Service manual

Power Supply
3-10
Rectifier/Filter
This circuitry rectifies and filters the AC voltage to produce a DC voltage for the rest of
the power supply.
Switch-Mode Battery Charger
The battery charger converts the output from the Rectifier/Filter to one of two levels. The
first level is called the float voltage and equals 6.85 VDC (nominal at 25 C). The second
level, called the overcharge voltage, is 7.4 VDC (nominal at 25 C). Both of these levels
are temperature compensated. The charger is capable of delivering a maximum of 1.5A at
its output in either the float mode or overcharge mode. The charger is a pulse width
modulated switching type that has a nominal switching frequency of 100 KHz.
The charger enters the overcharge mode when any one of the following conditions are
met: VBAT drops below 6.1V (nominal at 25 C), the instrument is turned on, a print
operation starts, or the instrument is plugged into AC power. The charger changes into the
float mode when the current into the battery is less than 80 mA.
Battery
The battery is a lead-acid 6 volt unit, providing 6.5 Amp-Hours of current when fully
charged.
VPRINT Boost Regulator
The VPRINT Boost Regulator converts its input voltage to the 26 V supply required for
operation of the thermal printhead.
Switched +6V Boost Regulator
The boost regulator is activated by turning the instrument On. It eliminates the heavy
transients induced into the +5V supplies by the printing subsystem during normal printing.
The output from this switching regulator is the input to both of the +5V Linear Regulators
when the instrument is On. When the instrument is in Standby mode, the battery powers
the unswitched +5V Linear Regulator.
5V Linear Regulators
There are two regulators, both linear, described here. The first is the unswitched +5V
regulator that provides power for circuitry active in the Standby mode. The System Gate
Array circuitry active in Standby mode includes the real time clock.
The second +5V Linear Regulator provides the bulk of the operating power for the rest of
the instrument when the instrument is On. This regulator is off when the instrument is in
Standby mode.