B245/B276/B277/B268/B269 Service Manual 18-Jan-06
225
Charge Roller Voltage Correction
Correction for Environmental Conditions
With a drum charge roller system, the voltage transferred from roller to drum varies with the
temperature and humidity around the drum charge roller. The higher the temperature or
humidity is the higher the applied voltage required.
To compensate, the machine uses the ID sensor to measure the effects of current
environmental conditions. For this measurement, the process control parameters are
balanced so that any small change in drum potential caused by environmental effects is
reflected in a change in the amount of toner transferred to the drum.
This measurement is made immediately after the ID sensor pattern for toner density control.
Immediately after making ID sensor pattern [A], the charge roller voltage stays the same, but
the development bias goes up to –600 V; as a result the drum potential is reduced to –650 V.
The laser diode is not switched on, and the drum potential is now slightly higher than the
development bias, so a very small amount of toner transfers to the drum. The ID sensor
measures the density of this pattern [B], and the output voltage is known as Vsdp. This
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