C
IRCUIT
A
NALYSIS
748213-P Rosemount Analytical June 1997
6-9
Model 755R Oxygen Analyzer
collects in the non-uniform magnetic field around the test body. Oxygen, because of
its paramagnetic qualities, gathers along the magnetic lines of flux and forces the
dumbbell of the test body out of the magnetic field.
A light source is focused on the test body mirror. As the test body moves out of the
magnetic field, the mirror distributes light unevenly on two photocells (BT1 and BT2).
The photocells create a current proportional to light. This current is converted to a
±
voltage by U1 and U2 located on the connector board in the detector housing. This
voltage is then presented to comparator U1 on the controller board. The output of U1
goes to U2. The output of U2 causes current to flow through the feedback loop
attached to the dumbbell.
This feedback current creates an electro-magnetic field that attracts the dumbbell and
mirror into the test assembly magnetic field until the mirror reflects light almost
uniformly on each photocell. A current proportional to the oxygen concentration in the
magnetic field of the test assembly has to be flowing through the feedback loop in
order to maintain balance and provide a reading of the oxygen content of a sample.
Resistances R7, R8 and the resistance of the wire in the feedback loop determine the
gain of amplifier U2. The mirror on the dumbbell is positioned by the amount of current
in the feedback loop. The mirror reflects light from the source (DS1) to the photocells
(BT1, BT2). This repositioning of the mirror is a form of mechanical feedback to the
input of the amplifier U1.
The net result is that the output of U1 could vary from 0 to -70 mV, or 0 to -7.0 V,
depending on the range of the instrument. R4, C3 and R5, C7 form damping circuits
for the input amplifier U1 and to smooth out noise that might be introduced by the
measurement source.
Diode CR2 is a low-leakage device. Its purpose in the circuit is to ensure that the
dumbbell and mirror are positioned correctly with respect to the photocells on initial
application of power.
If the dumbbell was out of position on start-up, the mirror might reflect light from the
source onto one of the photocells. If the photocell output was positive, the current in
the feedback loop would be in the wrong direction and its electromagnetic field would
cause the dumbbell to be further repelled from the permanent magnetic field. The
result would be error, not balance.
On application of AC power, capacitor C1 has no charge. The current will have to flow
through R2. Initially the full 30 V drop (the difference between the +15 VDC and -15
VDC power) will appear cross R2. The cathode of CR2 will be initially at -15 VDC.
The anode of CR2 will be some value more positive than -15 VDC. CR2 will conduct.
The input terminal of U1 will be negative and the current through the feedback loop
around U2 will cause the dumbbell and mirror to be positioned correctly in the test
body.
