Rosemount 755, Instruction Manual

Page: 72 / 94

M
ODEL
755 O
XYGEN
A
NALYZER
6-10
748183-H Rosemount Analytical
December 1997
the range has been selected by Range Switch SW1.
The output of U3 is picked off between R20 and R26 and brought into the final
amplifier. The wiper of potentiometer R20 picks off a potential that helps give a little
phase lead to the measurement circuit.
6.6 FINAL OUTPUT AMPLIFIER
The output of U3, in terms of 0 to +7.5 VDC for a zero to 100% change in
measurement span, goes to the resistance divider circuit: R20 and R26, see Figure 6-
7. The potential at the junction of R20, R26, and R29 for 100% of span would be
about +0.465 V. This value is applied to the noninverting terminal of U4. The output of
U4 causes Q1 to conduct. The voltage at the emitter of Q1 and its junction with R32
should vary from 0 to +5 VDC for a change of zero to 100% in measurement span.
When the voltage at the emitter is +5 volts, the junction between R30 and R32 is
±
0.465 V. This value is fed back through R31 to the inverting terminal of U4. This
feedback value balances the input.
As the input measurement varies, the 0 to +7.5 V output of U3 varies proportionately.
The junction of R20 and R26 changes between 0 and +0.465 V, causing the output at
the emitter to vary from 0 to +5 V. This causes the junction between R30 and R32 to
move between 0 and +0.465 V, to balance U4.
The wiper of R20 picks off a higher voltage value than that at the junction of R20 and
R26. Under stable conditions, the difference between these two values appears across
the capacitor C11, and the input to U4 is the value at the junction of R20 and R26. If
the measurement increases, the wiper of R20 immediately picks off a higher value,
which is transferred through C11 to U4, causing the output of Q1 to give a quicker
indication of a change at the meter or recorder. Capacitor C11 will charge up to the
new difference between the potential at the wiper of R20 and the junction R20, R26.
The amount of phase lead will depend on the R29, C11 time constant and the potential
difference during a change.
The +6.2 V zener diode (CR3), the +15VDC supply, and the 1.0K (R40) combine to
limit the output supplied by U4 to the base of Ql to -0.6 V and +6.2 VDC.
Since the output of the final amplifier is 0 to +5 VDC, R38 is used as a trim
potentiometer to set the correct amount of current (1 mA) through the output meter for
fullscale deflection.
Jumper J7 is used to select the output range value for a voltage input recorder. If a
current converter board or an alarm board is used, the output voltage value is supplied
to each board as an input signal.
The 6.2 VDC determined by zener diode CR3 is also the reference supply for the
alarm point adjustments on the alarm board.