Nellcor OxiMax N-550 Service Manual page 116

Technical Discussion
Power Supply PCB Theory Of Operation
Battery
Main PCB
106
The N-550 uses a switch mode power supply. This Power Supply provides the
DC power needed to charge the battery and to power the Main PCB.
Electomagnetic Static Discharge (ESD) protection is also provided by the
power supply.
AC power is passed through a step-down transformer. The transformer's
secondary winding is fused with two 2.0-amp fuse. If a short circuit occurs in
the DC circuitry, these fuses prevent the transformer from overheating. The
output of the transformer varies, depending on load and input. High frequency
noise from the AC line and from the Main PCB is filtered by R1 and C1
before passing through the bridge rectifier.
The bridge rectifier provides the DC power used in the N-550. The positive
output is the MAIN_DC of 20 volts DC. This positive voltage is used for the
battery circuit and to power the Main PCB.
A lead-acid battery is used in the N-550. It is rated at 12 volts DC, 1.2 amp
hours. When new and fully charged, the battery will operate the N-550 for two
hours. A new battery will last 15 minutes from the time the low battery alarm
is declared until the N-550 is shut down due to battery depletion.
The battery can withstand 400 charge/discharge cycles. Recharging the
battery to full capacity takes 11 hours in standby and 12 hours if being used.
Changing from AC to battery power does not interrupt the normal monitoring
operation of the N-550. When the N-550 is running on battery power, the data
port will be turned off.
The Main PCB is the heart of the N-550. All functions except the unregulated
DC power supply, display, and keypad reside on the main PCB. The following
text covers the key circuits of the main PCB.