CertiGuide to A+ (A+ 4 Real)  9  Chapter 11: Power Supplies, Surge Protection and Uninterruptible Power Supplies

Chapter 11: Power Supplies, Surge Protection and Uninterruptible Power Supplies 1 Power for PC Survival: Surge Protectors

In order to bench test an ATX power supply without having it connected to a motherboard, the A+ technician must provide a PS-ON control signal to switch the power supply ON. The PS-ON control signal is normally controlled through a momentary contact switch mounted on the front of the computer case. This front-panel-mounted switch is connected to the motherboard and provides contact closure to the power supply motherboard connector.

The A+ technician can test the power supply from outside of the computer case by grounding the PS-ON control signal located at Pin 14 of the power supply connector (figure 1). This is typically a green wire. Ground connections are located at Pins 3, 5, 7, 13, 15, 16, & 17. Ground wires are typically black.

Be aware that a power supply may display proper voltages when nothing is connected to it, yet these voltages can fall below acceptable levels when a load is applied.

The power supply output voltages should be measured with a load applied to give a realistic indication of its ability to deliver sufficient current when it is placed in service. This load may be simulated by placing a 10-ohm, 10 watt resistor across the wire where the voltage to be measured is, and Ground. If the voltage remains within tolerance with the simulated load applied, the power supply is "good to go".

Output voltages are best measured with a digital voltmeter that can display two decimal points of resolution. This is the easiest way to check that the outputs are within the required 5% tolerance. Table 27 shows the acceptable voltage ranges and wire color-codes for the ATX power supply.

The ATX power supply provides addition control signals to the motherboard connector. These are described below:

• 5VSB - This is a standby voltage that may be used to power circuits that require power input even during the powered-down state. The 5VSB pin should deliver 5VDC to operate circuits such as soft power control and "wake on LAN". This is why it is important to disconnect the power cord completely when servicing the computer.
  
  
• PW-OK - PW-OK is a power good signal and is switched ON by the power supply to indicate that the +5 VDC and +3.3 VDC outputs are above the undervoltage thresholds of the power supply.

The ATX motherboard connector has 20 pins, the signals of which are laid out as in Figure 408.

Figure 408: ATX Power Supply Motherboard Connector Signals

Chapter 11: Power Supplies, Surge Protection and Uninterruptible Power Supplies 1 Power for PC Survival: Surge Protectors

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