Rosemount 755R, Инструкция по эксплуатации

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C
IRCUIT
A
NALYSIS
6-2
June 1997 Rosemount Analytical 748213-P Model 755R Oxygen Analyzer
When the non-inverting terminal of comparator 2 is less positive (more negative) than
the inverting terminal, the transistor conducts and the output of the comparator is -15
V. This value is the output of the OR circuit.
Comparator 2 is biased at 0 volts on the inverting terminal. Comparator 1 is biased at
about 159 mV on the non-inverting terminal. Positive feedback or hysteresis is built
into each comparator circuit for stability or positive action. This is achieved by the 20
M resistances, R70 and R73.
An approximate 8 V peak-to-peak AC signal is applied to comparators 1 and 2. As the
signal starts going positive, comparator 2 transistor ceases conducting and
comparator 1 transistor is off.
When the signal exceeds the +159 mV on the non-inverting terminal, it turns on
comparator 1 and the output is -15 V. Comparator 1 stays on until the signal drops
below +159 mV, at which time the output will be the value of the OR bus.
As the AC signal goes negative with respect to ground, the transistor of comparator 2
conducts and the output is again -15 V. The output remains at -15 VDC until the
incoming signal crosses zero value and the positive signal causes the comparator 2
transistor to cease to conduct.
Summing the effects of the two comparators in the OR circuit results in no output from
the comparators for about 4
°
of the sine wave, 2
°
after the signal goes positive (0 to
2
°
) and 2
°
before the positive signal reaches 180
°
(178
°
to 180
°
).
During the period that neither comparator is conducting, the value on the OR bus is
the potential from the temperature-sensing bridge plus the effect of the ramp
generator, probably -1.88
±
0.03 V.
The on-off effect of the comparators to the OR circuit results in application of a
positive-going pulse (from -15 V to -1.89 V) to the temperature bridge at the rate of
120 pulses per second.
Capacitor C36 is added to the input circuit to delay the incoming AC signal so that the
pulses will occur at or just after the line frequency crossover point.
Circuits for a ramp generator and a temperature-sensing bridge are part of the case
heater control circuit (See Figures 6-2 and 6-3).
On initial application of power to comparator of Figure 6-2, no potential exists on the
inverting terminal because no charge exists on capacitor, C37. If the transistor of
comparator 3 does not conduct, +15V is at the output terminal. With +15V at the
output, the potential on the non-inverting terminals will be about
±
2.3 V because of the
resistance divider, R75, R76.