Railway signalling system electrical monitoring circuit - uses frequency comparison techniques, with one frequency representing normal operating state of signal circuit

摘要

The circuit is designed to permanently supervise the correct operation of an electrical supplied from a stable alternating voltage source (2). The supervisory circuit has a convertor (5), which converts the maximum value of current circulating in the circuit into a proportional voltage V. An astable oscillator (6) converts the voltage V into a voice frequency f proportional to V. The circuit has another oscillator (8) which emits a constant frequency F which is greater than the frequency f. The two frequencies f and F are combined in a mixer (7). A very narrow passband filter (9) is centred on one of the values F + fo or F - fo. fo being the value of the frequency f when the supervised circuits power consumption is normal. The filer (9) is connected to the output of the mixer (7). A rectifier (10) is connected to the filters output. The rectifier supplies a signalling relay (11) with rectified current.