Rosemount 755A, Инструкция по эксплуатации

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E
LECTRONIC
C
IRCUIT
A
NALYSIS
6-8
April 2000 Rosemount Analytical 245364-U Model 755A Oxygen Analyzer
C35
.01uF
-
+
AR7
Q5
DS1
R66
1.0
2
3
+15V
120 V
RMS
6.1 VAC
CR7
CR8
C31
2000uF
R65
4530
R64
14K
R63
7.5K
VR3
9.0V
C34
.01uF
T1
α
2.2V
Q4
α +8.5V BUS
+
6.1 VAC
Amplifier AR7 has a fixed value, approxi2.2VDC on terminal 3. The output of
AR7 is positive, causing Q4 to conduct. As Q4 conducts, electrons flow from the
center-tap of T1 to ground and from ground through DS1 for an input voltage to
terminal 2 of AR7, through R66 to develop a bias on the base of Q5, through Q4 to the
+8.5V bus, and back to the secondary. As Q5 conducts, some of the current going
through DS1 is shunted from the main current path, and goes through Q5, which acts
as a variable feedback resistance, goes to the positive output potential of AR7.
As DS1 ages, its light emission decreases and its resistance increases. The current
through DS1 tends to decrease, causing a decrease in the voltage drop across DS1
and the input potential to terminal 2 of AR7. Now the output AR7 will increase, causing
Q4 to conduct more current through R66. As the potential across R66 increases, Q5
will conduct more current, causing a further increase in current flow through DS1. The
net result is that the voltage across DS1 will remain uniform and the operation of Q4
and Q5 will adjust the gain of AR7 to maintain the light emission from DS1 uniform for
a long period of time.
Voltage fluctuations in the 115VAC supply could cause some variation in the amount
of current flowing through the bulb DS1. However, the voltage drop across DS1 would
cause AR7 to adjust Q4 and the voltage drop across R66 to adjust Q5. The net result
would still be uniform current flow through DS1 and uniform light emission.
F
IGURE
6-6. D
ETECTOR
L
IGHT
S
OURCE
C
ONTROL
C
IRCUIT
6.6 DETECTOR WITH FIRST STAGE AMPLIFIER AND
PRESSURE COMPENSATION CIRCUITS
Refer to Figure 6-7. The detector assembly consists of a test body suspended on a
platinum wire and located in a non-uniform magnetic field.
The test body is constructed of two hollow glass spheres forming a dumbbell shape.
They are filled and sealed with pure, dry nitrogen. Around the test body, a titanium
wire is chemically etched in order to form a feedback loop that can create a