T
HEORY
245364-U Rosemount Analytical April 2000
5-9
Model 755A Oxygen Analyzer
Zero suppression is obtained as the sum of (1) a jumper selectable, fixed value of 0%,
20%, 40%, 60%, or 80% and (2) a continuously adjustable value of 0% to 25%.
The scale expansion factor is jumper selectable for 1, 10, or 100.
Potentiometric output is jumper selectable for 0 to 10mV, 0 to 100mV, 0 to 1V, or 0 to
5VDC.
5.3.3 C
ASE
B
OARD
The Case Board contains power supply and temperature control circuits. The board is
mounted inside of the analyzer case (see Figure 1-2).
The various circuits operate on main power transformer T1. The two primary windings
of T1 are configured (at the factory) for operation on either 120 VAC or 220 VAC.
The Case Board consists of the following:
S
OURCE
L
AMP
P
OWER
S
UPPLY
This circuit provides a regulated output of 2.30 VDC to operate incandescent source
lamp DS1 within the optical bench assembly. One secondary of main power
transformer T1 drives a fullwave rectifier consisting of CR7 and CR8. The output of
DS1 is held constant by a voltage regulator circuit utilizing U7, Q4, and Q5.
±±±±
15V P
OWER
S
UPPLY
This circuit provides DC voltages required for various amplifiers and other circuits.
Fullwave rectifier bridge CR5 provides both positive and negative outputs. Each is
routed through an associated series-type integrated circuit voltage regulator, providing
regulated outputs of +15V and -15V.
D
ETECTOR
T
EMPERATURE
C
ONTROL
This circuit maintains the detector at a controlled temperature of 150
°
F (66
°
C).
Temperature is sensed by RT1, a resistance element permanently attached to the
detector assembly.
The signal from the sensors is applied to amplifier U6, which drives transistors Q2 and
Q3, thus controlling application of DC power from fullwave rectifier bridge CR6 to the
two heaters within the detector/magnet assembly; HR1, mounted on the top of the
magnet, and HR2, mounted permanently on the rear of the detector assembly.
C
ASE
T
EMPERATURE
C
ONTROL
This circuit maintains the interior of the analyzer case at a controlled temperature of
140
°
F (60
°
C). Temperature is sensed by a thermistor on the Control Board adjacent
to critical electronic components. The circuit provides on-off control of heater element
HR3 via TRIAC Q7. Heater HR3 is located in the heater/fan assembly.
