Testing an ATX Power Supply
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:
The ATX motherboard connector has 20 pins, the signals of which are laid out as in Figure 408.
Home - Table Of Contents - Contact Us
CertiGuide to A+ (A+ 4 Real) (http://www.CertiGuide.com/apfr/) on CertiGuide.com
Version 1.0 - Version Date: March 29, 2005
Adapted with permission from a work created by Tcat Houser et al.
CertiGuide.com Version © Copyright 2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.