IC6-A and IC6-B are voltage comparators having as their inputs a voltage corresponding
to the desired set point and the output of the pressure amplifier. When the pressure is lower
than the set points selected the amplifier switches from positive saturation to negative
saturation turning on the relay driver transistor and energizing the relay. Each comparator
has regenerative feedback which causes hysteresis of approximately 15% of a decade.
5.11 Thermocouple Circuit (Refer to Fig. 5-8)
The thermocouple circuit consists of a transducer power supply, two identical amplifiers
IC 11 and IC 12, and meter circuits. Only one circuit will be discussed since operation is
identical.
Power to heat the filament of the GPC 270006 transducer is regulated by a pair of back
to back zeners, CR3 I and CR32, whose output signal is an approximate square wave as shown
in Fig. 5-8. This signal is coupled to the thermocouple by transformer T2. R49 is used to
adjust the output of the thermocouple to 10 mV de when the pressure at the thermocouple is
less than I x 10-3 Torr. Amplifier ICI I is a low drift, inverting amplifier with a DC gain of
200. Initial adjustment of the amplifier is made by adjusting R54 for an output of +2.0V de
with the 270006 transducer at atmosphere. At a pressure of I x 10-3 Torr or lower the output
of the amplifier is reduced to zero due to the + 10 mV input from the thermocouple. The
amplifier output is used to power the meter.
5.12 Thermocouple Process Control (Refer to Fig. 5-9)
The thermocouple process control circuit consists of two identical voltage comparators
ICl3-A and ICl3-B, and relay drivers 021 and 022. Again only one channel will be discussed
since they are identical. The output of the thermocouple amplifier is compared with the
output of the set pot through resistors RA 12-1 and RA 12-2. Voltage comparator IC I3-A is at
negative saturation when the pressure is above the desired set point. When the pressure input
decreases to the desired set point the output of the amplifier switches to positive saturation
turning on the relay driver and energizing the relay. The comparator has regenerative
feedback which provides hysteresis operation.
The output of ICl3-A
is
also used to control the filament relay (refer to Fig. 5-9).
5-5
