GB2224909A - Balanced constant current feed for subscriber lines - Google Patents

Abstract

A telephone subscriber's line circuit has a transformer (TF) which does not carry DC, as its line-side winding is split with a mid-placed capacitor (C). The two legs of the line are each coupled to the DC terminals (0-50V) via current regulators (CR1, CR2) each preferably shunted by a high value resistor (Rs) which provides balance if the regulators are not identical. Results are best if the regulators, which may be commercially available constant-current devices - have positive temperature coefficients of pass current. <IMAGE>

Description

BALANCED CONSTANT CURRENT FEED FOR SUBSCRIBER LINES The present invention relates to telephone line circuits, and especially to arrangements for feeding direct current from the exchange via the telephone line to the subscriber's instrument.

Power feeding of such local loops has been done in several ways, ali of which are expensive in either space or cost. The traditional technique uses a split-winding transformer with a by-pass capacitor between the windings, the DC feed being via a pair of feed resistors. This calls for transformers with well-defined inductive characteristics to meet transmission performance without core saturation due to the high flux in the windings due to the line DC. Hence the components are large and tend to be a compromise in respect of low frequency transmission performance.

Some reduction in the size of the transformer can be effected by flux cancellation techniques, but this needs extra windings plus feedback to ensure that the net DC flux is low. Solid state arrangements have been used in some systems, but they are expensive and are not as robust as transformers when subject to over-voltages on the line.

An object of the invention is to provide a transformer-based arrangement in which the transformer does not carry any direct current, and wherein the transformer characteristics can be optimised in terms of size and transmission performance.

According to one aspect of the present invention, there is provided a constant-current line-feeding circuit, such as used in an automatic telephone exchange, in which the two legs of the line are interconnected via first and second transformer windings connected in series via a capacitor so that the windings pass AC but not DC, in which the positive leg of the line is coupled to the relatively positive terminal of the DC supply via a first current regulation device, and in which the negative leg of the line is coupled to the relatively negative terminal of the DC supply via a second current regulation device, the two current regulation devices being nominally identical and having positive temperature coefficients of pass current whereby the devices are thermally feedback balanced.

According to another aspect of the present invention there is provided a current line-feeding circuit, such as used in an automatic telephone exchange, in which the two legs of the line are interconnected via first and second transformer windings which are coupled in series via a capacitor so that the windings pass AC but not DC, in which the positive leg of the line is coupled to the relatively positive terminal of the DC supply via a first current regulation device, in which the negative leg of the line is coupled to the relatively negative terminal of the DC supply via a second current regulation device, the two current regulation devices being norminally identical, and in which the two current regulation devices are each shunted by a resistive impedance whose resistance is high compared with the line loop impedance, so that the current-regulation devices are feedback balanced due to the high impedances of the shunting resistive impedances.

Such arrangements are especially suited to situations in which a low-level wetting current is needed, but are also suitable for higher current cases such as direct exchange line.

Embodiments of the invention will now be described with reference to the drawings, in which Fig.l shows the basis of the invention, and Fig.2 is a preferred embodiment thereof.

In Fig. 1, the line transformer TF has two line-side windings to which are connected the two legs of the line loop. These two windings are connected in series via a DC blocking capacitor C. The resistor RL represents the line loop resistance which is relatively low when the subscriber's set is off-hook, but is of the order of thousands of ohms in the on-hook condition.

The other winding of the transformer gives access to the exchange.

Each line leg is connected to its terminal of the DC supply, usually the exchange battery, via the appropriate one of the two constant-current devices CR1, CR2, also known as current-regulators. The devices are nominally identical.

If CR1 and CR2 are simple fixed regulators, the usual mis-match causes the one with the lower current to determine the loop current, while the other saturates.

The net effect is to establish the correct DC conditions, but the line is badly unbalanced due to the low impedance to ground on one leg via the saturated regulator. It is possible to select matched pairs of regulators, but this is costly and would not guarantee tracking with temperature and voltage variation.

