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How to Test PSU DC Output Voltages to Determine if Faulty
The Internal ATX Power Supply converts the Alternating Current (AC) from the mains input to a Direct Current (DC) output that is required for the computer to work.The typical voltages supplied are 3.3 volts, 5.0 volts, -5.0 volts, 12.0 volts, -12.0 volts.
The 3,3 and 5volts are typically used by the digital circuits, whereas the 12-volt is used to power the motors found in Disk Drives and Hard Drives. It is also used to Power the CPU. The following tests involve testing the voltages across the HDD, Motherboard, CPU, CD/DVD ROM Drive Power Connectors.
These tests can be carried out with the Power Supply Fitted or removed from the computer.
NEVER ATTEMPT MAINTENANCE ON POWER SUPPLIES AS THEY CAN BE DANGEROUS
EVEN WHEN THE POWER REMOVED. If a fault is suspected with the PSU then the first step is to take VOLTAGE readings, then substitute the POWER SUPPLY with a good quality brand, with the same MAINS VOLTAGE and WATTAGE.
PSU and PSU Connectors Overview.
1. To Check The HDD 4-Pin Molex Power Connector Voltages:
The maximum voltage that will be measured will be no more than 12v/12.6v.
The Multi-meter should be set to DC 20 Volts as shown. The meter display will show 00.0 (i.e. - 0 Volts). NOTE: Any number after the decimal point can be ignored
The computer is best laid on its side when taking meter readings for ease of access.
Next open the case and power up the computer:
Care should be taken here as the fans will be spinning, certain components will be hot to the touch and observe ESD precautions.
The Anti-static wristband should be worn and attached to the case or an Earthing Point. (If you do not have one, simply earth yourself on the metal part of the computer case periodically)
Locate the HDD 4-pin Molex.
The Red Probe should be inserted into the right-hand side of the connector (Yellow Wire) The Black Probe should be inserted into one of the two centre holes (Both of which are negative)(Black Wires).
The reading should read 12.4V
NOTE: You can also take a reading between the connector and the computer case, the voltage reading will be the same (12.4V), as shown.
A reading should also be taken between the RED wire on the back of the HDD Molex and the computer case, this will show a reading of around 5V.
NOTE: This method can be repeated for testing the voltages for Red and Yellow wires on the CD/DVD Drive Molex Connector and for testing Molex Fan Connectors and CPU Connectors, some examples below.
2. To Check The Motherboard 24-Pin Power Connector Voltages:
Next we can check the voltages across the 24-Pin Motherboard connector, simply connect the RED probe to each colour in turn, whilst the Black Probe is touching the computer case, the readings will be similar to those shown.
NOTE: The BLACK and WHITE wires are not classed as COLOURS and are rated 0V, you need not test between the case and these connections.
You can of course test between these and any COLOUR in order to get a reading.
If we look at the relationship between the COLOURS and the VOLTAGE, we will see that ALL the COLOURS carry exactly the same voltage, as shown.
This is very useful to note, because if a Device has failed then you can check that it is operating at the correct voltage if you measure the same colour Wire on a different Device.
A few notes to consider when replacing or buying a new PSU:
Most entry level systems are usually supplied with a basic generic, unbranded PSU. If you are building a medium to high spec system, it is highly recommended that you use a branded PSU such as Antec, OCZ, Corsair etc.
Many engineers state that 10-15% of the system budget should be used on a quality PSU.
When replacing Power Supplies Always check the Voltage is set to the correct setting on the rear of the PSU, either 110v/115v or 220v/230v. Setting the Power Supply voltage switch to the incorrect Input Voltage could seriously damage the Power Supply and possibly cause damage to other components of your computer.
The correct voltage is determined by the country where the Power Supply is located. (I.E.- UK 220v/230v USA 110v/115v.
*****ALWAYS RECHECK YOUR WORK WHEN WORKING WITH POWER SUPPLIES*****
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