Canon LBP-2000 Service Manual page 61

CHAPTER 2
c. Detecting the toner level and the presence/absence of a cartridge
A toner level sensor is mounted inside the cartridge.
The Printer collects readings (Ant) of the toner level sensor mounted inside the cartridge and
the output (Dv) of the developing bias, and subjects them to A/D conversion.
The CPU checks the difference between the cartridge detection signal (TNRCHKD) and the toner
detection signal (TNRCHKT) occurring after A/D conversion when the developing bias is applied to
find out the level of toner and the presence/absence of a cartridge.
The presence/absence of a cartridge is checked only during a warm-up rotation interval. The
level of toner is checked while the developing bias is applied during a warm-up interval and during
an initial rotation interval. If the absence of toner is detected during control, retry control will be
executed a maximum of three times. (Note 1)
If the absence of toner is detected for the three retry operations, the Printer will indicate the
message 'READY TONER LOW' in its control panel.
Note: 1. If retry operation is executed three times as the result of toner level detection at power-
on, about 40 sec of wait time will be required.
2. As many as about 100 pages may be printed after the message 'READY TONER LOW' is
indicated (i.e., about 60 g of toner remains).
d. Transfer
DC controller PCB
/TRPWM
ASIC
/TRPFOT
TRCHG
/TRNFOT
TRCRNT
CPU
A negative bias, sheet-to-sheet bias, or positive bias is applied to the transfer charging roller to
suit the ongoing sequence. The negative bias is applied at such times as needed to move the toner
from the transfer charging roller to the photosensitive drum by way of cleaning the roller. The
sheet-to-sheet bias is applied at such times as necessary, is of a voltage lower than that used for
printing, and is used to prevent residual toner on the photosensitive drum from sticking to the
transfer charging roller. The positive bias is controlled as necessary, is a positive voltage, and is
used to transfer toner from the photosensitive drum to paper.
2 - 22
Transfer bias circuit
Comparison
detection circuit
circuit
generation circuit
Constant
current/constant
voltage switch circuit
Negative voltage
generation circuit
Figure 2-2-12
Positive current
Positive voltage
Superimposition
Tr T403
To transfer
charging roller