Auto Power Control (Apc) - Ricoh A193 Service Manual

3.4 AUTO POWER CONTROL (APC)

L D 5 V
IC 1
D A T A
D A T A
L E V E L
L D E R R
L D O F F
Even if a constant electric current is applied to the laser diode, the intensity of
the output light changes with the temperature. The intensity of the output
decreases as the temperature increases.
In order to maintain a constant output level, the output light intensity is
monitored through a photodiode (PD) enclosed in the laser diode. The
photodiode passes an electrical current that is proportional to the light
intensity. The output is not affected by temperature, so it faithfully reflects the
changes in the LD output, without adding anything itself.
Just after the machine is turned on, IC2 on the laser drive board excites the
laser diode at full power and the output of the photodiode is stored as a
reference in IC2. IC2 monitors the current passing through the photodiode
(PD). Then it increases or decreases the current to the laser diode as
necessary, comparing it with the reference level (LVL2). Auto power control is
performed during printing while the laser diode is active.
The laser diode level is adjusted on the production line. Do not touch the
variable resistors on the LD unit in the field.
SM
IC 3 IC 2
L D 1
L V L 2
P D
L D O F F
L V L 1
E R R
2-21
LASER EXPOSURE
LD Drive Board
LD
A193D521.wmf
(A193)