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High Frequency
Filtf~ring
Line input voltage through the power cord is applied to a high-frequency line filter. This filter
traps high frequency noise from both entering or leaving the power supply. It enables the output
voltages to be virtually free from voltage spikes that may appear at the input. The filtered output
is sent to the fuse and power switch.
Fuse
For both line voltage ranges, the same size fuse protects the power supply. Voltage selection and
fuse size are shown below:
Voltage Setting
120
V
ac
240 V ac
Line Voltage
Fuse Rating
85 to 129 V ac
8A Normal Blow
187 to 255 V ac
8A Normal Blow
In both cases, the line frequency is 48-66 Hz.
Low Frequency Filtering
When the power switch is turned on, voltage is applied to the Low Frequency Filtering circuits.
These circuits trap low frequencies entering or leaving the power supply. They also reduce ripple
in the output voltages. After filtering, the line voltage is sent to the Bridge Rectifier Section.
On-Off Switching
This operator-controlled switch connects power to the Low Frequency Line Filter. Located on
the computer (or bus expander) front panel, it is mechanically connected to the power supply by
a switch extension. When turned to the ON position, a green LED on the front panel is lit. The
LED is connected to the voltage outputs and indicates that all power supplies are operating.
Input Voltage Selection
A voltage select switch is set to either the 120 V ac or 240 V ac position, depending on the line
voltage. Voltage selection changes the input voltage to the bridge rectifier to make the bridge
output the same for either line voltage.
Rectification and Switching Control Circuits
Functions of the Rectification and Switching Control circuits are:
• Full-Wave Bridge Rectifier
• Switching Control Circuit
• Power Transfornler
Full-Wave Bridge Rectifier
Part of the input ac voltage is applied across a full-wave bridge network that outputs a DC
voltage.
Large storage capacitors charge up to this voltage and provide a constant energy
supply to the individual power supply circuits.
Functional Description 39
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Troubleshooting
