When you press the power button on the front of the computer it creates a short between two terminals on the motherboard which initiates the start of the PSU. The procedure described above can be used to determine if the motherboard is the cause of your start up problem, but isn't a accurate means of testing the PSU. The procedure outlined below is intended to test the PSU rail voltages to be certain that they fall within their proper perimiters. Please note, there are certain safety precautions which are outlined below which should also be applied if you are going to use the procedure in the previous post.Reading PSU Rail Voltages Caution
: Please read this 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.
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.
There are five different DC rail voltages which are color coded. The Black wires are always negative.Yellow +12VDC
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.
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
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 isnít 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. Maximum.........Minimum