GE MAC 5000, Service Manual

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Revision B MAC
™
5000 resting ECG analysis system 2-35
2024917-010
Equipment Overview: Theory of Operation
Lid Open Detection
A self-aligning connector routes power and motor signals from the power
supply compartment to the CPU board. When the lid is closed the
DOOROPEN signal is shorted to ground. When the lid is open a pull-up
resistor ensures a high level on DOOROPEN. Moe monitors this line to
detect lid open conditions that are reported to the system software to
avoid misinterpretation of motor fault indications. When the door is
open, the motor connections are lost and Larry receives no tachometer
feedback from the motor. Without knowing the cause of the lost
tachometer info, Larry can only respond with a paper jam condition.
Moe’s knowledge of the lid state is used to suppress this error message as
well as prevent further print operations.
AC Power Monitor
Moe senses the presence of AC power through a voltage divider (R102,
101) which drives the under-voltage detection comparator in the Battery
Charger (Vtrip = approx. 7V). The battery charger will not be enabled
unless the DC power supply voltage is above approximately 21V.
Battery Pack
The MAC 5000 uses a 15-cell nickel metal hydride (NiMH) battery pack
with integral thermal sensor for charge termination detection and self-
resetting thermal fuse for short circuit protection. Charge current and
normal system operating power are obtained from the AC power supply.
The charger circuitry monitors both battery charge current and power
supply output current. The battery is always charged at the maximum
rate possible but system power demands take precedence over charger
demands. The charger automatically reduces charge current as required
to keep the AC power supply output current within specified limits. In
the extreme (during printing) charging ceases and energy is taken from
the battery to meet peak system demands. When system power draw
declines, all excess power supply capacity is once again delivered to the
battery.
Battery Temperature Sensor
Moe uses a thermal sensor inside the battery pack to determine when to
terminate charge. During normal charge, the electrical energy obtained
from the power supply is stored in chemical reactions in the battery.
When the battery reaches full charge there are no more reactants
available in which to store chemical energy and the supplied charge
power is converted directly to heat. The sudden rise in pack temperature
caused by this release of heat is an indicator of full charge. When the
rate of pack temperature rise exceeds a certain threshold, charge is
terminated. This is the only normal charge termination mechanism.
Fully drained battery may give higher temperature rise for initial few
minutes. To avoid the premature termination of charge, the threshold is
set at a higher level for first five minutes. Abnormal conditions such as
battery or ambient temperatures beyond spec, or excessive pack voltage,
may also terminate charge. Once fully charged, the battery is
maintained by low duty cycle charge current pulses.