Oce 3121 Technical & Service Manual page 208

Switching the charging roller ON and OFF
The charging roller is switched ON and OFF by the charging roller ON/OFF
signal (HVPDC) from the DC control PBA.
The primary side of the main transformer (T101) on the power supply PBA is
controlled by the CPU (Q605). When the HVPDC signal is sent to the power
supply PBA, the CPU (Q605) generates the DC control signal (PPWM) to
switch ON the secondary side of T101. A DC bias is applied to the charging
roller.
Then, the CPU generates the AC control signal (PAPWM) following the com-
mands created via the serial communications with the DC control PBA. As a
result, the transformer T300 applies AC to the charging roller.
Controlling the charging bias
The output values of the DC and the AC applied to the charging roller are con-
trolled by:
the DC control signal (PDPWM)
I
the AC control signal (PAPWM)
I
These are pulse signals, generated by the CPU (Q605).
For the DC bias, the output voltage value is returned to the CPU (Q605). This
continuously varies the duty ratio of the DC control signal. Thereby controlling
it to a specific voltage (constant voltage control).
For the AC bias, the current value of the AC output is sent to the AC drive cir-
cuit.
The AC drive circuit uses the reference signal (AC control signal PAPWM) and
the value of the AC current to generate signals to drive the transformer T300.
These limit the AC bias to a constant current.
Correcting the Apparent Surface Voltage on the Drum
The apparent surface voltage of the drum fluctuates in relation to changes in
the ambient temperature inside the machine. This is corrected by varying the
charge voltage. The temperature is measured by the thermistor, positioned on
the rear frame of the copier. The data is sent to the CPU on the DC control
PBA.
The CPU and the DC control PBA compute the data, and the results are sent to
the power supply PBA in serial communication mode.
The power supply PBA, in response, uses the data to determine the DC charge
level.
502 TSM Océ 3121 Functional description