GE MAC 5000 Service Manual page 56

2-36
Equipment Overview: Theory of Operation
Absurdly low voltage readings from the battery temperature sensor
indicate an open thermistor. This is used as an indication that no
battery pack is present.
The sole purpose for resistor R153 is to protect Moe's ADC (AN3) pin in
the case where the temperature signal TBATTERY becomes
inadvertently tied to VBATT+. This can easily occur since the two pins
are adjacent. Should the short occur, resistor R153 will limit the current
and Moe's internal protection diodes will clamp the voltage to +3V-C.
Battery Voltage Sensing
Moe continuously monitors battery voltage during operation.
Excessively high pack voltages during charge will cause charge
termination. If battery pack voltage falls below a predetermined
threshold during operation, the battery gauge is immediately cleared to
zero and the main CPU is notified of the critically low voltage. System
software then initiates an orderly shutdown to protect the battery pack
and prevent loss of date/time.
Ambient Temperature Sensor
Extreme ambient temperatures are not favorable for battery charging.
Rapid changes in ambient temperature can cause premature or delayed
charge termination by altering the pack's temperature. Moe monitors
ambient temperature via the thermistor RT1 to ensure that charging
occurs only within the "safe" temperature range as well as to minimize
the effects of changing ambient temperature on charge termination
(particularly to avoid premature termination, which would give a false
"full" reading on the gas gauge).
The battery and ambient thermistors are the same type and value to
ensure reasonable tracking. Capacitors C155 and C156 filter noise from
the temperature sense lines.
Thermistor Bias Switch
To reduce quiescent power consumption when the system is turned off, a
switch disables bias current to the battery and ambient thermistors.
Q104, under control of MOE, switches the low side of the thermistor bias
networks.
Charge Light
Moe provides power to the amber charge light in the power supply
compartment. Moe communicates the current battery/charger state via
this light. Four conditions may be indicated:
1. Battery charged (light is off)
2. Battery needs charge (light blinks twice per second)
3. Battery is critically low (light blinks once per second)
4. Battery is charging (light on continuously)
NOTE
If the battery is so completely discharged that the
MAX782 VL output (+5V) falls out of regulation, the
charge light will remain off.
MAC™ 5000 resting ECG analysis system
2024917-010
Revision B