Theory
91369 Service Manual 3-8
Charger CPU IC — This IC is a pre-programmed microprocessor designed specifically for controlling the
charging of batteries. The CPU monitors the voltage and resistance of the batteries while charging and
determines when to turn the charger power converter ON or OFF, and can detect faulty batteries. The CPU
periodically performs maintenance cycles to keep batteries fully charged.
Charger Power Converter — The power converter consists of a LT1511 step-down converter and various
discrete components. It is turned ON and OFF under the control of the charger CPU IC. When ON, it charges
the batteries with the maximum current that is allowed by the charge rate control circuit.
Charge Rate Control — The minimum battery charge rate is always controlled by the battery itself. A battery
will only accept the current it is capable of converting to charge inside the cells. However, the control for the
maximum battery charge current is controlled in one of two ways. If the monitor is powered from the power
supply and the front panel power switch is OFF, the maximum charge current defaults to the maximum current
that the batteries can take, which is 1.8 A. If the monitor is powered from the AC/DC power supply and the front
panel power switch is ON, the CPU will have control of the maximum battery charge rate via an analog switch
and digital potentiometer. The CPU monitors the power consumed by the monitor electronics and adjusts the
charge current based on the amount of remaining power available from the power supply.
Battery Charging LEDs — A pair of simple op-amp oscillator circuits controls the operation of the two front
panel battery charge status LEDs. When either battery is charging, a signal from the charging circuit causes
that battery's oscillator to run, which causes the corresponding front panel LED to flash. The two front panel
battery charge status LEDs are independent of the front panel power LED. When external power is available,
the power LED is constantly lit; this LED is off when external power is not available.
Battery Fault/Interrupt Signal — If the battery voltage exceeds the upper limit, the charger CPU IC will power
OFF the charger and then attempt to charge again. If the battery voltage exceeds the upper limit again, a 1 Hz
signal will be sent to an MPC8270 CPU interrupt input (via the ISA bridge). The software will sense this 1 Hz
rate and will indicate a battery failure. If the battery voltage is too low, the process is the same.
Automatic Fan Control — The MPC8270 CPU monitors internal temperature via temperature sensors and an
A/D. If the temperature exceeds a preset limit, the CPU will run the fan at a normal speed until the temperature
is reduced. However, during battery charging, the battery chargers override the CPU and force the fan on at a
higher than normal speed. This allows the battery chargers to run at full output without overheating the unit.
Note:
The fan will not operate unless a battery is installed.
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