Rosemount 755, Instruction Manual

Page: 71 / 94

C
IRCUIT
A
NALYSIS
748183-H
6-9
Rosemount Analytical
December 1997
Resistances R5, R17 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 (DSI) to the photocells
(BTI, 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. R8, C3 and R6, and C2
form damping circuits for the input amplifier UI and to smooth out noise that might be
introduced by the measurement source.
R8, C3 and R6, and C2 form damping circuits for the input amplifier U1 and 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 C6 has no charge. The current will have to flow
through R18. Initially the full 30-V drop (the difference between the +15 VDC and -15
VDC power) will appear across R18. 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.
As the charge on C6 increases, the cathode of CR2 becomes more positive. When it
exceeds that on the anode, CR2 ceases to conduct and isolates the +15 VDC and -15
VDC power supply from the input circuit.
If the measurement span is zero-based (0% to 10% for example), a simple voltage
from front-panel ZERO potentiometer R10 may be added to the input of U2 to
counteract any electrical offsets that may occur because of any imbalance in the
detector and the photocells BTl and BT2. If the span is elevated (11% to 21% for
example) or, in other words, the zero is suppressed, a zero suppression module is
added to the circuitry around potentiometer R10. The modified potential from R10 is
added to the input value to U2 to accomplish a balance at the lower limit of the
particular measurement range.
Amplifier U3 receives the output of Ul (0 to -70 mV to 0 to -1.0 V) and amplifies this
value. The output of U3 is always 0 to +7.5 V. This is accomplished by RANGE Switch
SW1, which selects some portion of the output and supplies this value as feedback to
the input of U3. Adjustment of the input resistance R4 gives span trim adjustment once