Rosemount 755A, Инструкция по эксплуатации

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E
LECTRONIC
C
IRCUIT
A
NALYSIS
245364-U Rosemount Analytical April 2000 6-5
Model 755A Oxygen Analyzer
T1
FAN
MOTOR
MT2
RV1
MT1
HEATER
HR3
T1
NEUTRAL
HOT
120V
RMS
F2
Q7
Theoretically, at 135
°
F (57
°
C) the potential at the junction of RTR1 and R84 is
-1.85VDC. This is equivalent to a resistance of 21.2 K. By substituting a decade box
for the thermistor and placing 20.2 K into the bridge, the heater should be off. With
22.7 K, the heater should be full on.
Since the potential at the junction of R82 and R83 can vary between 1.85V and 1.92V
according to the 6 Hz ramp, and the potential at the junction of RT1 and R84 may vary
around or within these limits, depending on temperature, the error signal to comparator 4
may vary from 0 mV to some absolute value. The polarity of the error signal will depend on
the deviation from the desired temperature and the ramp value at the function of R82 and
R83.
The input from the OR circuit comparator (See Figure 6-1) is either -15VDC or the ramp
effect on the bridge. When -15V, the junction of R82 and R83 is also this value. The error
signal into comparator 4 is negatively large to the inverting terminal. Comparator 4 output
transistor does not conduct. The base of Q6 is positive; therefore, Q6 does not conduct
and a charge builds up on capacitor C38.
The input from the OR comparators 1 and 2, a form of multivibrator circuit, pulses 120
times a second. For about 100 microseconds the junction of R82 and R83 is some value
between -1.85V and -1.92V, depending on the ramp generator. For this brief period of
time (one pulse), comparator 4 compares the potential of junction R82, R83 with junction
RT1, R84 of the bridge circuit. If the temperature at RT1 is low, the potential at the
non-inverting terminal of comparator 4 is more negative and the output is -15V.
F
IGURE
6-4. C
ASE
H
EATER
C
IRCUIT