Canon NP2120 Service Manual page 73

F. Measuring the Document
Density
1. Outline
The copier has an automatic
mechanism, which controls the DC component of
the development bias according to the density of the
document. If the document is uniform in density, the
development bias to enable reproduction of fog-free
copies.
2. Mechanism
The scanner is moved forward about 70 mm from
the image leading edge with the scanning lamp kept
the output is sent to the DC controller.
Based on the output from the AE sensor, the DC
controller controls the DC component of the devel-
opment bias.
exposure (AE)
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Figure 3-31
C A N O N NPZt20/2020 NN. J A N . 1992 PRINTEO I N J A P A N IIMPRIME At, JAPON, d
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Figure 3-30
3. Fteading the AE Sensor Output
The AE sensor consists of a photodiode (Q702)
and operation amplifier (Q701); when the light re-
ceived by the photodiode (reflected by the docu-
ment) is strong, the output voltage (AE) of the
operation amplifier is low; on the other hand, if the
The DC controller reads the output (AE) of the AE
sensor based on the zero-cross signa1 (ZXDP) sent
by the lamp regulator.
Thelimitercircuitconsistsof Q701 (2) anddiodes
and serves to cut AE output of 5 V or more to prevent
be assumed; if the AE output rises above the setting
of VR2. it is lowered to maintain the setting.
D C controller P C B