IE83709B1 - A circuit for developing an AC ringing voltage from a DC voltage supply - Google Patents

Description

"A circuit for developing an AC ringing voltage from a DC voltage supply” The present invention relates to a circuit for developing an AC ringing voltage from a DC line voltage supply for applying to a telephone on an extension line of a private branch exchange (PBX).

In order to cause the ringer in a telephone to ring, an AC ringing voltage must be applied to the telephone. When a telephone is connected to a PBX by an extension line, it is essential that the PBX has the facility for applying the appropriate AC ringing voltage to the extension line. This, in general, requires the provision of a separate AC voltage supply which can be applied to the extension line, or alternatively, relatively complex circuitry for converting a DC voltage supply of the PBX into an appropriate AC ringing voltage. This, in general, adds significantly to the expense and complexity of a PBX.

There is therefore a need for a PBX, and in particular, for a circuit for use in a PBX, or indeed, in any other branch exchange, whether private, private and automatic or otherwise.

The present invention is directed towards providing a circuit for developing an AC ringing voltage from a DC voltage source.

According to the invention there is provided a circuit for developing an AC ringing voltage from a DC line voltage supply, the circuit comprising four first switch means connected in the form of a bridge to define a pair of input terminals for receiving the DC line voltage, and a pair of output terminals on which the AC ringing voltage is developed, the first switch means being connected to the input and output terminals so that each input terminal and each output terminal is connected to its adjacent output terminal or input terminal by one of the respective first switch means, and a control means for selectively and alternately switching the first switch means in pairs for alternately reversing the voltage on the output terminals for developing the AC ringing voltage on the output terminals, wherein the control means comprises four second switch means for controlling the respective first switch means.

In another embodiment of the invention each switch means is a semiconductor switch. Ideally, each switch means is provided by a transistor.

In one embodiment of the invention a detecting circuit is provided for detecting the on-hook/off-hook status of a telephone connected across the output terminals.

Preferably, the detecting circuit comprises a first impedance means, and a second impedance means connected in series across a secondary DC voltage, the second impedance means being connected to one of the input terminals of the bridge circuit so that current flowing through a telephone connected across the output terminals of the bridge circuit derived from the DC line voltage source flows through the second impedance means, the ratio of the impedance of the second impedance means to that of the first impedance means being such that when the telephone is in an on- hook state the voltage across the second impedance means is at a value on one side of a predetermined voltage, and when the telephone is in the off-hook state current flowing through the telephone and in turn through the second impedance means causes the voltage across the second impedance means to be on the other side of the predetermined voltage, the control means being connected to the second impedance means for monitoring the voltage developed across the second means.

In one embodiment of the invention the first and second impedance means are provided by first and second resistors.

In another embodiment of the invention the voltage across the second impedance means is less than the predetermined value when the telephone is in the on-hook state, and above the predetermined value when the telephone is in the off-hook state.

In another embodiment of the invention the current through the telephone is fed to the second impedance means through a third transistor, and the first impedance means is connected between the secondary DC voltage and the base of the third transistor.

In a further embodiment of the invention the second impedance means is connected between the emitter of the third transistor and ground of the DC line voltage supply.

Advantageously, the control means is connected between the second impedance means and the emitter of the transistor for monitoring the voltage across the second impedance means.

Preferably, the secondary DC voltage is derived from a voltage of logic circuitry of the control means. Advantageously, the secondary voltage is approximately 5 volts.

Preferably, the predetermined voltage is similar to the secondary DC voltage. in one embodiment of the invention the circuit is adapted for applying the DC line voltage to the output terminals under the control of the control means. Preferably, the circuit is adapted for reversing the polarity of the DC line voltage applied to the output terminals under the control of the control means.

In one embodiment of the invention the DC line voltage is connected to one of the input terminals of the bridge circuit through a high AC and low DC impedance input circuit. Preferably, a third switch means is provided in the input circuit for switching out the impedance in the input circuit for increasing the AC drive capability of the circuit. in one embodiment of the invention the DC line voltage is applied to one of the input terminals of the bridge circuit through a high impedance constant current output circuit.

