Novametrix Medical Systems OXYPLETH 520A, Service Manual

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Section 9
Electronic Theory of Operation
20
Model 520A Service Manual 5693-90-01
PRELIMINARY
effect of synchronizing the output of the pulse width modulator with the data
sampling operations of the analog board and keeps power supply switching spikes
from interfering with those operations.
Power Supplies
9.3.2
The secondary pins 7, 8, 9 of T1 form a center tap transformer, the voltage is
rectified by D2 and D4 then filtered by C10, C11, C12 and C13. The dual 12 volt
supplies, +V12 and -V12 which are generated from this voltage are regulated by
IC7 and IC8 respectively. The secondary winding of pins 5 and 6 of T1 are rectified
by D3 and filtered by C9, L1 and C20, this voltage designated as VCC (+5 volts)
acts as reference for IC9, supplies power for the opto isolator non-isolated side and
powers other circuitry on the board.
The secondary winding consisting of pins 2 and 3 are rectified and filtered by D1
and C1. The rectified voltage at this point is approximately 7 volts DC and is
regulated to 5 volts by IC2. This isolated supply powers the isolated portion of the
opto-isolators and the RS232 driver chip IC1. The unregulated voltage VRAWI is
sent to the rear panel connector J101.
The backup voltage (VBACK) is regulated by IC12 from the VIN supply.
Capacitors C22 and C27 serve as filters and D17 allows VCC to power VBACK
circuitry when the monitor is on. At this point D18 is biased off so IC12 is idle.
When the monitor is turned off and VCC collapses D18 is then forward biased and
IC12 now supplies VBACK circuitry, D17 at this time is reverse biased and
prevents power from reaching VCC.
The saturation sensor LEDs derive their power (LEDSRC) from the current
regulator IC32. (See sheet 3 on 2471 schematic, page 2 on 2775 schematic.)
Resistor R31 limits the maximum current draw to 45 mA (nominal draw 35 mA).
Regulator output is filtered by C85 and L2. The charge stored on capacitor C1
supplies the 290-350 mA peak currents that can occur when the sensor LEDs are
turned on. Diode D12 prevents the regulator output from exc7.5 volts while
the fuse, F2, provides current limit protection in the event of a regulator circuitry
failure. The RP10 (pins 1,8 and 5,6) divider network provides the CPU (via IC33)
with a means to monitor the LEDSRC status.
The +VA and -V12 supplies are regulated to +V5 (+5 volts) and -V5 (-5 volts) by
IC44 and IC43 respectively. These supplies are used by the 20 bit ADCs, the 8 bit
ADC and other circuits associated with them.
Voltage References
9.3.3
A +2.5 volt precision reference supply, generated by IC35 from the 12 volt +V12
supply, is used as a reference voltage for the ADC chips IC37 and IC34.