Novametrix Medical Systems OXYPLETH 520A, Руководство по обслуживанию

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2471 & 2775 Main Board
5-Jun-01 Model 520A Service Manual
19
PRELIMINARY
2471 & 2775 Main Board
9.3
The Main Board contains all the analog and digital circuitry that controls the sensor,
external communication and front panel display. The isolated power supplies,
microprocessor circuits and memory are all contained on this board. The earlier
2471 Main Board and newer 2775 Main Board are identical with exceptions noted
in the text.
Power On/Off Control Circuitry
9.3.1
The Model 520A power on/off control circuitry consists of the VBACK supply
(regulated by IC12), IC10, IC11 and the
POWER
key. (See page 4 2471 schematic,
page 4 on 2775 schematic.)
When the battery or AC Mains is first applied to the power supply board (via VIN
J102 pin 1), VBACK goes to +5 volts, provides power to IC10 and IC11, and
through the C26 and RP4 (pins 3,4) network at IC10 pin 8, sets IC10 pin 2 to a logic
Low.
The ON/OFF line is brought Low each time the front panel
POWER
key is pressed.
This sends the output at IC11 pin10 High. This Low-to-High transition clocks the
(#1) D flip-flop portion of IC10. The /Q1 output at IC10 pin 2 goes High and with
each successive press of the
POWER
key, this output toggles to the opposite level
(Low or High). A High turns the Model 520A on and a Low shuts it off.
While the /Q1 output at IC10 pin 2 is High, the MOSFET Q7 is turned on and pulls
the gate of MOSFET Q8 to ground, thus causing Q8 to conduct as well. With Q8
conducting, the currently active monitor power source—either the AC Mains
derived supply or the battery supply will flow through Q8 to the voltage input (pin
7) of the Pulse Width Modulator IC9. The output IC9 pin 6 will oscillate (at the
frequency set by R13 and C15). This causes Q5 to switch on and off and provide a
path to ground through the primary coils of T1 for the supply (Mains or battery) at
T1 pin 12. Current flowing in the primary is measured at IC9 pin 3 and the duty
cycle of the pin 6 output will vary with the load on the transformer.
Current flow in the transformer primary induces current in the three secondary coils
and creates the
±
12 volt analog supplies (+V12 and -V12), the VRAWI that powers
the isolated RS232 circuitry, and the +5 volt VCC supply that power the remaining
circuits in the monitor. The Model 520A turns on. The +V12 and -V12 supplies are
rectified and filtered by D2, D4, C10, C11, C12 and C13. The +V12 is regulated by
IC7 and the -V12 by IC8. The VDD supply is rectified by D3, filtered by L1, C9
and C20 and fused at F1, and in addition, a feedback loop to IC9 contains VR1
which is factory adjusted to produce a +5.00 volt
±
0.05 volt VCC supply (measured
under load).
Once the monitor powers up, a SYNC signal toggles Q9 on and off causing a timing
pulse to be transmitted across C19 and C15 to the input at IC9 pin 4. This has the