Advantageously, the second impedance means forms part of the high impedance constant current output circuit.

Preferably, the third transistor forms part of the high impedance constant current output circuit.

In one embodiment of the invention the control means comprises an electronic control circuit, and preferably, the control means comprises a microprocessor.

In a further embodiment of the invention the frequency at which the respective pairs of first switch means are switched for alternately reversing the voltage on the output terminals of the bridge circuit is determined by the control means.

Additionally, the invention provides a private branch exchange comprising a circuit according to the invention for developing an AC ringing voltage from a DC line voltage supply for applying to an extension line from the private branch exchange.

The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only with reference to the accompanying drawing which illustrates a circuit diagram of a circuit according to the invention for developing an AC ringing voltage from a DC line voltage supply ofa PBX.

The circuit according to the invention is indicated generally by the reference numeral 1 and comprises a pair of main terminals 2 and 3 across which the DC line voltage of the PBX is applied, the main terminal 2 being the positive terminal and the main terminal 3 being the ground terminal. In this embodiment of the invention the DC line voltage is forty volts approximately. The DC line voltage is applied across a pair of input terminals 5 and 6 of a bridge circuit 7 of a H-bridge 8, and as will be described below, the AC ringing voltage is developed across a pair of output terminals 10 and 11. An extension line 12 from the output terminals 10 and 11 applies the AC ringing voltage to a telephone 14 and connects the telephone 14 to other circuitry (not shown) of the PBX through the bridge circuit 7. The DC voltage is applied to the input terminal 5 from the main positive terminal 2 through a high AC and low DC impedance input circuit 15 which will be described below, and is applied to the input terminal 6 from the ground terminal 3 through a high impedance constant current output circuit 17, which will also be described below.

The bridge circuit 7 of the H-bridge 8 comprises four first switch means, namely, four first transistors Q1, Q2, Q3 and Q4 which are connected in the form of a bridge so that each input terminal 5 and 6, and each output terminal 10 and 11 is connected to its adjacent output or input terminal by one of the first transistors Q1 to Q4. In other words, the input terminal 5 is connected to the output terminals 10 and 11 by the transistors Q1 and Q3, respectively, and the input terminal 6 is connected to the output terminals 10 and 11 by the first transistors Q2 and Q4.

A control means for selectively and alternately switching the first transistors Q1 to Q4 in pairs for alternately reversing the voltage on the output terminals 10 and 11 for forming the AC ringing voltage comprises a control circuit, which in turn comprises a microprocessor 20 and four second switch means, namely, four second transistors Q5, Q6, Q7 and Q8 which are provided in the H-bridge 8. In this embodiment of the invention the first transistors Q1 to Q4 are switched in pairs whereby the first transistors Q1 and Q2 form one pair of first transistors, and the first transistors Q3 and Q4 form the second pair. The second transistor Q5 switches the first transistor Q1, the second transistor Q6 switches the first transistor Q2, the second transistor Q7 switches the first transistor Q3 while the second transistor Q8 switches the first transistor Q4. Control terminals 22, 23, 24 and 25 of the H-bridge 8 which are connected to the bases of the corresponding second transistor Q5 to Q8, respectively, are connected to and controlled by the microprocessor 20. Thus, a signal from the microprocessor 20 applied to the terminals 22 and 23 switches on the second transistors Q5 and Q6 for in turn switching on the first transistors Q1 and Q2. A signal from the microprocessor 20 applied to the terminals 24 and 25 switches on the second transistors Q7 and Q8 for in turn switching on the first transistors Q3 and Q4.

During the application of the DC line voltage to the telephone 14 through the output terminals 10 and 11 the appropriate pair of first transistors Q1 and Q2, or Q3 and Q4 are held switched on by the microprocessor 20. Additionally, the polarity of the DC line voltage applied to the telephone 14 through the output terminals 10 and 11 may be reversed for on-hook transmission signalling for, for example, prior caller display transmission. The reversal of the polarity of DC line voltage on the output terminals and 11 is achieved by switching the DC line voltage to the output terminals 10 and 11 through the alternative pair of first transistors Q1 and Q2, or Q3 and Q4.

