I!3OD
Referring to the summing junction
of
U16
it can
be
seen that the amplifier gains for any
particular input signal is determined by the ratio of feedback resistor
R60
(zone G12) and
the resistor in series with the input
signal. For example, the gain
of
the summing amplifier
for any external input signal at
TP19
(external perturbation signal) is
R60
divided by
R40.
Zener diodes
VR1
and
VR2
(zone H13,12) are employed
to
limit
the output
of
summing
amplifier at
TP17
to
+6v.
TP18 is the test point for servo ground.
It
should
be
noted that
servo ground is depicted on the schematic as
G(S) and logic ground is shown
as
G(L).
5.3.2.6 Positioner Power
Amplifier
The Positioner Power Ampl ifier is shown on Schematic
No.1
0281
0,
sheet 1 (zone G11).
This
amplifier is a linear feedback transconductance amplifier. It is referred to
as
a
transconductance
amplifier
because
its
output
current is proportional
to
the input voltage
to the power amplifier.
The
input to the power
amplifier
circuit
is
the output signal
of
the Summing
Amplifier
U16,
which
can
be
monitored at TP17. When the signal at TP17 is positive,
016
turns on,
causing
019
to conduct which, when relay
K1
(zone
C10)
is energized, connects
-20v
through
R125,
the positioner coil via J205 and
R110
to ground. Conversely, when the
signal at
TP17
is negative,
015
turns on, causing 018 to conduct which, when relay
K1
(zone
C10)
is energized, connects +
20v
through
R114,
the positioner coil via J205 and
R110
to
ground.
J205 (zone
C7)
connects the positioner coil
to
the Servo PCBA, thus establishing a path
for current through the
coil
and corresponding series resistors. The voltage drop across
R110.
which is proportional
to
the current through
R110,
is fed back
to
the power
preamplifier
U17
(zone G11).
U17
compares the input voltage and the feedback voltage
which is
proportional
to
the current through the coil, and turns
off
016
or
015
and
019
or
018
when the output current at
TP5
is equal to some constant of proportionality times the
input
voltage at TP17. Potentiometer
R111
(zone F12) is provided
to
adjust and vary
this
transconductance of the power amplifier.
014
(zone H10) and 017 (zone
E10)
limit
the maximum output current of the amplifier. The
voltage drop across
R125
or
R114
and the value of
R122
and
R124
or
R115
and
R113
determine the maximum value
of
current through
019
or
018,
respecti~eiy.
Additional current
limiting
is provided by
VR1
and
VR2
which
limit
the voltage at
TP17
to
+
6v,
and the adjustment
of
R111.
This value
of
current is normally adjusted lower than
that provided by the
limiting
values
of
014
and 017.
Resistors
R118
and
R119
(zone
F1
0)
provide
an
internal voltage feedback loop in the power
ampl ifier stage.
R-C
networks
C12
and
R117,
C13
and
R120,
C29
and
R203
assures the
high frequency
stability
of the power amplifier and eliminates high frequency oscillations.
5.3.2.7 Position Transducer Lamp Failure Detector
J203 pins 10 and 12 shown on Schematic No. 102810, sheet 1 (zone E12), connect the
Position Transducer Lamp
to
the Servo PCBA. In normal operation, the lamp is driven from
+ 10v with resistor
R128
in series between the lamp and ground. When the lamp is
operating
normally, i.e., illuminated, the voltage drop across
R128
turns on 020. Thus, the
Position Transducer
Failure signal (SPTFG) at pin
18
of
J202 is low. Should the lamp fail,
020
is turned off and SPTFG goes high, thereby detecting the lamp failure.
5-8
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