GB2063017A - Telephone system subscriber battery charging - Google Patents

Abstract

A system for charging subscribers' batteries from a central telephone exchange without interference to normal telephone activity. During charging the subscriber's battery 4 is periodically disconnected from the line 3 by a switch 10 and line voltage sensed by a sensor 7 to ascertain whether a call is to be made or received. Line voltage equal to central battery voltage indicates non-use and causes line to be reconnected to subscriber's battery. Charging current pulse sensor 12 at exchange disconnects central battery 5 from line 3 on absence of charging pulses on the line or presence of ringing voltage on subscriber's exchange equipment 1. <IMAGE>

Description

SPECIFICATION Telephone system subscriber battery charging This invention relates to telephone systems and more specifically to a system for charging batteries located at subscriber's installations from a central exchange. In particular, but not solely, the present invention is concerned with the charging of batteries for powering subscribers' carrier equipment in a so called "one-plus-one" carrier system. That is, a system where a second subscriber circuit is added to an existing subscriber line or cable pair.

When a battery located at a subscriber terminal is to be charged from the exchange battery via the subscriber's line two conditions in particular must be met to ensure normal operation of the subscribers telephone is not effected. First, the line must not be shunted by the battery or associated charging circuits at either end when a call is in progress.

Second, the line must present a high resistance when a ringing signal is being sent to the subscriber from the exchange so as to obviate seizure of the subscribers line relay thus tripping off the ringer before the subscribers bell has sounded.

Prior art systems meet the first requirement, and attempt to meet the second requirement, by monitoring the line current in terms of the voltage drop across a series resistance and when this increases upon the subscribers receiver being lifted (microphone current) the battery and charging circuits are disconnected from the line. To provide a measurable voltage change under limiting line conditions where the loop resistance will be about 1500 ohms the charging current cannot be greater than about 10 milliamps. Even if the charging current is limited to this value a ringing signal can still result in line seizure. The low charging current permitted by the prior art systems is particularly unsatisfactory in countries where the existence of free calling zones encourage more frequent and more prolonged calls which result in reduced recharging time and a greater demand on the battery.

It is therefore an object of the present invention to provide a battery charging system for charging subscribers' batteries from a central exchange in a telephone system which will meet the abovementioned desiderata.

Accordingly in one aspect the invention may be said to consist in a method for charging subscribers' batteries from a central exchange in a telephone system comprising: connecting the exchange power supply to the subscriber's line, connecting the subscriber's battery to said line to which his telephone receiver equipment is permanently connected Periodically disconnecting the subscriber's battery from said line and sensing the line voltage at the subscriber's terminal, reconnecting the battery to the line if the line voltage is equal to the exchange power supply voltage, sensing the current in said line at the exchange and if charging current pulses are not detected disconnecting the subscriber's line from the exchange power supply and connecting it to the subscriber's exchange equipment, monitoring for ringing voltage on the subscriber's exchange equipment and if ringing is detected disconnecting the exchange power supply from said line.

In another embodiment the invention may be said broadly to consist in battery charging apparatus for charging subscribers' batteries from a central exchange in a telephone system comprising: a first switch which connects the exchange power supply to the subscriber's line, a second switch which connects the subscriber's battery to said line to which the subscriber's telephone receiver equipment is permanently connected, means at the subscriber's installation which periodically cause said second switch to open, a voltage sensor at the subscriber's installation which senses the magnitude of the voltage on said line, said voltage sensor causing said second switch to close only if the line voltage is greater than a predetermined value, a current pulse sensor at the exchange which senses the current in said line, said current pulse sensor causing the subscriber's line to be held in connection with the exchange power supply if charging current pulses are detected but connecting said line to the subscriber's exchange equipment if charging current pulsing ceases, and a ringing voltage detector at said exchange which detects ringing voltage on the subscriber's exchange equipment and disconnects the exchange power supply from said line on detection of ringing voltage.

