Mallinckrodt Nellcor N-20 Service Manual page 64

Technical Supplement
Figure 9-28: Ambient Temperature Circuit
U5 is a precision-temperature sensor. It outputs (PR_TEMP) a voltage proportional to the ambient
temperature, which is 10 mV per degree centigrade. For example, at a room temperature of 25 °C, the
U5 output would be 250 mV. U5 is used whenever an optional printer is installed. Because the printer
is a thermal printer, ambient temperature must be compensated for.
9.10.19 Battery Voltage
The battery voltage circuit is illustrated in Figure 9-29.
The analog input voltage range of the CPU is 0-5 VDC. Because the battery voltage may be as high
as 6.2 V, R69 and R70 form a voltage divider to decrease the measured battery voltage to a usable
level. The gain is 0.75; thus, if the battery voltage was 6 V, then the voltage of BAT_VOLT would be
6 × 0.75 which equals 4.5 V.
The software has the ability to determine when battery power is too low. If the software determines
that the battery voltage is too low to provide accurate information, the software generates an audible
signal and automatically switches the unit off. If an optional printer is installed, the battery voltage
data are used to compensate for battery voltage changes that can affect printout quality.
9.10.20 Battery Type
The battery type circuit is illustrated in Figure 9-30.
TP2
VRECHARGE
TEST
N-20
AUX
PCB
The unit can operate with either disposable or rechargeable batteries. Battery type input is digital; a
high input informs the CPU that rechargeable batteries are in use. If rechargeable batteries are used,
9-26
TEMP SENSOR
U5
1
VCC VCC
2
TEMP
3
GND
LM35
VBAT
R69
15.8K
BATTERY
1%
BAT_VOLT
VOLTAGE
SENSE
R70
47.5K
1%
Figure 9-29: Battery Voltage Circuit
VCC
CR27
1N914
R100
15K
R101
150K
Figure 9-30: Battery Type Circuit
PR_TEMP
C115
0.1uF
TP73
C62
.01UF
BAT_TYPE