TM 55-4920-429-13
C. The next stage, a programmable gain amplifier, is controlled by a logic decoder. The signal is
scaled and may be inverted and offset, depending on its type. Thus the signal at U14-9 or U14-3 is nor-
malized to a range of 0 to 5 volts dc.
d. If the signal is less than 0.76 volt, the lower-limit threshold detector turns on and closes relay K2.
Relay K2 lights the STATUS B indicator. If the signal is greater than 4.60 volts, the upper-limit
threshold detector turns on and closes relay K1. Relay K1 lights the STATUS A indicator. If the nor-
malized signal for either state is between these upper and lower limits, neither status indicator will
light.
1-14. Ac/Dc Amplifier.
(See FO-2, FO-3, and FO-8.), The ac/dc amplifier provides scaling of all input signals to less than one-
fourth of initial amplitude. With the external signal path closed, gain of the amplifier is -0.215. With
the external signal path open and resistor R62 connected to logic ground or to +7.5 vdc, the gain of the
amplifier is -0.150. Diodes CR6 and CR8 limit the difference between its two input pins to 0.6 volt dc.
Diodes CR5 and CR7 limit the difference between the two input pins and logic ground to 8.6 volts dc.
1-15. Ac to Dc Converter.
(See FO-2, FO-3, and FO-8.) The ac to dc converter rectifies, amplifies, and filters the 400 Hz ac input
signals. This enables monitoring by the dc threshold detectors. The output of the circuit is a negative dc
signal, equivalent to the rms value of the ac input signal. The ac to dc converter circuit consists of two
amplifier stages. The first stage is a half-wave rectifier/amplifier. The second stage is an integrating
amplifier that provides full-wave rectification and filtering of the ac input signal.
1-16. Programmable-Gain Amplifier.
(See FO-2, FO-3, and FO-8.) The programmable-gain amplifier provides final scaling of the input
signal before it is applied to the threshold-detector circuits. The scaling is done directly or through
offset/inverter amplifier. The gain of the amplifier is a function of which one of its four input resistors
is switched into the input signal path:
G a i n=
Where R1 = R78, R88, R89, or R90
With R78 selected, gain= -1.0 (Program 1).
With R88 selected, gain = -1.9217 (Program 2).
With R89 selected, gain = -2.1047 (Program 3).
With R90 selected, gain = -6.6566 (Program 5/7).
Capacitor C20 minimizes electrical noise in the dc input signal.
1-17. Offset/Inverter Amplifier.
(See FO-2, FO-3. and FO-8.) The offset/inverter amplifier provides offset inversion of scaled input
signals for program 5. (Refer to table 1-2. ) This offset inversion correlates the active and inactive states
of input signals and the limits of the threshold detector circuits. The active state of input signals is 0
volt. The inactive states are +7.5 volts dc and +24 volts ac. With 0 volt input tO the offset/inverter am-
plifier, its output applies +5.48-volt dc to the threshold-detector circuits to cause the STATUS A indi-
cator to come on. Any positive value of input signal greater than 0 volt is proportionally subtracted
from the +5.48 volt dc offset output of the amplifier through inverter action. For example: application
of +5.16-volt dc (scaled value for a 24-volt ac inactive state) results in a +0.32 volt dc signal. This input
to the threshold-detector circuits causes the STATUS B indicator to come on.
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