Direct current compensation circuit for transformers

#1. Field of Invention

#This invention relates to a circuit for direct current compensation of miniature transformers.

#2. Description of Prior Art

#The invention will now be described with reference to the accompanying drawing in which the single FIGURE shows a schematic representation of an embodiment of the invention.

#With reference now to the figure, there is schematically shown in separation transformer 10 having windings 12 and 18 connected in a subscriber loop 20, compensation winding 16, and winding 22 in a crosspoint loop 26. A conventional power source, -V, is connected to the winding 12 via a standard feed impedance network comprising diode 90, resistor 92 and capacitor 94 and to the compensation winding 16 via a resistor 42. The power source, -V, may be a -48 volt central office battery commonly used in telephone systems. Resistors 50 and 52 are serially connected between ground potential and the winding 18. A transistor 32 has its base terminal connected to the node between resistors 50 and 52, its emitter coupled via resistor 54 to ground potential, and its collector coupled via photo emissive means or light emitting diode 13 to compensation winding 16. The light emitting diode 13 is coupled to a photo transistor 14 which is in turn connected to a transistor switch of a type well known in the art and comprising transistor 62 and biasing resistors 56, 58 and 60. A capacitor 40 and a diode 34 are connected in shunt across resistors 50 and 52. The resistor 52 in combination with the resistor 50 acts as a feed impedance for the subscriber loop. The capacitor 40 is selected so that alternating current signals will bypass the resistors 50 and 52 and the diode 34 provides reverse voltage protection to the compensation circuit.

#Direct current signaling is employed in the subscriber loop to indicate whether the subscriber line is idle or busy, to indicate service requests and to transmit called number information in the form of dial pulses. With such signaling the subscriber loop is opened to indicate an idle condition, closed to indicate a busy condition or closed and opened in sequence to indicate dial pulses. Each time the loop is closed, a direct current I.sub.1 flows in the windings 12 and 18. The windings 12 and 18 will produce a magnetic field in the transformer 10 in a first direction. Ideally, the current I.sub.1 is of fixed magnitude. In practice, the current I.sub.1 may vary.

#The crosspoint network which is not shown requires a direct current bias. The bias current, I.sub.2, flows through the winding 22 which is arranged so that the magnetic field it produces is opposite in direction to the field produced by windings 12 and 18. Thus, the magnetic field produced by winding 22 partially compensates for the magnetic field produced by the winding 12 and 18. For example, if windings 12 and 18 have a total number of turns W.sub.1 and winding 22 has a number of turns W.sub.2, then the net magnetic field (expressed in ampere-turns) produced in the transformer is I.sub.1 W.sub.1 - I.sub.2 W.sub.2. Since W.sub.1 and W.sub.2 are fixed and I.sub.2 is a constant, the net magnetic field will be zero for only one value of I.sub.1. With a compensation current I.sub.3 flowing in winding 16 having W.sub.3 turns, the net magnetic field will be zero if