JPS5825757A - Communication terminal circuit - Google Patents

Abstract

PURPOSE:To make it unnecessary to provide an independent power source for compensating current and make a used transformer small-sized and improve the capacity of a circuit, by using effectively the DC current itself flowed to a hybrid circuit. CONSTITUTION:A transformer 1 is the hybrid transformer and has terminals 21 and 22, which are connected to a telephone set through a subscriber's line, and terminals 23, 24, and 25 connected to a four-wire circuit. The DC current indicating the operation state of the telephone set is supplied to lines through windings 1-1 and 1-2 of the transformer 1 from a power source 5 when the telephone set is hooked off, and simultaneously, it is stopped by a capacitor 2 so as to exert no bad influence upon the voice signal, and the current shunted through resistors 3 and 4 is detected by a DC current detector 6. At this time, since the magnetic field generated by windings 1-1 and 1-2 is cancelled by the compensating current flowed to windings 1-3 and 1-4, the DC magnetic field in the iron core has only a little residual.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a communication terminal circuit used in a line circuit or an outgoing trunk circuit between exchange equipment and transmission equipment.

It has been well known that transformers are indispensable for line circuits and trunk circuits. Generally, the iron core of this type of transformer has magnetic saturation characteristics, and as the magnetic field increases, the increase in magnetic flux density decreases. Therefore, when a direct current and an alternating current are superimposed and applied to the windings of a transformer, the main inductance of the transformer with respect to the alternating current decreases as the direct current increases. The relationship between main inductance and DC current varies greatly depending on the design of the transformer, but in order to prevent the main inductance from decreasing too much even when a large DC current is passed, the cross-sectional area of the iron core must be made large. It is necessary to reduce the magnetic flux density.

On the other hand, the frequency characteristic of the insertion loss of a transformer is 1%, and the loss for low frequencies increases as the main inductance decreases. Therefore, in order to obtain a transformer with good loss-frequency characteristics, it is necessary to use a transformer with a large main inductance, that is, a large transformer with a large core cross-sectional area.

By the way, in the above-mentioned line circuit or trunk circuit, the line is monitored by direct current flowing through the transformer. For example, in a line circuit, when a telephone installed in a subscriber's premises is taken off-hook, a direct current flows through the line circuit. Off-hook is detected by detecting this current through the line circuit. Thus, in line circuits, it is essential to flow direct current through the transformer. For this reason, conventionally, a large core has been used to prevent the transformer core from being saturated by the monitoring direct current. Therefore, the transformer used in this type of circuit has the disadvantage of being very large and necessarily expensive.

An object of the present invention is to eliminate the above-mentioned drawbacks and provide a small and inexpensive communication terminal circuit.

According to the present invention, a capacitor is connected in series between two windings of a primary winding, and a first resistor is connected between one end of the capacitor and one end of a DC power supply. 1 current mirror circuit is connected.

A first compensation additional winding wound around the same magnetic core as the primary winding is connected between one end of the output side of the first current mirror circuit and one end of the capacitor.

Further, a second current mirror circuit is connected between the other end of the capacitor and the other end of the DC power supply via a second resistor, and one end of the output side of the second current mirror circuit and the other end of the DC power supply are connected to each other via a second resistor. A communication terminal circuit characterized in that a second compensating additional winding wound around the same magnetic core as the primary winding is connected between the other end of the capacitor and the other end of the capacitor.

Next, a communication terminal circuit according to the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a circuit diagram of an embodiment in which the present invention is applied to a line circuit. In this figure, transformer 1 is a hybrid transformer.

Terminals 21, 22 are connected to a telephone (not shown) through the subscriber line and terminals 23 +-2 are connected to a four-wire circuit.
4, 25. On the other hand, when the telephone is turned off 7 degrees, the direct current indicating the operating status of the telephone is supplied from the power supply 5 to the line through the windings @1-1 and 1-2 of the transformer 1, and at the same time is converted into an audio signal. The current that is blocked by the capacitor 2 and shunted through the resistors 3 and 4 so as not to have an adverse effect on the current is detected by the DC current detector 6.

