GE MAC 5500 Service Manual page 51

Revision E
Equipment Overview: Theory of Operation
Moe enables / disables the charger via CR102. When Moe pulls the CHRGTRL
line low, CR102 sinks current from the Battery Charger's VC pin shutting down the
error amplifier and disabling switching. R119 ensures that the charger remains off
when Moe is starting up.
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, R103)
which drives the under-voltage detection comparator in the Battery Charger (Vtrip =
approximately 7V). The battery charger will not be enabled unless the DC power
supply voltage is above approximately 21V.
Battery Pack
The MAC 5500 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.
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.
MAC 5500 resting ECG analysis system
2020299-020
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