Samsung SCX-5315F Service Manual page 159

Circuit Description
3-8-4-2. 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 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 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 prop-
erty 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 overshoot 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.
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Samsung Electronics
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