Xerox WorkCentre Pro 412 Service Manual page 112

Circuit Description
5-10-6-4 The Concept of Fixed Lamp Control
For fixed lamp control, the logic unit "fuser on" control signal and SMPS unit DC power must be sup-
plied. This circuit turns on only when "fuser on" sends the signal and the DC power is supplied.
The following explains how the fixed lamp control circuit works.
logic unit "fuser on" sends trigger current to triac driver PC3 LED, then the infrared ray is detected by PC3
photo detector. Next, YC3 triac is conducted.
The conducted current sends trigger input to triac SY1 gate. At this point, SY1 is conducted and AC
power is supplied to fixed lamp. Lamp is turned on and temperature rises.
As this fixed lamp control circuit uses the AC voltage ("+" and "-" are repeated) as the power supply, it
used two-way triac (SY1), which has advantage over one-way SCR considering the price, size and reli-
ability.
Triac's gate can be triggered by either forward or reverse signal. Once triac is turned on, it will not be con-
trolled by gate signal, but will be continuously on until the current between major terminals decreases
below the holding current. In other words, you cannot turn it off with reverse signal unlike SCR.
This property is called current-voltage threshold rise rate (commutation: dv/dt). In AC power control
application, triac has to turn off conduction in each zero crossing or switch it twice in each cycle. This
switching operation is called commutation. It is possible to turn off the triac at the end of half cycle by elim-
inating the gate signal when the load current (IL) is gained at the level equal to or lower than holding
current. When triac commutes off-line, the direction of the voltage of the both ends of triac will be
reversed and increase up to the maximum value of line voltage (VAC). At this point, the width of rise rate
will be determined by dv/dt and overshoot voltage, by the circuit. When triac commutes off-line, the volt-
age of both ends of triac will have the same voltage as the line voltage.
I
L
Inductive I
L
V
T
V
AC
G
Inductive Circuit
5-74 3/03 WorkCentre Pro 412, FaxCentre F12 &
WorkCentre M15/M15i