Threshold detector for monitoring supply voltage - has variable delay circuit and uses comparison with reference to provide warning output if threshold is exceeded

摘要

The threshold detector includes a filter (12) which receives an alternating voltage (v1) provided by an input circuit (10). This input circuit comprises a transformer (11) whose primary is connected to the supply to be monitored. A controllable fraction (v1) of the voltage available on the secondary is applied to the filter input (12). The filter output is connected to the input of a rectifier (13) (which may be half wave or full wave), whose output is connected to the first input (14a) of a comparator (14). The comparator receives on its second input (14b) a reference value (vs) whose amplitude is determined as a fraction of the threshold fixed for the monitored supply. The comparator output is fed to a temporary memory circuit (15) with a constant delay, and then to a circuit having variable delay (16). The output and this circuit forms the threshold detector output.