Patent GB-1150864-A (GB)
Title: Crosspoint Operating Circuit
IPC: H04Q3/00
Filed: 1968-01-11
Abstract: 1,150,864. Automatic exchange systems. INTERNATIONAL STANDARD ELECTRIC CORP. 11 Jan., 1968 [23 Jan., 1967], No. 1729/68. Heading H4K. Cross-point relay windings such as Dr, Cr, are energized from a constant current source (transistor T and Zener diode Z, Fig. 1) and the contacts of at least those cross-points nearest the junctors are operated under currentless conditions. Contacts mj controlled by a marker prepare a path from the current source to a selected junctor. Marker contacts md, mc prepare energizing circuits over diodes GD, GC. Transistors TJ, TD, TC are switched to complete the prepared paths. Winding Dr is energized to earth over diode GD and operates contacts dr to prepare an energizing circuit for the next winding Cr. Before Cr can be energized over dr, however, the marker must disconnect the earth on the control wire over diode GD, this being done by cutting off transistor TD after a time sufficient for contacts dr to have settled down. As shown in Fig. 1, operation of Cr brings up cut-off relay Cor in the line circuit. The contacts j in the junctor put earth on the control wire to hold the connection and allow the marker to withdraw. As relay Cr is adjacent the line circuits and is consequently not employed as often as Dr, the contacts cr are worked in a current active circuit but they can be worked in currentless conditions if the cut-off relay Cor has a diode to earth on its line circuit side and a contact which isolates the winding from line circuit battery until the contacts cr are made and the energizing circuit of Cr is disconnected by the marker. The relay windings are protected from voltage spikes by shunt resistors. A diode GJ2 strapped to battery shorts the series-connected windings when they are released from the junctor and damps the decay of current to protect the releasing contacts in the control wire. Instead of placing the problem of timing the disconnection of the successive operating circuits in the marker each operating circuit may find earth over a capacitor on the marker side of diodes GD, GC, the current to energize a cross-point relay winding charging the capacitor which back-biases the diode GD or GC after a time sufficient for the cross-point relay contacts to settle. The rising voltage on the capacitor means that the contacts dr; cr in the control wire operate with some current available to the circuit. The voltage in the capacitor may be held at earth to preserve currentless contact operation by means of the circuit in Fig. 5. The capacitor C is connected with a transformer TR in the feedback path of an amplifier AMP with a bootstrap action so that the capacitor charges with the potential V1 at earth until the amplifier saturates, feedback over the transformer ceases, diode G conducts and the voltage V1 steps upward. Before the capacitor is employed in a further marking function it is discharged over contact s and resistor R4. If the transformer is omitted and capacitor C is connected between input and output of the amplifier, Fig. 4 (not shown), the voltage V1 stays at earth until the amplifier saturates after which it increases with time.