Samsung ML-920 Service Manual page 172

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 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 sup-
plied 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 controlled by gate sig-
nal, 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 eliminating the gate signal when the load cur-
rent (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.
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<Inductive Circuit>
Repair Manual
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Samsung Electronics
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