Rosemount 755, Инструкция по эксплуатации

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C
IRCUIT
A
NALYSIS
748183-H
6-7
Rosemount Analytical
December 1997
C35
.01uF
-
+
U7
Q5
DS1
R66
1.0
2
3
+15V
120 V
RMS
6.1 VAC
CR7
CR8
C31
2000u
F
R65
4530
R64
14K
R63
7.5K
VR3
9.0V
C34
.01uF
T1
a
2.2V
Q4
α
+8.5V BUS
+
6.1 VAC
CASE BOARD
tap of T1 to ground and from ground through DS1 for an input voltage to terminal 2 of
U7, through R66 to develop a bias on the base of Q5, through Q4 to the +8.5 V 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 U7.
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 U7. Now the output U7 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 U7 to maintain the light emission from DS1 uniform for a
long period of time.
Voltage fluctuations in the 120 VAC supply could cause some variation in the amount
of current flowing through the bulb DS1, however the voltage drop across DS1 would
cause U7 to adjust Q4 and the voltage drop across R66 to adjust Q5. The net result
would still be uniform current flow through DSI and uniform light emission.
F
IGURE
6-5. D
ETECTOR
L
IGHT
S
OURCE
C
ONTROL
C
IRCUIT
6.5 DETECTOR WITH FIRST STAGE AMPLIFIER
The detector assembly consists of a test body suspended on a platinum wire and
located in a nonuniform magnetic field, Figure 6-6.
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
counteracting magnetic force to the test body displacement caused by oxygen
concentration in the test assembly magnetic field.
Attached to the center arm of the test body dumbbell is a diamond-shaped mirror.
Attached to the mirror are two separate platinum wires in tension with the supports for
the test body. The supports are isolated from ground and are electrically connected to