Xerox WorkCentre Pro 412 Service Manual page 64

Circuit Description
5-8-4-2. The Concept of Fixed Lamp Control
For fixed lamp control, the logic unit "fuser on" control signal and SMPS unit DC power must be supplied. 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 U501 LED, then the infrared ray is detected by U501
photo detector. Next, U501 triac is conducted.
The conducted current sends trigger input to triac THY501 gate. At this point, THY501 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 (THY501), which has advantage over one-way SCR considering the price, size and reliability.
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 switch-
ing operation is called commutation. It is possible to turn off the triac at the end of half cycle by eliminating 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 over-
shoot voltage, by the circuit. When triac commutes off-line, the voltage 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
5-38 WorkCentre Pro 412
August 2001
Launch Issue