CertiGuide to A+ (A+ 4 Real) - Testing an ATX Power Supply
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CertiGuide to A+ (A+ 4 Real)
Chapter 11: Power Supplies, Surge Protection and Uninterruptible Power Supplies
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Chapter 11: Power
Supplies, Surge
Protection and
Uninterruptible Power
Supplies
1
Power for PC Survival:
Surge Protectors
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
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CertiGuide to A+ (A+ 4 Real) - Testing an ATX Power Supply
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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.
Table 27: ATX Voltage Specifications
Nominal Voltage Wire Color Minimum Voltage Maximum Voltage
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+5 V
Red
4.75 V
5.25 V
-5 V
White
-4.75 V
-5.25 V
+12 V
Yellow
11.4 V
12.6 V
-12 V
Blue
-11.4 V
-12.6 V
+3.3 V
Orange
3.135 V
3.465 V
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.
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CertiGuide to A+ (A+ 4 Real) - Testing an ATX Power Supply
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Figure 408: ATX Power
Supply Motherboard
Connector Signals
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Chapter 11: Power
Supplies, Surge
Protection and
Uninterruptible Power
Supplies
1
Power for PC Survival:
Surge Protectors
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