E
LECTRONIC
C
IRCUIT
A
NALYSIS
245364-U Rosemount Analytical April 2000
6-7
Model 755A Oxygen Analyzer
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
and increases rapidly as the temperature decreases. R59 in this bridge circuit represents
the setpoint value for temperature. Suppose that, at temperature, resistance of the bridge
(R55, R56, R59 and RT1) equals 149 K.
If the temperature goes down, RT1 increases in resistance and causes the junction of RT1
and R59 to go positive in voltage value. Since R55 and R56 are of equal resistance, their
junction is at zero volts. Therefore, terminal 3 of AR6 is more positive than terminal 2 and
the base of Q2 is positive. Q2 conducts, allowing alternating current to flow through
heaters 1 and 2. The voltage drop across the heaters, when completely cold, would be
about 20VAC and, when controlling, would be AC of very low amplitude.
As the temperature increases, the resistance of RT1 decreases and the junction point
between RT1 and R59 becomes less positive. Terminal 3 of AR6 becomes less positive
with respect to terminal 2. The output of AR causes Q2 and Q3 to conduct less. When
terminal 3 equals terminal 2, or is less than terminal 2, the output of AR6 is zero or less.
Q2 and Q3 do not conduct and the heater would not be supplying heat energy to the
detector.
F
IGURE
6-5. D
ETECTOR
H
EATER
C
ONTROL
C
IRCUIT
6.5 DETECTOR LIGHT SOURCE CONTROL CIRCUIT
Refer to Figure 6-6. 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
appro8.5V bus to the current-limiting Darlington configuration of Q4. Q4
controls the basic amount of current through DS1.
