C
IRCUIT
A
NALYSIS
748213-P Rosemount Analytical June 1997
6-7
Model 755R Oxygen Analyzer
F
IGURE
6-4. D
ETECTOR
H
EATER
C
ONTROL
C
IRCUIT
6.5 DETECTOR LIGHT SOURCE CONTROL CIRCUIT
Refer to Figure 6-5. The detector light source control circuit maintains the light output
from the bulb (DS1) as uniform as possible, regardless of voltage fluctuations or aging
of the bulb.
The power source for the light bulb is a center-tapped secondary of transformer T1.
This AC voltage is rectified by CR7 and CR8 and filtered (C32), presenting an
appro8.5 V bus to the current-limiting Darlington configuration of Q4.
Q4 controls the basic amount of current through DS1.
Amplifier AR7 has a fixed value, approxi2.2 VDC 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
+8.5 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 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
25 VAC
C31
.01uF
-
+
U6
R56
149K
Q2
CR12
120 V
RMS
Q3
R60
100
R61
2.0
R62
1K
R58
5M
R88
5M
R55
700K
R59
700K
RT1
+15V
-15V
2
3
6
HR1 +2
F1
CR6
WO4
