Introduction; Explanation Of Circuits According To Function; Drum Surface Potential Control Circuit - Printronix L5535 Maintenance Manual

Drum Surface Potential Control Circuit

Explanation Of Circuits According To Function

The following pages provide an introduction to the structure of the main
electrical circuits of this printer.
Drum Surface Potential Control Circuit
Circuit action begins when the controller sends the MC board a command to
set the surface potential. This command contains a byte of data that
represents the desired drum surface potential. The CPU on the MC board
stores this data byte in the MC board data memory as a reference value, for
later comparison with data representing the measured drum surface potential.
Meanwhile, the drum surface potential sensor is continuously generating an
AC signal (approximately 640 Hz) whose amplitude corresponds to the
surface potential on the drum. The sensor sends this signal to the surface
potential measurement circuit on the drum surface potential sensor (DSPS)
board. In the surface potential measurement circuit, a low-offset operational
amplifier amplifies this weak signal from the sensor, then an absolute value
circuit rectifies and smooths the signal, before sending it to the MC board.
Finally, an analog-to-digital (A/D) converter on the MC board converts the
signal from an analog signal to a byte of digital data, then sends this
measurement data byte to the gate array.
The CPU on the MC board compares this measurement data byte with the
reference data byte previously stored in the data memory. Based on that
comparison, the CPU generates a byte of control data and sends it to the
main charger current control circuit on the MC board.
In the current control circuit, a digital-to-analog (D/A) converter converts this
byte of digital control data to an analog signal, then an operational amplifier
scales the signal to a range of 0 to +12 volts (+HV1 CONT), before sending it
to the high-voltage power supply.
When the MC board determines that the main charger should be energized, it
asserts the main charger remote ON/OFF (+HV1 REM) signal to the high-
voltage power supply, and the high voltage power supply sends current
(+HV1) to the main charger. The magnitude of this current corresponds to the
control voltage (+HV1 CONT), and falls within the range of 100 to 1000 µA.
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