GB2182527A - Telephone circuit - Google Patents

Abstract

Modern telephones often include electronics which needs a low current supply when the telephone is on-hook. This is usually supplied via a bleed resistor which by-passes the hook-switch when on-hook. This has disadvantages, so the present invention overcomes these disadvantages by replacing it with a constant-current active circuit using transistors and resistors. The constant current circuit uses two transistors (T1 and T2) and two resistors (R1 and R2), connected as shown. <IMAGE>

Description

SPECIFICATION Telephone circuit This invention relates to circuit arrangements for use in apparatus such as telephone subscribers' instruments connected to a telephone line.

Telephone instruments and other apparatus connected to telephone subscribers' lines are becoming increasingly complex and thus often contain circuits or memories which need to be maintained operative when the apparatus is off-line, i.e. on-hook in the case of atelephone instrument. Many telephones administrations allow a small current, or bleed, to be drawn when on-hook to provide a supply in such cases. The conventional wayto do this is to draw the bleed currentvia a high value resistor.

While the use of such a bleed resistor is simple it has disadvantages. Thus if two or more instruments are connected to a single line, the bleed current taken by one instrument while another is on-line, i.e. off-hook, is severely reduced since the line voltage may then fall from the nominal exchange voltage of 48 volts to as little as 9 volts. If this happens the bleed current is inadequate for correct working. Asecond disadvantage is that some small PABX's use lower supply voltage, e.g. 24 volts. Hence instruments intended to work on the normal 48 volt exchange supply only take half the designed current when connected to a 24 volt supply. This would necessitate the supply of special instruments for such exchanges.A third disadvantage is that some administrations although allowing the use of bleed currenttestthe system at a voltage higher than the normal exchange voltage. To complywith this need at this highervoltage, the current available in normal operation may be much reduced.

Conventional solutions involve batteries in the subscriber's instruments, or local mains connections, and these are expensive. Hence it is an object of the invention to provide an arrangement in which the abovementioned disadvantages are minimised or overcome.

According to the invention, there is provided a supply circuit arrangement for a telephone subscriber's instrument, or other apparatus connected to a telephone line, which arrangement when in use by-passes the hook-switch or its equivalent to supply currentto the instrument or apparatus when in the on-hook or inoperative condition, wherein the arrangement includes active semi-conductor means so proportioned as to pass a current of suitable value forthe supply ofthe instrument or apparatus when in the on-hook or inoperative condition.

An embodiment ofthe invention will now be described with reference to the accompanying drawing, in which Fig. lisa block diagram of a conventional telephone subscriber's instrument, and Fig. 2 is an active bleed circuit, according to the invention, for use in a set such as that of Fig. 1.

The blocks shown in Fig. 1 comprise protection elements A, polarity guard B, hook-switch C,transmission circuit D, signalling circuit E, power feeding circuitry F and a bleed circuit G. The polarity guard B is usually a diode or transistor bridge and it ensures that the direct currentfrom the line has the correct polarity irrespective ofthewaythe instrument is connected to the line. The bleed circuit is shown connected directly into the signalling circuit, but it could also be connected to the power feeding circuit.

As already mentioned, in the conventional case the bleed is a simple resistor, the approximate bleed currentthen being given by:- lb = V, RB where VL is the line voltage and Rb is the bleed resistance.

In the present arrangement, the resistor is replaced by an active circuit, which may be ofthe constant current source type, and an example is shown in Fig. 2.

In this, transistorT1 is biassed into conduction via a resistor Tri, so the current in T1 increases until the voltage developed across another resistor R2 causes T2 to conduct. It then diverts currentfrom the base of T1, and the circuit stabilises with the currentthrough T1 limited to that which just causes T2 to conduct. The approximate bleed current derived via such a circuit is given by:- lb = VBE R2 where VBE is the base-emitter voltage at which T2 begins to conduct. Thus the bleed current is substantially independent of line voltages.

Hence the current supply to the electronic circuitry in the sub-set is stabilised at a level suitable forthat circuitry. This applies also when two telephone instruments are connected in parallel tothe line, which is significant since such arrangements are becoming more fashionable.

This bleed circuit may, if desired, be disabled when off-hook since it is then shorted out by the hook-switch or its equivalent.

Thus the electronics in the instrument can receive a suitable current level both when the instrument is in parallel with another instrument, and when connected two a low voltage PABX. It will also comply with administrations' requirements forthe supply of bleed current atvoltages higherthan those normally provided by exchanges, The bleed circuit can be a separate unit which is thus applicable to a variety of existing instruments or it can be incorporated into a chip which also contains some at ieast of the electronic circuitry used in the instrument.

