Power Supply Pcb Theory Of Operation - Nellcor NPB-190 Service Manual

Technical Supplement
S6

POWER SUPPLY PCB THEORY OF OPERATION

S6.1 Battery Circuits
S-4
The NPB-190 uses an unregulated linear power supply. This power supply
provides the DC power needed to charge the battery and to power the User
InterFace PCB (UIF). Electro Static Discharge (ESD) protection is also provided
by the power supply.
AC power from the PEM is passed through a step-down transformer, T2, which
has two primary and two secondary windings. If switch SW1 on the back of the
monitor is in the 120 VAC position, the primary windings are in parallel. The
primary windings are in series if SW1 is in the 240 VAC position.
Each secondary winding is fused with a 2.0 amp fuse (F1 and F2). If a short
circuit should occur in the DC circuitry, these fuses prevent the transformer from
overheating. The output of the transformer varies, depending on load and input.
Voltage measured between the outlet of a secondary winding and ground can be
from 6 to 20 VAC. High frequency noise from the AC line and from the UIF
PCB is filtered by C6 and C8 before passing through the bridge rectifier.
The bridge rectifier provides the DC power used in the NPB-190. The positive
output is the Main_DC ranging from 7 to 18 VDC. This positive voltage is used
for the battery circuit and to power the UIF PCB.
Two circuits are included in this section of the Power Supply PCB. One circuit is
used to charge the battery, and the other circuit provides battery protection.
Charging Circuit
The Power Supply will charge the battery any time the NPB-190 is connected to
AC power even if the monitor is not turned on. The voltage applied to the battery
is 6.8 ± 0.15 VDC and is current limited to 400 ± 80 mA.
Battery Protection
Two types of battery protection are provided by the Power Supply; protection for
the battery and protection from the battery.
SW2 is a resettable component that protects the battery. SW2 opens and turns
the charging circuit off if the temperature of the battery rises above 50∞ C. If the
output of the battery exceeds 2.5 amps, F3 opens. F3 protects the battery from a
short to ground of the battery output. F3 cannot be reset.
Protection from the battery is provided in case the battery is connected
backwards. Should this happen, the output of the battery is shorted to ground
through CR3. This provides protection for other circuits in the monitor.