The US&S AFO-IIC track circuit system is designed to provide train detection in territories without insulated joints. It represents the latest development in US&S' highly reliable series of AFO-II systems. AFO-IIC offers even greater flexibility in track circuit selection than before, and incorporates design improvements that further enhance compatibility with other track circuit equipment.
An AFO track circuit is composed of a transmitter, receiver and receiver relay. The transmitter and receiver use a dc power source for operation. The transmitter introduces an audio signal of a specific assigned frequency into the track through two wires connected directly to the rails. The point where wires are connected defines one end of the AFO-IIC track circuit. The AFO-IIC receiver only responds to a specific assigned frequency. The receiver is also connected to the rails with two wires and this point defines the other end of the AFO-IIC track circuit. Upon receiving the proper frequency, the receiver detects, amplifies and rectifies the signal to provide an output to operate an external relay such as the US&S PN-150BH. The contacts of the relay are then employed in the same fashion as conventional track relay contacts. The AFO-IIC series includes a transmitter and a receiver. Each consists of a sheet steel enclosure, with printed circuit board(s) attached to the top cover. Standard AREMA terminals are provided on both units for external connections. The receiver includes a screwdriver-adjustable sensitivity control. Both units are designed for shelf, wall or rack mounting using standard mounting hardware.
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