IE41644B1 - Improvements in or relating to power supply circuits - Google Patents

Abstract

1472442 Sub-station circuits STANDARD TELEPHONES & CABLES Ltd 30 July 1974 33570/74 Heading H4K A power supply arrangement for maintaining power to the circuitry of a telephone set during loop disconnect signalling comprises a capacitor C charged from the exchange battery via a rectifier bridge 3 and an arrangement of a Zener diode ZD and transistors TR1, TR2. The arrangement of the Zener diode and transistors is such as to ensure that the voltage across capacitor C is mainteined at or below a predetermined limit: To improve the response of the circuit to low line voltages and currents the slope resistance of the Zener diode can be reduced by connecting a parallel arrangement of a resistor and the collector base junction of a transistor in series with the diode, the collector emitter junction of the transistor being connected in the line loop (Fig. 2, not shown). The rectifier bridge 3 may be a diode bridge, or a transistor diode bridge as shown in Fig. 3. The load L may be the circuitry of a pushbutton type dialling apparatus of the type in which the actuation of a push-button causes loop-disconnect impulses to be transmitted.

Description

This invention relates to power supplies, and in particular to power supplies for operating a batteryless telephone subset.

A major problem in the design of a batteryless push-button telephone subset is that of obtaining sufficient power from the line to operate the circuitry and to maintain power during the break periods of outpulsing. Restrictions on the maximum and minimum permissible line currents and on the line voltage developed across the subset add to the difficulty.

The object of the invention is to minimise or to overcome these disadvantages.

According to the invention there is provided a circuit for providing electrical power to a batteryless telephone subset of the type employing loop disconnect signalling via a subscriber loop to the exchange and for maintaining power to the subset during the break periods of outpulsing, the circuit, when in use, being connected across the legs of the loop so as to couple the loop to the subset, the circuit including a full wave rectifier for rectifying the loop current though the subset, a capacitor connected in series with a diode across the d.c, output of the rectifier, a pair of transistors arranged as a cascaded amplifier in parallel with the capacitor and diode and with its output connected to the junction of the capacitor and diode, and a Zener diode for limiting the base potential of the input transistor of the amplifier so as to limit the voltage across the capacitor to a predetermined maximum, in which, when the subset is coupled to the loop via the circuit and the capacitor is charged to its maximum voltage, and in which, during the break periods of outpulsing, the subset draws current from the charge stored in the capacitor.

Embodiments of the invention will now be described with reference to the accompanying drawing in which:Fig. 1 shows a telephone subset power supply connected to a subscriber loop. - 2 41644 Fig. 2 shows a modified power supply of the type shown in Fig. 1, and Fig. 3 shows a rectifier bridge circuit for use in the circuits of Figs. 1 and Referring to Fig. 1 the power supply is operated from the exchange battery V via the legs 1 and 2 of the subscriber loop each of which legs includes a line ( resistance R. The power supply includes a full wave rectifier 3, such as a diode bridge, feeding a storage capacitor C and a voltage control circuit comprising transistors TRI and TR2, diode D and Zener diode ZD. The full wave rectifier 3 ensures that the power supply is unaffected by change of polarity of the line voltage.

When the power supply is first connected to the loop via terminals TI and T2 the storage capacitor C is fully discharged and the voltage developed across the terminal TI and T2, V], is determined by the bridge rectifier voltage drop plus the Zener voltage of Zenor diode ZDI plus the base-emitter voltage drops of transistors TRI and TR2. The voltage V2 applied to the load L, the subset circuitry, is equal to V-| minus the bridge rectifier voltage drop.

The line current I around the loop is defined by the difference between the exchange battery voltage V and divided by the resistance of the loop. The current i through the load is equal to V2 divided by the load resistance. Thus a current of I - i is available to charge capacitor C through transistor TR2.

Capacitor C becomes charged to a voltage equal to V2 minus the saturation 20 voltage drop of TR2, about 0.2 volts in the case of a silicon transistor, and thereafter surplus current is passed through transistor TRI so as to maintain the voltage V2 constant.

When outpulsing occurs and the contacts 4 open, the load current i is supplied from the charge stored in the capacitor C. Under this break condition the voltage V2 - 3 41644 across the load is equal to the voltage across the capacitor minus the forward voltage drop of diode D. At the end of the break period (67 msec, in the British Post Office system) the contacts reclose, capacitor C is recharged and the load voltage returns to its original value.

, In some applications the voltage supply to the load may be smoothed in a manner which does not introduce a time delay to the initial connect condition.

A further Zener diode may be employed for this purpose. In order to improve the response of the circuit to low line voltages and currents, such as may occur at the end of a long subscriber loop, the effective slope resistance of Zener diode ZD may be reduced by a further transistor TR5 connected across the Zener diode as shown in Fig. 2.

Fig. 3 shows a diode-transistor bridge rectifier which may be employed in place of a conventional four diode bridge in the circuits of Fig. 1 and Fig. 2. This rectifier has a lower forward voltage drop than the four diode bridge, thus allowing it to handle low voltages and currents, but the reverse voltage rating is limited to the base-emitter breakdown voltage of transistors TR3 and TRA. Resistors RI and R2 serve to limit the base current of transistors TR3 and TR4 respectively.

It should be noted that although the contacts 4 are shown schematically 2o in Figs. 1 and 2 as mechanical or electromechanical, they may of course be replaced by an electronic switching arrangement.

Claims (7)

1. A circuit for providing electrical power to a batteryless telephone subset of the type employing loop disconnect signalling via a subscriber loop to the 25 exchange and for maintaining power to the subset during the break periods of outpulsing, the circuit, when in use, being connected across the legs of the loop so as to couple the loop to the subset, the circuit including a full wave rectifier - 4 41644 for rectifying the loop current though the subset, a capacitor connected in series with a diode across the d.c. output of the rectifier, a pair of transistors arranged as a cascaded amplifier in parallel with the capacitor and diode and with its output connected to the junction of the capacitor and diode, and a Zener·diode for limiting the base potential of the input transistor of the amplifier so as to limit the voltage across the capacitor to a predetermined maximum, in which, when the subset is coupled to the loop via the circuit and the capacitor is charged to its maximum voltage, and in which, during the break periods of outpulsing, the subset draws current from the charge stored in the capacitor.

2. A circuit as claimed in claim 2, and in which the effective slope resistance of the Zener diode is reduced by means of a third transistor connected across the Zener diode.

3. A circuit as claimed in claim 1 or 2, and in which said full wave rectifier is a diode bridge.

4. A circuit as claimed in any one of the proceeding claims, and in which said full wave rectifier is a diode-transistor bridge.

5. A circuit for maintaining electrical power to a telephone subset of the loop disconnect signalling type during the break periods of outpulsing substantially as described heroin with reference to Fig. 1 or to Fig. 2 of the accompanying drawings.

6. A circuit for maintaining electrical power to a telephone subset of the loop disconnect signalling type during the break periods of outpulsing substantially as described heroin with reference to Figs. 1 and 3 or to Figs. 2 and 3 of the accompanying drawing. 5 41644

7. A telephone subset when fitted with a power supply circuit as claimed in any one of the preceeding claims.