TM 10-6665-297-13&P
1-3
CONTROL MODULE (continued)
1-3.1.2 Signal Amplifier (continued)
sensor is a part. A differential voltage then appears between the two amplifier inputs and amplification
occurs. The resulting amplifier output is applied to the percent L. E. L. meter and to the warning, alarm,
recorder, and failure detection circuits that follow.
1-3.1.3 Test Circuit. (Figure 1-9) The test circuit is provided to simulate gas alarm conditions by pushbutton
action. When the TEST pushbutton on the control module front panel is depressed, a circuit is closed
from ground through potentiometer R22 to the + input of differential amplifier U3 of CD800/830 or U1
of CD802/832. The result is an imbalance between the two inputs to U3 or U1 such as would be caused
by the detection of a 100- percent L. E. L. gas-air mixture at the detector. A screw-adjustment setting of
the RANGE potentiometer R22 controls the signal level to the amplifier to drive the indicating meter to
full scale with the TEST push- button depressed. Since the signal displayed on the meter is also a
function of signal amplifier gain, as established by the GAIN adjustment potentiometer (R10 for
CD800/830; R9 and R13 for CD802/832), RANGE control R22 may require slight readjustment if the
gain setting (U3 for CD800/830; U1 for CD802/832) is altered during a calibration check. When the
proper setting of R22 has been made, the ALARM and WARN lamps and relays respond to operation
of the TEST pushbutton as if in an actual alarm condition, so that the operation of the control module
circuitry and external warning devices may be evaluated for purposes of maintenance.
1-3.1.4 Warning Circuit. (Figure 1-10) The control module warning circuitry operates from the amplified
detector output and an adjustable reference voltage. The reference voltage is usually set at a level
equivalent to an amplified detector signal representing a gas-air mixture of 20 percent of the Lower
Explosive Limit (20% L. E.L.). Both the amplified detector output and the reference voltage are applied
to a voltage comparator (U4 for CD800/830; U3 for CD802/832). If the signal level is below the
reference voltage, the base of transistor Q5 is negative, and Q5 is in the "off" state. When the
amplified signal exceeds the reference voltage level, a positive voltage of sufficient magnitude to "turn
on" transistor Q5 causes current to flow through the transistor to energize the warning light and relay.
Any audio or visual warning devices connected to the relay output terminals may be controlled in this
manner. The warning reference volt- age level is set by means of a screw adjustment on WARN
potentiometer R26 for CD800/830 or R48 for CD802/832. Transient pulses are prevented from
energizing the warning circuit by means of a time delay circuit described below.
23-9617
7/1/76
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