Nellcor N-180 Service Manual page 33

N-180 Pulse Oximeter Service Manual .
point and controls transistor Q8 to maintain this point at a voltage that
represents a battery voltage of 13.8 V. Variable resistor VR1 adjusts the
voltage at this point.
The circuit composed of transistor Q11 and associated resistors
constitutes a current-limiting circuit that has foldback characteristics.
Battery charge current flows through R54. The voltage drop across R54
is proportional to charge current. Transistor Q11 is normally turned off
by reverse bias generated by the R49/R50 divider. If the charge current
increases so that the drop across R54 exceeds the drop across R49 Q11
turns on. This allows additional current to flow through VR1/R53
raising the voltage at the sense point. Transistor Q10 reacts by reducing
the drive to Q8, which in turn reduces the charging voltage, whereupon
the cycle continues,
Should the battery or battery input circuit encounter a short, the voltage
at the sense point (R53 and R54) drops; accordingly, Q10 attempts to
increase the charge current. To prevent this increase and resultant
overheating, a short circuit protection circuit is employed. Transistor
Q12, normally held in conduction by the battery voltage value, shuts off.
This results in Q9 turning Q8 off.
m DC power supplies and battery charger
The DC power supplies consist of two unregulated DC sources derived
from two individually isolated secondary windings of the power
transformer. These supplies are used for the battery charger and
various regulators.
Unregulated power for the main power supply comes from a 20 VAC
isolated secondary. The AC is coupled onto the CPU PCB via connector
CNS, filtered by inductor L1, and rectified by bridge D9. The resulting
DC is from 15 to 25 V depending on the line voltage value. Three-
terminal device Q4 regulates the DC at a maximum of 24 V. This
voltage is passed through D2 and becomes VPS (TP4), which is used to
power regulator Q3, providing a highly regulated +5 V for processor
circuit power. The 24 VDC output from Q4 is also used by the battery
charger circuits. Diode D3 isolates the battery from VPS when AC power
is used.
In the absence of AC power, the 12-volt battery potential is coupled
through D3 to supply regulator Q3. In this case D2 isolates the main
power supply from the battery potential, reducing unnecessary current
drain.
AC voltage for the RS-232 power supply comes from an 8 VAC isolated
secondary. The AC is coupled onto the CPU PCB via connector CN7 and.
__rectified by bridge D10. The resulting DC will be from-7-to-9-V--——---—-----—-
depending on the line voltage value. Three-terminal device Q7 regulate
. the DC at 5 V. This voltage is isolated from all other grounds and
powers only optical isolators PH1/PH2 and RS-232 device U10.
RS-232 Port and Associated Isolators (Sheet 3 of 4)
The CPU PCB processor U9 has two internal modems (modem 0 and 1), but
only modem port 0 is used. This port accommodates four RS-232 signals as
follows:
3-18