Railroad track circuit

Claim 2

Track circuit apparatus for a stretch of railroad track traversed at times by railroad cars, comprising in combination, a. a source of alternating current energy having an additional center tap connection, b. a load impedance connected across the rails of said track stretch, c. a balancing impedance, d. said source and said load and balancing impedances being connected in a bridge circuit network, balanced when a no-rail shunt condition exists, each portion of said source, as divided by said center tap, and each impedance forming a separate leg of said bridge circuit, e. indication means coupled to said load impedance and responsive to shunt and no-shunt rail conditions for indicating the occupancy or nonoccupancy, respectively, of said stretch by a car, f. delay means coupled across opposite balance points of said bridge network and responsive to each shunt condition across said rails for establishing a predetermined delay period, g. said delay means also coupled to said indication means for enforcing the full predetermined delay period after each rail shunt condition, prior to the subsequent response of said indication means to a no-rail-shunt condition.

Claim 3

Track circuit apparatus as defined in claim 1 in which, a. said load impedance comprises a load resistor connected across said rails and the primary winding of a first control transformer connected in parallel with said resistor through said rails, said parallel circuits together forming one leg of said bridge network, b. said indication means comprises a track relay and a first transistor, series connected across the secondary winding of said first control transformer, c. said first transistor controlled by said delay means for completing the energizing circuit for said relay to provide a nonoccupied indication only when said predetermined delay period has expired after a no-rail-shunt condition exists.

Claim 4

Track circuit apparatUs as defined in claim 2 in which the combination further includes, a. a second control transformer having its primary winding connected between said center tap terminal of said source and the opposite terminal of said bridge network, and in which, b. said delay means comprises a second transistor and a timing capacitor, c. the secondary winding of said second control transformer coupled to said second transistor for actuating that transistor to its conducting condition when a rail shunt condition occurs, d. said capacitor connected in a charging circuit across the secondary of said first control transformer for charging to a selected voltage level during said predetermined delay period, e. said second transistor having connections for discharging said capacitor when that transistor is in its conducting condition, f. said capacitor coupled to said first transistor for actuating that transistor to complete said energizing circuit for said track relay only when said selected voltage level of capacitor charge is reached.

Claim 5

Track circuit apparatus as defined in claim 3 in which, said timing capacitor is coupled to said first transistor by a Zener diode having a preset breakdown voltage equal to said selected charging voltage level.

Claim 6

Track circuit apparatus as defined in claim 4 in which, a. said balancing impedance is an adjustable resistor, b. said alternating current source is a track transformer having its primary winding connected across a source of alternating current energy and a center tapped secondary winding providing equal output from each portion of that winding.

Claim 7

Track circuit apparatus as defined in claim 1 in which, said indication means comprises a track relay and a first switching semiconductor device, the winding of said relay and the conducting path of said first device connected in a series circuit which is coupled to said load impedance for at times energizing said track relay, b. said delay means coupled to said first semiconductor device for switching on said series circuit to energize said track relay only when said predetermined delay period has been completed after a no-rail-shunt condition begins, c. said track relay having connections for controlling the display of a nonoccupied and an occupied indication as said relay is energized and deenergized, respectively.

Claim 8

Track circuit apparatus as defined in claim 6, in which said delay means comprises, a. a timing capacitor, b. a charging circuit for said capacitor coupled to said load impedance for charging said capacitor to a preset voltage level during said predetermined delay period, c. a second switching semiconductor device coupled to said bridge network and responsive to the occurrence of a rail-shunt condition for switching its conducting path to its on condition, d. said second device having connections for discharging said capacitor when said second device is in its on condition, e. said timing capacitor coupled to said first device for switching on said series circuit only when said preset voltage charging level is reached.

Claim 9

A balanced bridge track circuit arrangement for a stretch of railroad track, comprising in combination, a. first and second equal secondary track transformer windings having a common connection and forming two legs of said bridge circuit, b. a balancing impedance connected to form a third leg of said bridge circuit, c. a load impedance connected to form a fourth leg of said bridge circuit and also connected across the rails of said track stretch, d. a primary winding of said track transformer connected to a source of alternating current energy, e. a first control transformer having a primary winding connected across the rails of said track stretch for forming a parallel path in the fourth leg of said bridge circuit, f. a second control transformer having a primary winding connected between said common connection of said track transformer sEcondary windings and the opposite terminal of said bridge circuit formed by the common connection of said third and fourth legs, g. indication means coupled to the secondary winding of said first control transformer and responsive to the flow of current in said bridge circuit during a nonshunted condition of the rails of said track stretch for recording a nonoccupied indication of said track stretch, and h. delay means coupled to the secondary winding of said second control transformer and responsive to the flow of current in said bridge track circuit when the track rails of said stretch are initially shunted for controlling said indication means to record a stretch occupied indication, i. said delay means being further responsive to a nonshunted rail condition subsequent to a rail shunt for establishing a predetermined delay period and coupled to said indication means for delaying the recording of a nonoccupied indication until the expiration of said delay period.

Claim 10

A balanced bridge track circuit arrangement for a stretch of railroad track, comprising in combination, a. first and second equal secondary track transformer windings having a common connection and forming two legs of said bridge circuit, b. a balancing impedance connected to form a third leg of said bridge circuit, c. a load impedance connected to form a fourth leg of said bridge circuit and also connected across the rails of said track stretch, d. a primary winding of said track transformer connected to a source of alternating current energy, e. a first control transformer having a primary winding connected across the rails of said track stretch for forming a parallel path in the fourth leg of said bridge circuit, f. a second control transformer having a primary winding connected between said common connection of said track transformer sEcondary windings and the opposite terminal of said bridge circuit formed by the common connection of said third and fourth legs, g. indication means coupled to the secondary winding of said first control transformer and responsive to the flow of current in said bridge circuit during a nonshunted condition of the rails of said track stretch for recording a nonoccupied indication of said track stretch, and h. delay means coupled to the secondary winding of said second control transformer and responsive to the flow of current in said bridge track circuit when the track rails of said stretch are initially shunted for controlling said indication means to record a stretch occupied indication, i. said delay means being further responsive to a nonshunted rail condition subsequent to a rail shunt for establishing a predetermined delay period and coupled to said indication means for delaying the recording of a nonoccupied indication until the expiration of said delay period.

Claim 11

A track circuit arrangement as defined in claim 8 in which said delay means comprises, a. a timing capacitor, b. a semiconductor device having its control elements coupled to the secondary winding of said second control transformer for triggering to its conducting condition when a rail shunt condition exists, and having connections for discharging said timing capacitor when said device is in its conduction condition, and c. a charging circuit for said timing capacitor having connections to the secondary winding of said first control transformer for charging said capacitor to a selected voltage level during said predetermined delay period, d. said capacitor further coupled to said indication means for delaying the response to a nonshunted rail condition until said selected voltage level is obtained.

Claim 12

A track circuit arrangement as defined in claim 9 in which said indication means comprises, a. a track relay having connections for providing when energized a nonoccupied indication for said stretch and when deenergized an occupied indication, b. another semiconductor device, c. the winding of said relay and the conduction path of said other device being connected in series across the secondary winding of said first control transformer for energizing said relay when said other device is in its conducting condition and a nonshunted rail condition exists, and in which, d. said timing capacitor is coupled to the control elements of said other device for triggering said other device to its conducting condition only when said capacitor is charged to said selected voltage level.