GB1302235A - - Google Patents

Abstract

1302235 Temperature alarms SHORROCK DEVELOPMENTS Ltd 26 Aug 1971 [17 Sept 1970] 44415/70 Heading G4N [Also in Division H3] A temperature alarm system utilizes a junction diode D3 as a temperature sensor and is provided with an automatic fault indicator. As shown, an increase in temperature in the region of the silicon junction diode D3 causes reduction in voltage at termiinal 4 of an operational amplifier A3 which results in a decrease in output voltage from terminal 6. If this decrease is sufficient to cut-off Zener diode D11, transistor TR3 will be cut-off so that the voltage at point E will turn on transistors TR5, TR6 and light the alarm lamp L1. TR5, TR6 remain latched until reset by contacts K. A similar effect will occur if the rate of increase in temperature is excessive, due to the effect of capacitor C2 and field effect transistor TR4. The system is applicable as a fire alarm or as a rate of temperature rise indicator in a refrigerator. Automatic fault indication.-A multivibrator TR7, TR8 feeds short-duration pulses into the system, the pulses being too short to light the alarm lamp L1. Capacitor C4 prevents the test pulses from latching TR5, TR6. Capacitor C5 is normally charged via resistor R6 but becomes discharged by TR9 by each test pulse. In the event of a fault the failure of test pulses will allow C5 to charge sufficiently to break down Zener diode D10 and turn on TR10 and fault indicator lamp L2. Modifications. Fig. 1 (not shown) relates to a simpler arrangement in which the voltage across the sensing diode D3 is fed to the non- inverting input of the amplifier A3 whilst the test pulses are fed to the inverting input and the rate detecting capacitor is shunted across the Zener diode equivalent to D11 of Fig. 2. Relays (not shown) may be used to operate an audible alarm or to feed a stored message into a line connected to the fire station.