1. A supply circuit arrangement for a telephone subscriber's instrument, or other apparatus connected to a telephone line, which arrangement when in use by-passes the hook-switch or its equivalent to supply current to the instrument or apparatus when in the on-hook or inoperative condition, wherein the arrangement includes active semiconductor means so proportioned as to pass a current ofsuitable value for the supply ofthe instrument or apparatus when in the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Telephone circuit This invention relates to circuit arrangements for use in apparatus such as telephone subscribers' instruments connected to a telephone line. Telephone instruments and other apparatus connected to telephone subscribers' lines are becoming increasingly complex and thus often contain circuits or memories which need to be maintained operative when the apparatus is off-line, i.e. on-hook in the case of atelephone instrument. Many telephones administrations allow a small current, or bleed, to be drawn when on-hook to provide a supply in such cases. The conventional wayto do this is to draw the bleed currentvia a high value resistor. While the use of such a bleed resistor is simple it has disadvantages. Thus if two or more instruments are connected to a single line, the bleed current taken by one instrument while another is on-line, i.e. off-hook, is severely reduced since the line voltage may then fall from the nominal exchange voltage of 48 volts to as little as 9 volts. If this happens the bleed current is inadequate for correct working. Asecond disadvantage is that some small PABX's use lower supply voltage, e.g. 24 volts. Hence instruments intended to work on the normal 48 volt exchange supply only take half the designed current when connected to a 24 volt supply. This would necessitate the supply of special instruments for such exchanges.A third disadvantage is that some administrations although allowing the use of bleed currenttestthe system at a voltage higher than the normal exchange voltage. To complywith this need at this highervoltage, the current available in normal operation may be much reduced. Conventional solutions involve batteries in the subscriber's instruments, or local mains connections, and these are expensive. Hence it is an object of the invention to provide an arrangement in which the abovementioned disadvantages are minimised or overcome. According to the invention, there is provided a supply circuit arrangement for a telephone subscriber's instrument, or other apparatus connected to a telephone line, which arrangement when in use by-passes the hook-switch or its equivalent to supply currentto the instrument or apparatus when in the on-hook or inoperative condition, wherein the arrangement includes active semi-conductor means so proportioned as to pass a current of suitable value forthe supply ofthe instrument or apparatus when in the on-hook or inoperative condition. An embodiment ofthe invention will now be described with reference to the accompanying drawing, in which Fig. lisa block diagram of a conventional telephone subscriber's instrument, and Fig. 2 is an active bleed circuit, according to the invention, for use in a set such as that of Fig. 1. The blocks shown in Fig. 1 comprise protection elements A, polarity guard B, hook-switch C,transmission circuit D, signalling circuit E, power feeding circuitry F and a bleed circuit G. The polarity guard B is usually a diode or transistor bridge and it ensures that the direct currentfrom the line has the correct polarity irrespective ofthewaythe instrument is connected to the line. The bleed circuit is shown connected directly into the signalling circuit, but it could also be connected to the power feeding circuit. As already mentioned, in the conventional case the bleed is a simple resistor, the approximate bleed currentthen being given by:- lb = V, RB where VL is the line voltage and Rb is the bleed resistance. In the present arrangement, the resistor is replaced by an active circuit, which may be ofthe constant current source type, and an example is shown in Fig. 2. In this, transistorT1 is biassed into conduction via a resistor Tri, so the current in T1 increases until the voltage developed across another resistor R2 causes T2 to conduct. It then diverts currentfrom the base of T1, and the circuit stabilises with the currentthrough T1 limited to that which just causes T2 to conduct. The approximate bleed current derived via such a circuit is given by:- lb = VBE R2 where VBE is the base-emitter voltage at which T2 begins to conduct. Thus the bleed current is substantially independent of line voltages. Hence the current supply to the electronic circuitry in the sub-set is stabilised at a level suitable forthat circuitry. This applies also when two telephone instruments are connected in parallel tothe line, which is significant since such arrangements are becoming more fashionable. This bleed circuit may, if desired, be disabled when off-hook since it is then shorted out by the hook-switch or its equivalent. Thus the electronics in the instrument can receive a suitable current level both when the instrument is in parallel with another instrument, and when connected two a low voltage PABX. It will also comply with administrations' requirements forthe supply of bleed current atvoltages higherthan those normally provided by exchanges, The bleed circuit can be a separate unit which is thus applicable to a variety of existing instruments or it can be incorporated into a chip which also contains some at ieast of the electronic circuitry used in the instrument. CLAIMS

1. A supply circuit arrangement for a telephone subscriber's instrument, or other apparatus connected to a telephone line, which arrangement when in use by-passes the hook-switch or its equivalent to supply current to the instrument or apparatus when in the on-hook or inoperative condition, wherein the arrangement includes active semiconductor means so proportioned as to pass a current ofsuitable value for the supply ofthe instrument or apparatus when in the on-hook or inoperative condition.

2. An arrangement as claimed in claim 1, and which includes a firsttransistorwith its collector connected to an input terminal, a first resistor connected between the input terminal and the base of the transistor, a second transistor having its base connected to the emitter of the first transistor and its collector connected to the base of the first transistor, and a second resistor connected between the emitter ofthe firsttransistor and an output terminal, which outputterminal is also connected tothe emitter ofthe second transmitter.

3. An arrangement as claimed in claim 1 or 2, and which is disabled when the instrument or apparatus is in the off-hook or operative condition.

4. Atelephone subscriber's instrument which includes electronic circuitry which needs to be supplied with electrical current when the instrument is on-hook, and a bleed circuitwhich by-passes the hook-switch or its equivalent when the instrument is in its on-hook condition, said bleed circuit providing a current supply for said electronic circuitry when the instrument is in the on-hook condition, wherein the bleed circuit includes a transistorised constantcurrent circuitso proportioned asto pass a currentofsuitable valueforsaid electronic circuitry in the on-hook condition of the instrumentfor a wide range of line voltage conditions.

5. An instrument as claimed in claim 4, and when the bleed circuit is disabled when the instrument is off hook.

6. An instrument as claimed in claim 5, and wherein the bleed circuit has an input terminal connected to the line side of the hook switch or its equivalent and outputterminal connected to the electronic circuitry to be supplied, and includes a first transistor whose collector is connected to the input terminal, a first resistor connected between the input terminal and the base of the transistor, a second transistor having its base connected to the emiter of thefirsttransistor and its collector connected to the base ofthe first transistor, and a second resistor connected between the emitter of the first transistor and the outputterminal, which output terminal is also connected to the emitter of the second transistor.

7. An instrument as claimed in claim 5 or 6, and wherein said disablementofthe bleed circuitwhen the instrument is in the off-hook condition is effected by short-circuitry bythe hook switch or its equivalent.

8. Atelephonesubscriber's instrument, substantially as described with reference to the accompanying drawing.