Rosemount 755, Инструкция по эксплуатации

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M
ODEL
755 O
XYGEN
A
NALYZER
6-4
748183-H Rosemount Analytical
December 1997
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, +15 V is at the output terminal. With +15 V at the
output, the potential on the noninverting terminals will be about
±
2.3V because of the
resistance divider R75, R76. Capacitor C37 will now start to charge positively through
R78. When the positive potential across C37 and at the inverting terminal of
comparator 3 exceeds the potential on the noninverting terminals, the transistor
conducts. The output is -15 V. A full 30 V drop appears across R77. The potential on
the noninverting terminal will now be about -2.3 V. Now C37 will discharge through
R78 until its potential exceeds that on the noninverting terminal. At that time.
comparator 3 will switch polarity and start charging C37 again. The result is that the
potential across C37 will vary almost linearly with time and form a ramp signal of about
6 Hz.
As the potential across C37 increases and decreases linearly, it affects the potential at
the top of the bridge circuit between R82 and R83 through R74. Because of the ramp
action charging and discharging C37, the potential between R82 and R83 varies
approximately from -1.85 V to -1.92 VDC.
The temperature-sensing device, RT1, in the bridge circuit is a thermistor. The bridge
is designed to control the temperature in the case at 135
°
F (57
°
C). When the
temperature is 135
°
F (57
°
C), the resistance of the thermistor RT1 will be at its lowest
and the potential at the junction of RT1 and R84 should be the same as the junction of
R82 and R83. Comparator 4 in Figure 6-4 does not allow pulses from the OR circuit
(comparators 1 and 2), to operate Q6 or TRIAC Q7 in the case heater, see Figure 6-5.
Theoretically at 135
°
F (57
°
C) the potential at the junction of RT1 and R84 is -1.85
VDC. 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 1.85 V and 1.92
V 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, Figure 6-1, is either -15 VDC 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 form multivibrator circuit, pulses 120 times
a second. For about 100 microseconds the junction of R82 and R83 is some value
between -1.85 V and -1.92 V, 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