At this time, the magnetic field generated by the DC current of windings 1-1, 1-2 in transformer 1 is
Since it is canceled by the compensation current flowing through -3.1-4, the DC magnetic field in the iron core becomes only a very small residual difference.

As a means for supplying this compensation current to the additional windings 1-3, 1-4 of the transformer 1, a pair of transistors 7 and 8. Or transistors with similar characteristics? Two sets of current mirror circuits are used, each consisting of one individual. Note that the above resistor 9.10. and 11.12
is inserted for the purpose of improving the current mirror effect when the line current is large, and is not essentially necessary. Now, when a DC current flows through the subscriber line, the same current flows through the series-connected windings @1-1.1-2, but the resistor 3.4 and the DC current detector 6 are A current that is exactly 1/2 of the direct current flowing through line 1-1.1-2 will flow. This is due to the current mirror effect acting in two current mirror circuits.
This is because 1/2 of the direct current flowing through the windings -1 and 1-2 is shunted to the supplementary four conventional additional windings 1-3 and 1-4, respectively. In order to make the compensation current flowing through the compensation windings 1-3 and 1-4 exactly equal to the IA of the DC current flowing through the transformer windings 1-1 and 1-2, connect it to a current mirror circuit. resistors 9, 10 and 1
This is possible by selecting the values of 1 and 12, and the transformer windings 1-1 and 1-2 and the additional compensation winding 1-
By setting the turns ratios of 3 and 1-4 to 1:2, the DC magnetic field in the transformer core can be kept to a very small compensation residual. Note that the DC current detector 6 uses a photocassiller, and detects the flow of DC power into the subscriber line.

Identification is achieved by receiving the detected light with a separately provided light receiver. In this example, the DC current detector 6 is inserted between the resistor 4 and the paired transistor 87, but it goes without saying that it may be inserted between the resistor 3 and the paired transistor 7.

According to the above embodiment, the magnetic field caused by the current in the second winding 1-1 can be compensated by the winding 1-3, and the magnetic field caused by the current in the first winding 1-2 can be compensated by the winding @1-4. Special subscriber line 1 For example, when a public telephone or some public telephones are connected, a ground fault occurs due to the resistance value of the telephone, causing a current difference between the two windings on the primary side of the transformer. Even if this occurs, the compensation effect will not be lost.

As is clear from the above explanation, according to the present invention, the direct current magnetic field of the transformer can be easily compensated for by utilizing the direct current flowing through the high voltage circuit without providing a separate power supply for the compensation current. Of course, the compensation performance does not deteriorate even though there is a difference in DC current between the two primary windings, and the compensation circuit can also utilize semiconductor technology.

Miniaturization of the transformer used, performance of the circuit and

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment in which the present invention is applied to a line circuit. In the figure, 1 is a transformer and 2 is a capacitor. 3.4, 9, 10, 11.12 are resistors, 5 is a power supply. 6 is a DC power supply detector, and 7 and 8 are a pair of transistors. 1-1.1-2 is a primary winding of the transformer, and 1-3.1-4 is an additional winding for DC magnetic field compensation.

Claims (1)

[Claims] 1. A capacitor is connected in series between the two windings of the primary winding, and a first current mirror is connected between one end of the capacitor and one end of the DC power supply via a first resistor. a first compensating additional winding wound around the same magnetic core as the primary winding is connected between one end of the output side of the first current mirror circuit and one end of the capacitor; Further, a second current mirror circuit is connected between the other end of the capacitor and the other end of the DC power supply via a second resistor, and one end of the second current mirror circuit is connected to the output side of the second current mirror circuit. and a second compensating additional winding wound around the same magnetic core as the primary winding is connected between and the other end of the capacitor'f
! :Characteristic communication terminal circuit.