Novametrix Medical Systems MARS PO2 TECH 2001, Service Manual

Page: 16 / 85

Section 4
Theory of Operation 2775 Main Board
10
Model 2001 Service Manual Rev. 01
AC Mains Operation
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 2775 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. See Power On/Off Control Circuitry
on page 3.
Battery Operation
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 2775 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.
2775 Main Board
The 2775 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 as well as the communication interface to
the DSP board.