Xerox WorkCentre M20 Service Manual page 175

When THV-EA is 'L', Q302 turns on and the VCE electric potential of Q302 will be formed and The changes in transfer output required by each load is controlled by PWM-DUTY.
sends IB to TRANS auxiliary wire through R311, C305 and VR302 via current-restricting resis-
tance R314, and eventually Q303 will turn on. When the current is accepted to Q303, Q303's Ic
increases to the current proportional to time through the T301 primary coil, and when it
reaches the Hfe limit of Q303, it will not retain the "on" state, but will turn to "off". As Q303
turns 'off', TRANS N1 will have counter-electromotive force, discharge energy to the second-
ary coil, send current to load and output THV(-) voltage through the high voltage output enable,
which is comprised of Regulation-circuit. Refer to Figure 8.
Figure 8 THV signal
Environment Recognition
THV voltage recognizes changes in transfer roller environment and allows the voltage suitable
for the environment in order to realize optimum image output. The analog input is converted to
digital output by the comparator that recognizes the environmental changes of the transfer
roller. It is to allow the right transfer voltage to perform appropriate environmental response
considering the environment and the type of paper depending on this digital output by the pro-
grams that can be input to the engine controller ROM.
This environment recognition setting is organized as follows: First, set the THV(+) standard
voltage. Allow 200M load to transfer output, enable output and set the standard voltage 800V
using VR201. Then set 56 (CPU's recognition index value) as the standard using VR302. This
standard value with CPU makes sure that the current feedback is 4A when output voltage is
800V and load is 200M. If the load shows different resistance value when 800V is output, the
current feedback will also be different and thus the index value will also be different. according
to the index value read by CPU, the transfer voltage output will differ according to the preset
transfer table.
General Procedures and Information
March 2004
GP 16
Work Centre M20 Family 6-31