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4
4.1
2543 Main Board
4.1.1
4.1.2
Rev. 00
Electronic Theory of Operation
The electronic theory of operation of the Model 509 Pulse Oximeter monitor is detailed
in the subsections below. See Schematics and Assembly Drawings on page 37. for
accompanying information.
Power Supplies and Voltage Reference
Refer to schematic sheet 3. Power for the monitor enters at J101 when SW1 is closed
(switched ON), the power is routed to the 2581 Power board through J102. Power then
returns to the board as an analog (V8.1) and a digital (VDD) supply. Diode D1 protects
against reverse bias, fuse F1 protects against over current conditions.
The LEDSRC supply which supplies the sensor's LEDs is current regulated by IC2 and
further filtered by L3, C8 and C9. It is supplied by the analog supply V8.1 from the 2581
board. The VDD digital supply from the 2581 board is filtered by L2 which creates the
+VA supply. The -VA supply is created by IC3 which is a charge pump inverter. The +VA
and -VA supplies are needed by the bipolar analog circuits in the monitor.
Refer to schematic sheet 2. A positive reference voltage VREF2.5 is developed by IC8,
which is a +2.5 volts DC reference derived from the +VA supply. A negative reference
voltage is developed by IC23A (pin1) by inverting the +2.5 volt supply (schematic sheet
3). This negative reference is -VREF at TP6. The analog to digital converters IC9, IC10
and IC11use the VREF2.5, the -VREF (-2.5V DC) is used by the digital to analog
converter IC22.
Sensor LED Drive Circuits
See schematic sheet 2. When the RDLED signal goes low (logic 0), Q3 turns off and
the VLED signal is divided down by R23 and R25, at IC7A (pin 3). FET Q2 is in turn
driven on by IC7A (pin 1). Current will flow from LEDSRC (J200 pin 7) through the red
LED in the sensor, through Q2, then through R20 to ground.
When RDLED returns high (logic 1), Q3 is biased on, forcing IC7A pin3 to ground
potential, this results in 0 volts at the output of IC7A (pin 1). FET Q2 is biased off, and
as a result, the Red LED in the sensor is off.
The Infrared LED drive circuit operates in the same manner as the Red LED drive
discussed above. The IRLED signal activates Q4 which controls IC7B, this in turn
controls Q8. The source of Q8 will control the Infrared LED of the sensor.
Refer to schematic sheet 3. The VLED line voltage is derived from IC23B pin 7 which
is controlled by the Digital to Analog Converter IC22. When the DACCS line is brought
Low IC22 is enabled. The data on lines D0-D7 now control the output of IC22 which in
turn control IC23Bs output on pin 7(VLED).
Model 509 Service Manual
7
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