Note that during the unbalanced condition referred to above, there is more power dissipation in the regulator acting as a current limiter than in the saturated one. Thus there is a temperature difference between the regulators, and this can be used as a feedback mechanism to increase the threshold of the limiting regulator. This can be done using a programmable current regulator having predictable positive temperature coefficient. Such devices are commercially available an example being the National Semiconductor LM334 - which is programmable for currents up to lOmA by a single resistor - 15 ohms for 5 mA.

This device has a linear positive temperature coefficient of 0.336%/K., the maximum equivalent temperature error for each device being t 6K. Thus the maximum temperature difference needed between two devices for them to have identical pass currents is 12K. Assuming a thermal resistance of 0.25K/mW, this corresponds to a power difference of only 48mW, which at 5 mA needs a voltage "headroom" of less than 10V to achieve balance.

The basic arrangement of Fig. 1 can be used with regulators CR1 and CR2 having positive temperature coefficients, but there is some risk of instability.

This can be overcome by shunting each regulator with a large value resistor Rs as shown in Fig 2. A typical value for Rs is 470 kohms, though resistances in the range of Skohms to 500 kohms can be used. The resistor has little effect on transmission performance, and the bleed current through it is usually of little significance. The resultant current will be a function of the ambient temperature, but this should be acceptable for the typical 50K temperature range encountered in telephony.

The method described, whereby shunt resistances are added to the current regulator, is also applicable where the regulators have no significant temperature coefficient. Balancing still occurs due to the current through the shunt being a function of the voltage across the regulator. Hence the regulator which determines the current has a higher voltage drop across it than the one which is saturated. The shunted current then tends to balance the voltage across the regulators.

In a typical case where a nominal 5mA line current is needed, it should be possible to use current regulators with a specified accuracy of +28. This means that a current of up to 0.2mA (4% of 5mA) has to be passed across the regulator with the lower current set point in excess of that being shunted across the regulator with the higher set point. If a voltage mismatch of up to 10 volts is acceptable, the shunt resistors need to have a value defined by 10/0.2, which gives 50 kohms to guarantee automatic balancing for worst case tolerancing of the regulators. Such shunt resistors are an order of magnitude less than those needed when thermal feedback is present, which may cause some degradation in transmission performance.

The current regulators used in the present circuits are of the National Semiconductors LM334 type This is a transistor circuit which is in effect based on a circuit of the long-tailed pair type. However, other forms of current regulators can be used.

Thus our circuit provides automatic balancing of the current regulators so that acceptable transmission performance is maintained. The value of the shunt resistance Rs is a function of several parameters including regulator tolerance, voltage headroom and temperature coefficient. In particular, a significant positive temperature coefficient of pass current for the regulators CR1 and CR2 provides a novel and effective control mechanism.

Claims (4)

1. A constant-current line-feeding circuit, such as used in an automatic telephone exchange, in which the two legs of the line are interconnected via first and second transformer windings connected in series via a capacitor so that the windings pass AC but not DC, in which the positive leg of the line is coupled to the relatively positive terminal of the DC supply via a first current regulation device, and in which the negative leg of the line is coupled to the relatively negative terminal of the DC supply via a second current regulation device, the two current regulation devices being nominally identical and having positive temperature coefficients of pass current whereby the devices are thermally feedback balanced.

2. A constant-current line-feeding circuit as claimed in claim 1 and wherein each current regulation device is shunted by a respective resistive impedance.

3. A constant-current line-feeding circuit, such as used in an automatic telephone exchange, in which the two legs of the line are interconnected via first and second transformer windings which are coupled in series via a capacitor so that the windings pass AC but not DC, in which the positive leg of the line is coupled to the relatively positive terminal of the DC supply via a first current regulation device, in which the negative leg of the line is coupled to the relatively negative terminal of the DC supply via a second current regulation device, the two current regulation devices being nominally identical, and in which the two current regulation devices are each shunted by a resistive impedance whose resistance is high compared with the line loop impedance, so that the current-regulation devices are feedback balanced due to the high impedances of the shunting resistive impedances.

4. A constant-current line feeding circuit, substantially as described with reference to Fig. I or Fig. 2 of the accompanying drawings.