Turning now to the output circuit 17, resistors R1, R2, R3 and R4 together with a third transistor Q9 together form a constant current circuit for connecting the input terminal 6 to the ground terminal 3. A detecting means for detecting the on-hook/off- hook status of the telephone 14 comprises a detecting circuit 26 having a first and second impedance means, namely, a first resistor R8 and a second resistor provided by the resistor R3 and the third transistor Q9. A secondary voltage, which in this embodiment of the invention is the five volt logic voltage supply of the microprocessor 20, is connected to a secondary terminal 28 of the detecting circuit 26. The first resistor R8 is connected between the secondary terminal 28 and the base of the third transistor Q9. The second resistor R3 is connected between the emitter of the third transistor Q9 and the ground terminal 3. The ratio of the resistance of the second resistor R3 to the first resistor R8 is such that when the telephone 14 is in the on-hook state with no current flowing through the telephone 14, and in turn no current flowing through the output circuit 17 from the bridge circuit 7, the voltage developed across the second resistor R3 is significantly less than a predetermined voltage, which in this embodiment of the invention is the logic five volt voltage, and when the telephone 14 is in the off-hook state, the current flowing through the telephone 14 and from the input terminal 6 through the resistor R3 is such as to increase the voltage across the resistor R3 to a voltage approximating to the logic five volt voltage. The microprocessor 20 is connected at 29 between the second resistor R3 and the emitter of the third transistor Q9 for monitoring the voltage developed across the second resistor R3 so that the microprocessor 20 can determine the on and off-hook state of the telephone 14.

The input circuit 15 is a high AC and low DC impedance circuit for increasing the impedance of the circuit when the DC line voltage is being applied to the telephone 14, and comprises a fourth transistor Q10 and associated resistors and capacitors.

Current from the DC line voltage supply flows through the collector of the fourth transistor Q10 and through a resistor R10 to the input terminal 5 of the bridge circuit . A third switch means, namely, a fifth transistor Q11 in the input circuit 15 is provided for shorting out the impedance in the input circuit 15 so that the positive main terminal 2 can be connected directly to the input terminal 5 of the bridge circuit 7 through the fifth transistor Q11. Shorting out the impedance of the input circuit 15 by the fifth transistor Q11 while the AC ringing voltage is being developed across the output terminals 10 and 11 of the bridge circuit 7 allows the voltage applied across the input terminals 5 and 6 of the bridge circuit 7 to be increased, thereby increasing the AC ringing voltage. A sixth transistor Q12 operated under the control of the microprocessor 20 through a control terminal 30 is provided for operating the fifth transistor Q11.

The circuit 1 is capacitively coupled by a coupling circuit 31 to the switching matrix (not shown) of the PBX in the case of analogue switching, and to the codec (not shown) in the case of digital switching. The coupling circuit 31 comprises capacitors C1 to C4 for coupling the circuit 1 to the switching matrix or codec. Overload protection diodes D1, D2, D3 and D4 are provided for connecting the circuit 1 to the switching matrix or the codec. Terminals 32 and 33 connect the coupling circuit 31 to the positive DC line voltage, in other words to the main terminal 2 through decoupling capacitors C5 and C6 for providing EMC (electro-magnetic compatibility) suppression.

In use, the voltage developed across the second resistor R3 is monitored by the microprocessor 20 for determining the on-hook/off-hook status of the telephone 14.

