GB800163A - An electrical circuit automatically responsive to a pre-determined change, or rate of change of a condition, e.g. temperature - Google Patents

Abstract

800,163. Radiation-sensitive electric systems. SPECIALTIES DEVELOPMENT CORPORATION. July 10, 1956 [Sept, 26, 1955], No. 21311/56. Class 40 (3). [Also in Group XXXVIII] An alarm system which automatically responds to a change of condition, e.g. heat, light, humidity, or mechanical force, is particularly described as a fire alarm and comprises fixed resistors 12, 13, an adjustable resistor 15, and a temperature-responsive resistor 16, bridge connected as in Fig. 1, the diagonals being connected by a polarized relay 17 provided with protecting varistors 36, 37 and a slugging condenser 42, the relay being responsive to a predetermined high temperature, e.g. 700‹ F., obtained by a slow rate of rise, the system also comprising a time constant network 39, 40, 41 which is responsive to a rapid rate of temperature rise above say 600‹ F. The values of the resistors are such that at normal temperatures the bridge is unbalanced in one direction and at alarm temperatures the bridge unbalances in the opposite direction. The relay 17 operates at the predetermined high temperature to switch over a blade 22 to make with contact 20 so that a relay 24 is energized to make contacts 27, 28 in an alarm circuit. The time constant circuit may also operate the blade by energizing the relay 17 if a rapid rise of temperature takes place above 600‹ F. In a modification the sensing resistor 16 may have a positive coefficient of resistivity rather than a negative coefficient, that is, the resistor becomes more conductive as the temperature decreases so that the network serves to detect abnormally low temperatures and may be used to control heating or refrigerating systems.