(b) The output of U6-1 is then fed to U6-6 where it is inverted. The
dryer ramp may be viewed at TP2. After adjusting the developer ramp, R2 may be
adjusted to produce an amplitude of 3 volts p-p. This ramp has a period of 2 seconds
and goes from 4 to 7 volts
(2) Calibration amplifier. U7A and U8A are CMOS op-amps. They are used
dryer and to add the offset (calibration) voltage to the error signal for the dryer and
(a) The voltages present at TP-5 and TP-3 represent the amplified
error voltage of the dryer and developer, respectively, in normal operation. After the
unit has reached thermal equilibrium, the voltage will vary between 4 and 7vdc, with
respect to TP-1 (signal ground).
(b) Resistor R-3 controls the dryer scale off-set, and is used to cause
the dial scale for the dryer temperature to agree with the actual temperature of the air.
Similarly R-4 is used to calibrate the developer temperature.
(c) When the unit is warming up, and the temperature error is large,
the voltage at TP-5 and TP-3 will approach zero. The respective power circuits will call
for full heat, as limited by the current sharing circuit.
(3) Temperature comparator. Amplifiers U7B and U8B are configured as
high gain comparators. Resistors R21 and 24, respectively, provide hysteresis which
limits oscillations and provides some latching action.
(a) You may observe the operation of the dryer and developer
comparators at TP-6 and TP-4. Refer to figure 4-7 to see typical wave forms.
(b) Whenever the ramp voltage, which is applied to the inverting input
of the comparators (U7B and U8B) exceeds the DC input voltage from the previous
stage of this circuit (U7A and U8A), the output of the comparator goes low, and heat will
be called for, if the remaining logic conditions permit.
(4) Power sharing circuit. Power sharing is provided to prevent both the
developer and dryer heater from being turned on simultaneously. In addition, power
sharing provides preferential heating to the developer solution during start-up. This is
necessary due to the large volume of solution that must be heated. The dryers warm up
relatively quickly, at which time the power is apportioned, during the remaining
operation of the processor, according to the factory setup of the board. A series of
inhibit signals are present to ensure that power sharing takes place.