A constant current cleaning power pack is used. --20µA
for the cleaning roller bias and --10µA for the scavenging
roller bias are applied. The larger negative bias for the
scavenging roller attracts the positively charged toner
from the cleaning roller to the scavenging roller.
To make the cleaning roller bias constant, a varistor is
used. The bias is approximately --83 volts, and
excessive current is discharged through the varistor.
As the cleaning carrier wears out, the electrical
resistance of the carrier increases. Because a constant
current power pack is used, the scavenging roller bias
increases. If the carrier is new, the scavenging roller is
approximately --340 volts. (If the varistor is removed, the
cleaning roller bias is approximately --170 volts.)
For the cleaning roller bias, if the varistor is not used and
the cleaning carrier wears out, too much bias voltage will
be applied to the toner collected by the cleaning roller.
Because a high negative voltage is applied to the
positively charged toner, the collected toner on the
cleaning roller might be re-attracted to the OPC surface
which is given a positive charge by the PCC. This will
appear on copies, especially on the leading edge, as a
faint dirty background. The varistor prevents this.
NOTE
If the copy volume is extremely low and if often only
a few copies are made from an original, the period
of the main motor rotation for each copy becomes
longer. This motor also drives the cleaning unit and
the cleaning carrier wears out faster than usual. In
such a case, no cleaning bias voltage is applied
around the end of the cleaning carrier life time
(400,000 copies) because all current is discharged
through the varistor due to too high resistance of
the carrier. As a result, a dirty background appears
on copies.
If this occurs, replace the cleaning carrier with new
one earlier than usual. Otherwise, remove the
varistor from the cleaning unit. For such a low copy
volume customer, the toner re-attraction to the OPC
does not occur even without the varistor.
Varistor
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