Novametrix Medical Systems OXYPLETH 520A, Service Manual

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Section 9
Electronic Theory of Operation
18
Model 520A Service Manual 5693-90-01
PRELIMINARY
The filtered, switched and fused output of the Power Entry Module is fed to the
primary coils of the rear panel mounted system transformer, T301. The secondary
output from T301 is rectified by D1 (bridge rectifier) and filtered by C1. The loaded
DC voltage at this point is approximately 20 volts.
The 20 DC volts is fed to the main board through fuse F301 to connector E302, and
is switched to the battery charging regulator IC1 (pin 5) through Q1B. Biasing for
Q1B is accomplished by C2, R1, R2, C8 and Q1A when AC power is applied. When
running on battery power Q1B is biased off by R2 and Q1A, this prevents the
battery from trying to power the battery charger regulator and IC2 that informs the
monitor of the loss of AC.
The output of switching regulator IC1 pin4 is rectified and filtered by D4, C4 and
L1 then fed to the battery through current sense resistor R3 and fuse F302 to
VBAT+ (J302 pin 1). The battery float charge voltage is maintained at 13.2 volts
except for fast charge which is regulated at 14.4 volts. The output is also monitored
for over current conditions. These parameters are controlled by IC3 and associated
circuitry. When the battery charge current exceeds 120mA of current IC3 pin 7 goes
high which biases Q2 on, this in turn shorts out R12 which affects the feedback
control (FB) to IC1 (pin 1). With R12 shorted out the control resistors R14 and R13
set the output voltage to 14.4 volts. When the charge current lowers IC3 pin 7 goes
low which biases Q2 off, this puts R12 back into the feedback control which now
consists of R12, R13 and R14 setting the voltage to 13.2 volts. When more than
600mA of current flows through R3, IC3 pin 1 shorts IC1 pin 2 to ground which
shuts IC1 off until its next switching cycle, when the current reaches a safe level
IC3 pin 1 allows IC1 to remain on.
The voltage rectified by D1, D2 and filtered by C2 is fed to IC2 as VCH (Voltage
Charge). The output of this 5 volt regulator provides the LINEST (Line Status)
signal to the main board at E302 pin 3. With AC applied, LINEST is high. LINEST
goes low when the AC is disconnected. The LINEST line is also routed to the power
on/off circuitry.
Battery Operation
9.2.3
Without AC power there will be no secondary voltage rectified by D1. Power for
the monitor will be supplied from the battery at J302 pins 1 (VBAT+) and 2
(VBAT-). The battery power will conduct through D3 and F301 to VIN at E302 pin
1 to the main board. R2 and Q1A bias Q1B off in this condition which prevents
power from reaching IC1, IC2 and IC3. The output of IC1 is also protected by D5
which is now reverse biased, the bridge D1 is also reverse biased and prevents T301
from discharging the battery. With no voltage at IC2 the LINEST will be low which
indicates to the main board that there is no AC power.