Abstract: A device for detecting the unbalance .DELTA.i between the two fractions of the return traction current flowing in the two rails of a railroad track, in the presence of a signalling current in the respective track circuit, includes a linear combination circuit (3) that receives two signals (i.sub.1 and i.sub.2) which are proportional to the two currents actually present in the two rails and obtains both the difference (i.sub.1 -i.sub.2) and the sum (i.sub.1 +i.sub.2) thereof. A demodulating circuit (4), fed with the difference signal, takes from it the modulating signal of the signalling current and feeds it to a sample generator circuit (5), which generates a sample function M(t). The sample function is supplied to two successive sampling circuits (106 and 206), one of which also receives the difference signal and produces a sampled difference signal M(i.sub.1 -i.sub.2), while the other also receives the sum signal and produces a sampled sum signal M(i.sub.1 +i.sub.2).
Abstract: The receiver of an audio frequency (AF) track circuit for a track section of an electric propulsion railroad is coupled across an associated impedance bond by the primary and secondary windings of a saturable transformer. The control winding of this transformer receives energy signals from a voltage comparator network which compares input signals from two pick up coils, one positioned in inductive relationship with each rail at the receiver end of the section. These coils are tuned to respond to a selected ripple characteristic of the rectified DC propulsion energy, e.g., the sixth harmonic of the basic AC power supply source.