Novametrix Medical Systems OXYPLETH 520A, Service Manual

Page: 26 / 133

Section 9
Electronic Theory of Operation
16
Model 520A Service Manual 5693-90-01
PRELIMINARY
the AC Mains must be connected and the front panel
POWER
key pressed before the
monitor will turn back on.
AC Mains Operation
9.1.2
The AC Mains voltage enters the monitor at the rear panel Power Entry Module
(PEM). This device contains a built in RFI power line filter, a double-pole single-
throw switch that opens and closes both AC input lines, fuses, and an input voltage
selection card.
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 2471 main board through fuse F301 to connector
E302, and is switched to the battery charging regulator IC1 (pin 5) through Q1.
Biasing for Q1 is accomplished by D2, R1 and R2 when AC power is applied. When
running on battery power Q1 is biased off by R1, R2 and D3, 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 J302
pin 1 (VBAT+). 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 switched by Q1 is also 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. See
Power On/Off Control Circuitry on page 19.
Battery Operation
9.1.3
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