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Electronic Theory
of Operation
The electronic theory of operation of the Model 520A Pulse Oximeter monitor is
detailed in the subsections below. Refer to Section 19, Schematic and Assembly
Drawings, on page 83 for further information.
2472 Power Supply Board
9.1
The 2472 Power Supply Board contains the circuitry needed to power the monitor
from the AC Mains (line voltage). The power supply board also connects to the
battery and contains the battery charging circuitry.
AC Mains and Battery Operation Overview
9.1.1
The Model 520A can be powered from its internal 12 volt battery or from the AC
Mains. The green ^
(sine wave shaped) front panel indicator illuminates when the
line cord is connected and the rear panel power switch is in the "|" (On) position.
This indicates that AC Mains power is reaching the power supply, that the battery
is being charged, and that if the monitor is turned on, it is being powered from the
line voltage.
If AC Mains power is removed by unplugging the line cord or setting the rear panel
power switch to the "0" (Off) position, the monitor will operate for up to four hours
from its internal 12 volt lead-acid battery. As the battery voltage runs low («11.5
volts), the red E# (battery indicator) on the front panel illuminates. At this point, the
AC Mains should be reconnected to power the monitor and charge the battery.
If the monitor continues to be powered from a battery in a low voltage situation, at
approximately 11 volts, a continuous alarm sounds for thirty seconds while the
Message Center displays BATTERY
EXHAUSTED
CONNECT
LINE
CORD. If this
alarm/message is ignored, the monitor displays will shut down and the battery
indicator will flash on and off about every 5 seconds. If AC power is now restored,
the monitor will re-initialize (go through the power up and self-test routines) and
resume normal operation. However, continued battery operation will eventually
activate a hardware low voltage circuit (=8.5 volts) that shuts the monitor off to
prevent damage to the battery. Once the unit is shut down with the hardware circuit,
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