to 20volts

Insert the red probe into the orange wire connector on the ATX block and the Black probe into any black wire connector. Then take your reading.
Reattach your ATX connector

Option2 – Sometimes the contacts are visible on the top side of the ATX connectors.

Set your multi-meter to 20v. With the PC on, try and insert the red probe in the top of the orange cable on the ATX block and the black probe into a black connector. Take your reading.

Option 3 –It is possible to purchase an ATX extension cables from all good electrical components retailers.

Simply remove about 5mm of sleeve on both the orange and black cables on the extension.

Fit the modified extension cable between the Motherboard and the PSU ATX connector.

Turn on PSU and set multi-meter to 20volts

Touch the bared section of the orange cable with the red probe and the black cable with the black probe. Then take you reading.
When testing is complete, remove the extension cable and reconnect the PSU ATX connector to the Motherboard.

Whichever option you take, your reading should be between 3.14v and 3.47v

Measuring resistance (ohms)

Resistance is a measurement of power between two points. Used normally for testing resistors we can also test for good or bad cables where there is a possible break.

1. Ensure there is no power supply. Unplug PSU completely.
2. Set the multi-meter to ohms and touch probes to test.
3. To test cable for example, place one probe into one cable (say pin 1) and the other into the other end.
4. You should get a signal which is near enough the same measurement as the test depending is there are any resistors between.

In summary..

Suspected broken cable? Set multi-meter to ohms.
Check for dead PSU? Set multi-meter to volts.

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

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