Printer Switch; Battery Switch; Acon Detector - HP PageWriter XL M1705B Service Manual

Printer Switch
This block consists of two separate circuits: the printer switch and
the voltage reference.
The printer switch provides battery power, VBAT, to the printhead
and printer motor. The signal name for the switched power is
VPRINT. The switch, which consists of three power FETs, closes
when +5READY goes high. A voltage divider scales down VPRINT
to produce VBATMEAS. The print circuitry uses VBATMEAS to
update the front panel battery level indicator, to initiate low battery
shutdown if necessary, and to provide printhead voltage to the printer
gate array.
Battery Switch
This circuit connects the battery (VBAT) to VIN when the
cardiograph is not connected to AC power. This provides power to
the +5 and patient module voltage regulators and to the preview
power supply. The switch, which consists of three FETs, closes when
SWITCHON (f rom the ACON circuit) is high.
A diode in the circuit isolates VIN from the battery during printer
current surges.
ACON Detector
This circuit generates the SWITCHON signal that controls the
battery switch, and the CHARGEROFF signal that controls the
battery charger. When the cardiograph is running on battery
power, the CHARGEROFF and SWITCHON signals are high,
which disables the battery charger and enables the battery switch,
respectively. When running on AC power, CHARCEROFF and
SWITCHON are low, enabling the battery charger and disabling the
battery switch. The ACON circuit samples AC voltage directly off
of the main transformer secondary, rectifes it, and compares it to a
reference voltage from the backup supply. ACON is high when AC
power is on.
ACONLED drives the AC indicator LED on the front panel.
5-20 Theory of Operation