Fail Sensing and Relay Circuit

TM 10-6665-297-13&P 1-3.1.6 Delay Circuits (continued) tor C4 for warning circuit signals, and between the equivalent alarm circuit resistor R6 and capacitor C5 for alarm circuit signals. Here the capacitors must build up enough charge to trigger unijunction transistors Q1 of the warning delay circuit and Q3 of the alarm delay circuit, which subsequently fire silicon-controlled rectifiers Q2 and Q4, whereupon the warning and alarm signals remaining after a nominal 2-second delay are released to activate the relays. Circuits in CD802/832 control modules are similar to the above in philosophy, but employ four operational amplifiers in a common integrated circuit in the place of unijunction transistors and silicon- controller rectifiers. 1-3.1.7 Fail Sensing and Relay Circuit . (Figure 1-12) In normal operation, CD800/830 Fail relay K3 is energized to provide a closed contact through which warning and alarm circuits operate the warning and alarm relays if potentially dangerous gas conditions should occur, Operation of relay K3 is controlled through two transistors (Q7 and Q8) which switch off when certain circuit failures cause a lower-than normal voltage at Zener diode CR11 and at the bases of Q7 and Q8. When this happens, relay K3 is de- energized to switch off the warning and alarm circuitry, thus preventing false warnings and alarms until malfunctions in the foregoing circuits are corrected. Fault conditions most likely to drop the base voltage to turn off transistors Q7 and Q8 include detector sensor element failure, detector cable open or short circuit, improper setting of the VOLT adjust potentiometer R2, and a 10-percent- less-than-zero meter indication. Fail relay K1 of the CD802/832 module operates similarly, depriving SR3, Q3, Q4, and Q5 of power during the failure mode. 1-3.1. 8 Alarm and Warning Reset Circuit. To assure attention even to transitory warning and alarm gas conditions, the warning and alarm relays K1 and K2 of CD800/830 modules remain on once energized until manually reset. This happens because contacts 7 and 11 of relays K1 and K2 close in response to gas condition signals to provide direct paths to ground through diodes CR2 and CR3 and the normally closed ALARM RESET pushbutton switch. This latching action prevents both relays from de-energizing until the ALARM, RESET pushbutton is momentarily depressed. Conversely, depressing the pushbutton will not clear either the warning (K1) or the alarm (K2) relays if the warning and/or alarm circuits through these relays remain closed in response to gas conditions, since these circuits then energize the relays regardless of the open or closed state of the reset circuit. It follows that if the gas condition is below the high-alarm point but above the low-alarm point, only the high-alarm relay will be de-energized (reset), and the warning will remain. 23-9617 7/1/76 1-18