Table 3-6 Explanation Of Heater Operation Flowchart - Canon NP-70 Service Manual

NP•70
3-55
Block
Actions Occumng
As the copier is turned ON, the START operation is begun. Simultaneously, the
heater control circuit begins operation to permit heater operation until the correct
temperature is reached.
SW_
"ON"
Initially, as the heater temperature is low, the circuit responds by generating a "HI"
output. from Q40. Due to the +20VDC level of Q40 ~6 ,capacitor C28 begins
charging.
STEP 1H0
As the charge of C28 reaches the operation voltage level sufficient to cause
unijunc-
tion transistor Q41 to turn-0N referred to here as the Q41 Gate), conduction begins
through Q41 and C28 immediately discharges.
The flow of current (during the above mentioned discharge) causes the development
of a pulse in the primary winding of transformer TP.
The pulse is immediately
felt in the secondary winding of the transformer and results in the turning ON of
triac Q42 for 1 /2 alternation of the input A. C. voltage.
It should be noted that the discharge time of C28 is almost instantaneous and, thus,
Q41 is only ON for a short period. As soon as Q41 turns OFF, C28 begins charg-
ing again to resume the pulse generation cycle.
STEP 2H0
As soon as the heater temperature reaches the correct level, Q40 responds by de-
veloping a "LO" output of OVDC. Consequently, capacitor C28 can no longer
charge to turn-on Q41 to develop a trigger.
Operation of the heaters (H2 & H3 only) will be prevented until the temperature
drops below the correct range.
STEP 3H0
The operation of the heater control circuit is now fixed until the temperature of
the Heater Assembly drops sufficiently to signal a change to the "LO" temperature
state.
END
Table 3-6 Explanation of Heater Operation Flowchart
© CANON INC. Second Printing,lune, 1974