GB1141730A - Monitoring circuit - Google Patents

Abstract

1,141,730. Threshold circuits. ATOM-ENERGI A.B. 6 July, 1967, No. 31250/67. Heading H3T. [Also in Division G1] A circuit for detecting when the amplitude of a signal has exceeded a predetermined level for an appreciable time (e.g. for monitoring the neutron flux in a reactor) comprises switching means 2 connecting the neutron detector 1 to an amplifier 3 and two-state circuit 4, the output of the latter operating the switch 2 and a further two-state circuit 5 which is coupled to an output circuit 6 and alarm 7. Under normal conditions a signal is produced from a radioactive source in the detector 1 and amplified by 3 to a level which is insufficient to make a Zener diode 47 conduct. Transistor 48 is therefore non-conducting and transistor 54 is conducting. If the neutron level rises above a predetermined value, material in the detector is converted into a short-lived isotope (¢-life 1 minute) which emits beta rays and increases the signal from the detector 1. Transistor 48 then conducts and relay 23 operates to disconnect the amplifier from the detector at contacts 21. Transistors 48 and 54 are therefore cut off, and at the same time, capacitors 64a and 64b are charged through contact 22. The cutting off of transistor 48 de-energizes the relay and the signal from the detector is re-applied to the amplifier 3. If the signal has dropped to its former level no further action occurs, but if the signal is still at its high level transistor 48 will conduct again. However, the charge on capacitor 46 will prevent the relay from operating and transistors 4 and 5 will be held in a conducting state, maintaining transistors 62a and 62b conductive. The charge on capacitors 64a and 64b (duplicated for reliability) will then cause the alarm 7 to operate. Thus a double operation of circuit 4 is necessary to give the alarm. The duration of the high signal needed to operate the alarm depends on the time-constant of the CR circuit 45/46, e.g. 3-4 seconds, this being about twice the time taken for the sequence of circuit changes. The relay 23 may be replaced by an electronic switch and tubes may be used instead of transistors. The circuit may be tested by feeding pulses from a generator 85 to the amplifier 3 and measuring the power consumption in a unit 86 which gives a different alarm.