The present invention provides a charging system which does not load the line when a call is being made or received irrespective of the line length, battery condition, or variation of handset microphone current, by primarily making use of the fact that the line voltage at the subscriber's terminal will rise to the exchange battery voltage if the line is not in use. Thus by periodically disconnecting the charging elements from the line and sampling the line voltage at the subscribers terminal it is possible to ascertain line use and only reconnect the charging elements if the line is not in use.

The preferred form of the invention will now be described with reference to the accompanying drawing which show in block diagram form a battery charging circuit for a telephone subscriber carrier system.

In the drawing the blocks shown on the left hand side represent components located at a telephone exchange and the blocks shown on the right hand side represent components located at a subscriber's installation. Signals destined for the subscriber are routed onto the subscribers line 3 by line relay 1 and the subscriber's telephone is indicated diagrammatically by symbol 2. The remaining components enable the subscriber's battery 4 to be charged via line 3 from the 50 volt exchange power supply 5.

The operation of the charging system will be explained by describing how the charging system responds to the various conditions which can exist on the subscribers line commencing with the state where the subscriber's hand set has just been replaced after use. A relay 6 at the exchange still connects line 3 to the subscriber's line relay 1. Since neither microphone current nor charging current is flowing in the line the voltage at the subscriber's terminal rises to the exchange power supply voltage of 50 volts. A line voltage sampler 7 at the subscriber's terminal detects the terminal voltage increase to 50 volts and enables a gated multivibrator 8 which in turn triggers a monostable multivibrator 9 the output pulse of which closes a charging switch 10 so as to connect battery 4 to the line via 3 line voltage rectifier 11.This device simply renders the subscriber's unit insensitive to line polarity. The charging switch 10 remains closed for the duration of the output pulse of the monostable multivibrator. At the commencement of this pulse the current change in line 3 is detected at the exchange by a charging pulse detector 12 which triggers a retriggerable monostable multivibrator 13 which activates relay driver 14 for the duration of the monostable output pulse to cause the relay switch 6 to connect line 3 directly to the exchange power supply. The system state which then exists is that for normal charging conditions.

Each time the subscriber's monostable 9 times out the charging switch 10 is opened, the subscribers terminal line voltage rises, he rise is detected by the line voltage sampler 7 and the multivibrator 8 is again triggered to recommence the monostable timing cycle. The system charging state is thus periodically interrupted momentarily to enable the line voltage to be sampled. This process continues until a change in the line condition takes place. At the exchange the charging pulses, that is the current pulses which occur while the monostable is timing, are detected by the charging pulse detector 12 and the retriggerable monostable 13 is maintained in its triggered mode since its natural period is preset to be slightly longer than that of monostable 9. Thus relay 6 is kept energized and the line remains connected directly to the exchange power supply.

The charging system assumes a new state when the subscriber lifts his handset thus altering the line conditions. Upon lifting the handset additional current is drawn from the line and accordingly when monostable 9 times out (this will occur before the retriggerable exchange monostable 13 times out) charging switch 10 is opened but because of the handset current the line voltage at the subscriber terminal cannot increase to 50 volts and the gated multi-vibrator 8 is not enabled by the line voltage sampler 7. Thus the charging switch 10 remains open. Shortly after monostable 13 also times out and the relay driver 14 releases relay 6 and connects line 3 to the subscriber's exchange equipment. At this time dial tone is returned to the subscriber.

The system conditions are now such that the subscriber may dial a number in the normal way but in order to prevent voltage sampler 7 responding to the dialing pulses these are detected by a sampler inhibitor 15 which inhibits the action of voltage sampler 7.The dialing pulses on line 3 are ignored by the exchange charging pulse detector 12 by an appropriate setting of that detector's time constants.

When the subscriber has completed his call and replaces the handset the system reverts to the charging state previously described.

Afurther line condition exists if ringing originates from the exchange destined for the subscriber. The ringing pulses are first sensed at the exchange unit by the ringing voltage detector 16 which causes a switch 17 to interrupt the 50 volt supply to the subscriber's line. In addition the ringing voltage detector 16 inhibits the operation of charging pulse detector 12. When the subscriber's monostable 9 times out charging switch 10 is opened and shortly afterwards the exchange retriggerable monostable 13 times out to release relay 6 and feed the ringing pulses onto line 3 which at this stage is loaded only by the telephone 2 in the normal way. The ringing voltage is detected by the sampler inhibitor 15 which prevents voltage sampler 7 from detecting the ringing pulses and confusing them with a change to full line voltage.When the subscriber's handset is lifted the system assumes the call state previously described. However the dialing pulse inhibit function previously described in association with that state does not arise. Again, on completion of the call the system reverts to the charging state.

