C
IRCUIT
A
NALYSIS
6-4
June 1997 Rosemount Analytical 748213-P
Model 755R Oxygen Analyzer
F
IGURE
6-3. R
AMP
G
ENERATOR
C
IRCUIT
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 non-inverting terminals, the transistor conducts. The output is -15 V. A
full 30 V drop appears across R77.
The potential on the non-inverting terminal will now be about -2.3 V. C37 will not
discharge through R78 until its potential exceeds that on the non-inverting 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 (See 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 RTR1 and R84 is
-15V
R74
590K
-
+3
R75
210K
R76
37.4K
C40
2200uF
C37
1.0uF
Q6
R80
10K
R79
10K
R81
56.2
C38
.18uF
R87
10K
R78
249K
R77
10K
TO
COMPARATOR
R83
63.4K
R84
169K
R82
9.09K
RT1
-15V
-15V to 1.88V
±
0.3V
+15V
+15V
-15V
INPUT FROM
MULTIVIBRATOR
-15V
OFF
OFF
OFF
-2.3V
+2.3V
6 Hz
T2
