Reading and Testing Desktop PSU Rail Voltages
Caution: Please read the following before continuing.
* Since it will be necessary for your computer to be on during this procedure, you need to be aware that you will be working with live 12Volt DC potentials, which if handled improperly may lead to electrical shock.
* There are electronics inside the case that are very susceptible to electrostatic discharges. To protect your computer, touch the metal of the case to discharge yourself of any electrostatic charges before touching any of the components inside.
* If you are not comfortable doing this procedure, then I would suggest that you not use this tutorial. The risks involved are minimal, but are there nevertheless. Anyone who uses this tutorial will be doing so at their own risk.
The following is a brief description of the three most common tools used to read voltages or test a PSU.
There are two devices commonly used to read the rail voltages: a PSU tester, and a multimeter.
The PSU tester is the easiest to use since all that is necessary is to plug the different connectors into the tester and read the results on the LCD display. The problem with most of these is that they only perform a pass/fail test. They will not provide you with actual voltage readings.
There are a variety of multiple meters, but this tutorial will address Analog and Digital multimeters. The advantage of these meters is that you will be able to obtain accurate real time voltage readings.
For those of you who wish to know more about multimeters there is an excellent article in Wikipedia.
An Analog multimeter is a little more complicated to use. Both Analog and Digital multimeters need to be set to the appropriate voltage, but with an Analog multimeter, you will need to choose the voltage range and must read the proper scale.
The Analog multimeter uses a needle display which moves from 0 across the scale until it reaches the voltage being tested. This multimeter has five major linear divisions with multiple scales to read a variety of ranges. An example would be three different ranges. The first is graduated in increments of 0 through 5, the second, 0 through 10, and the third, 0 through 25. Each of these ranges are subdivided into divisions that are graduated into tenths. In order to read 12 volts the 0 through 25 range would be the appropriate one.
Because DC voltage has positive and negative potentials this device is polar sensitive, this means that if you reverse the two probes when reading a positive DC voltage it will read as a negative voltage. This is actually necessary to read negative DC voltages. The two probes are differentiated by their color, Black (negative), and Red (positive). To read a positive DC voltage, the correct probes must be used with their corresponding potentials (positive to positive and negative to negative).
With the probes being used normally to read a negative DC voltage, the needle moves from the 0 to the left, "pegging" the needle. By reversing the probes you can properly read the negative voltages.
The Digital multimeter (DMM) is much simpler to use. As was mentioned previously, you will need to set the appropriate voltage. One of the advantages is that the DMM has an LCD display with a numeric readout, so there are not any multiple scales to read. Another advantage is that most DMMs are autoranging when reading voltages, which means that you will not need to set the range with these DMMs. A DMM will read both positive and negative DC voltages and display them correctly. When reading a negative voltage, a minus sign will appear on the display before the numeric value. This still is a polar sensitive device, so you will still need to use the positive and negative probes with their corresponding potentials.
The following instructions are to be used to isolate the PSU from the motherboard for testing purposes.
Shut down your computer, then unplug the power cable from the rear of the computer. To reduce the possibility of any shock press and hold the power button for thirty seconds to discharge any capacitors still holding a charge.
The connector of the PSU which connects to the motherboard is readily recognizable by the large number of wires in the bundle. To disconnect it you will need to press on the plastic clip to disengage it and then pull the connector up and away from the motherboard. Please take notice of the location of the locking tab and the notch on the socket of the motherboard, this will only connect one way as it is keyed. This wire bundle will have a memory of the way it has been installed and will want to bend back that direction, you may have to play around with it to find a position that the connector will stay in the same position while you run the test.
From the top left to right the pins are 13-24, the bottom from left to right are 1-12.
Below are the pinouts for the 20 and 24 pin ATX form factor connectors.
Please notice that there are PSUs with 24 pin and 20 pin connectors, the location of the green wire in the 24 pin connector is #16, and the green wire in the 20 pin connector is #14. If you look at the connector with socket side facing you and the clip on the top the number one pin will be on the bottom left corner. This makes the pin out for the 24 pin connector from left to right 13-24 on top, and 1-12 on the bottom. The pin out for the 20 pin connector from left to right is 11-20 on top , and 1-10 on the bottom. If you look at the connectors you notice that these are sockets that fit over the pins on the motherboard where the PSU cable attaches, this is where you will place the jumper. For a jumper you will need a piece of solid wire about the size of a paper clip (20-22 awg), preferably a wire with insulation. It will need to be large enough to fit firmly into the socket so that it will not need to be held in place while testing. You are at risk of electrical shock if you are holding the jumper when you power up the PSU. Insert one end of the jumper into the socket of the Green wire, and insert the other end into the socket of any Black wire. Once the jumper is in place plug the cord back in.
There are five different DC rail voltages which are color coded. The Black wires are always negative.
There are only three voltages that can be measured easily without disconnecting the 20/24 pin connector from the motherboard: +12V, +5V, and +3.3V.
The +12V and +5V voltages can be read from a four pin Molex power connector.
Four pin Molex power connector
The same voltages can be taken from a four pin SATA power connector, but in order to read the +3.3V you will need to read this from a five pin SATA power connector as seen below.
Five pin SATA power connector.
To read these voltages you will need to insert the Black (-) probe into any of the black sockets, and insert the Red (+) probe in the different colored voltage sockets. To read the voltages from a SATA power connector it is easiest to insert the probes into the bac k of the connector where the wires enter. Unfortunately the sockets of the modular SATA power connectors are not accessible from the back, so the readings will need to be made from the socket side. Some probes are going to be too large to fit in these sockets, so you may need to insert a piece of wire into the socket of which you want to read the voltage of and place the probe on this for your reading. To reduce the potential of creating a short I would suggest taking the ground potential from another connector so that the two wires will remain physically separated.
Caution: It is very important to make sure that you don't allow the two probes to touch each other when taking the voltage readings. This will cause a short which could damage the PSU or other components.
The following is optional.
To get accurate readings of the rail voltages it is important that there be a load on the PSU. In order to do this I would suggest downloading Prime95 and run the Just Stress Test for this purpose. This program was designed to be used by overclockers to put a full load on the RAM and CPU to determine the stability of their overclocking. Because of this it will put stress on the CPU and RAM which will create higher than normal temperatures. For this reason I would suggest not running this program any longer than is necessary. I would also suggest that an inspection be made of the interior of the case to make sure that there isn’t an accumulation of dust which would impede adequate cooling. Pay special attention to the heat sink and fan assembly on the CPU. If there is a dedicated graphics card with a fan installed on it, look at this fan as well.
Readings should not have variances larger than +/- five percent.
Once you have finished the test shut down the computer. With the computer off remove the jumper from the 24 pin connector and reattach it to the motherboard.
Edited by dc3, 28 November 2014 - 12:23 PM.