The charging current is variable between 20 and 60 mA according to line conditions. This charging current is more appropriate for the extended telephone use typical in free calling areas such as New Zealand. A current limiting circuit is normally incorporated in the exchange unit to set the maximum charging current when the system is used with a low resistance subscriber line. Since the battery charging current, say 60 mA, is much higherthan desirable for continuous "trickle" charging the voltage sampling inhibit circuit 15 is arranged to be triggered when a preset battery voltage is attained. This inhibits the operation of line voltage sampler 7 and maintains charging switch 10 in the open position.

Claims (10)

1. A method for charging subscriber's batteries from a central exchange in a telephone system comprising: connecting the exchange power supply to the subscriber's line, connecting the subscriber's battery to said line to which his telephone receiver equipment is perma- nently connected, periodically disconnecting the subscriber's battery from said line and sensing the line voltage at the subscriber's terminal, reconnecting the battery to the line if the line voltage is equai to the exchange power supply voltage, sensing the current in said line at the exchange and if charging current pulses are not detected disconnecting the subscriber's line from the exchange power supply and connecting it to the subscriber's exchange equipment, monitoring for ringing voltage on the subscriber's exchange equipment and if ringing is detected disconnecting the exchange power supply from said line.

2. Battery charging apparatus for charging sub scriber's batteries from a central exchange in a telephone system comprising: a first switch which connects the exchange power supply to the subscriber's line, a second switch which connects the subscriber's battery to said line to which the subscriber's telephone receiver equipment is permanently connected, means at the subscriber's installation which periodically cause said second switch to open, a voltage sensor at the subscriber's installation which senses the magnitude of the voltage on said line, said voltage sensor causing said second switch to close only if the line voltage is greater than a predetermined value, a current pulse sensor at the exchange which senses the current in said line, said current pulse sensor causing the subscriber's line to be held in connection with the exchange power supply if charging current pulses are detected but connecting said line to the subscriber's exchange equipment if charging current pulsing ceases, and a ringing voltage detector at said exchange which detects ringing voltage on the subscriber's exchange equipment and disconnects the exchange power supply from said line on detection of ringing voltage.

3. Apparatus according to claim 2 including at the subscriber's installation means for inhibiting the operation of said voltage sensor, said inhibit means having one or more inputs and being responsive to prescribed voltage conditions on said one or more inputs, one said input being connected to said line and being responsive to ringing voltage.

4. Apparatus according to claim 3 wherein said inhibit means has a second input connected to the subscriber's battery and being responsive to overcharge voltage.

5. Apparatus according to any one of claims 2 to 4 wherein said voltage sensor triggers a first monostable circuit on sensing excess voltage and said second switch is actuated by the output of said monostable circuit to remain in the closed state only for the duration of the monostable output pulse.

6. Apparatus according to claim 5 wherein said charging current pulse sensor triggers a retriggerable second monostable circuit having a time constant slightly longer than said first monostable circuit on sensing charging pulses and said first switch is actuated by the output of said second monostable circuit to maintain the connection between the exchange battery and the subscriber's line only while the output remains in the triggered state.

7. Apparatus according to any one of claims 2 to 6 wherein said ringing voltage detector actuates interrupt means in series with the connection between the exchange battery and said first switch.

8. Apparatus according to claim 7 wherein said ringing voltage detector inhibits said charging pulse sensor on detection of ringing voltage.

9. A method of charging subscribers' batteries from a central exchange in a telephone system substantially as hereinbefore described with reference to the accompanying drawing.

10. Battery charging apparatus for charging subscriber's batteries from a central exchange in a telephonesystem substantially as hereinbefore described with reference to the accompanying drawing.