The AC ringing voltage is applied to the telephone 14 through the extension line 12 from the output terminals 10 and 11 of the bridge circuit 7 under the control of the microprocessor 20 which selectively and alternately switches the second transistors Q5 to Q8 in the pairs Q5 and Q6, and Q7 and Q8 for in turn switching the first transistors Q1 to Q4 in the pairs Q1 and Q2, and Q3 and Q4. The DC line voltage is applied to the output terminals 10 and 11 under the control of the microprocessor 20 by selecting and switching on one of the pairs of first transistors Q1 and Q2, or Q3 and Q4, depending on the polarity at which the DC line voltage is to be applied to the output terminals 10 and 11 of the bridge circuit 7. Should it be desired to reverse the polarity of the DC line voltage appearing on the output terminals 10 and 11 of the bridge circuit 7 for, for example, signalling, the alternative pair of the first transistors Q1 and Q2, or Q3 and Q4 is switched on, while the other pair of the first transistors Q1 and Q2, or Q3 and Q4 is switched off.

The advantages of the invention are many. Firstly, the circuit according to the invention provides a relatively low cost circuit for developing an AC ringing voltage from a DC voltage supply with no electro-mechanical components being required, the circuit is suitable for either discrete or printed circuit board mounting, or indeed for ceramic hybrid implementation. The circuit furthermore allows for maximum drive voltage to a telephone during the application of the AC ringing voltage. The circuit can be used for either the ringing or non-ringing state of the telephone in the off- hook state. The circuit can be operated to comply with the safety extra low voltage requirement once the extension feed voltage is kept below 42.6 volts. As discussed above the circuit according to the invention may be used to provide reversals of the telephone for signalling purposes. A further advantage of the circuit according to the invention is that insertion losses of the circuit are relatively low. A further advantage of the invention is that the same circuit is used for developing and applying the AC ringing voltage and for detecting the on-hook/off—hook status of a telephone, and no further on-hook/off-hook detection circuitry is required.

While the first and second switch means have been described as being provided in the form of a H—bridge, the first and second transistors may be replaced by optoisolators where isolation is a requirement. However, the use of the first and second transistors has an advantage over the use of optoisolators due to the fact that optoisolators tend to be relatively more costly than transistors, and do not operate within the same tight specification to which transistors are capable of operating.

Claims (25)

1. A circuit for developing an AC ringing voltage from a DC line voltage supply, the circuit comprising four first switch means connected in the form of a bridge to define a pair of input terminals for receiving the DC line voltage, and a pair of output terminals on which the AC ringing voltage is developed, the first switch means being connected to the input and output terminals so that each input terminal and each output terminal is connected to its adjacent output terminal or input terminal by one of the respective first switch means, and a control means for selectively and alternately switching the first switch means in pairs for alternately reversing the voltage on the output terminals for developing the AC ringing voltage on the output terminals, wherein the control means comprises four second switch means for controlling the respective first switch means.

2. A circuit as claimed in Claim 1 in which each switch means is a semiconductor switch.

3. A circuit as claimed in Claim 1 or 2 in which each switch means is provided by a transistor.

4. A circuit as claimed in any preceding claim in which a detecting circuit is provided for detecting the on-hook/off—hook status of a telephone connected across the output terminals.

5. A circuit as claimed in Claim 4 in which the detecting circuit comprises a first impedance means, and a second impedance means connected in series across a 13 secondary DC voltage, the second impedance means being connected to one of the input terminals of the bridge circuit so that current flowing through a telephone connected across the output terminals of the bridge circuit derived from the DC line voltage source flows through the second impedance means, the ratio of the impedance of the second impedance means to that of the first impedance means being such that when the telephone is in an on—hook state the voltage across the second impedance means is at a value on one side of a predetermined voltage, and when the telephone is in the off-hook state current flowing through the telephone and in turn through the second impedance means causes the voltage across the second impedance means to be on the other side of the predetermined voltage, the control means being connected to the second impedance means for monitoring the voltage developed across the second means.

6. A circuit as claimed in Claim 5 in which the first and second impedance means are provided by first and second resistors.

7. A circuit as claimed in Claim 5 or 6 in which the voltage across the second impedance means is less than the predetermined value when the telephone is in the on-hook state, and above the predetermined value when the telephone is in the off- hook state.

8. A circuit as claimed in any of Claims 5 to 7 in which the current through the telephone is fed to the second impedance means through a third transistor, and the first impedance means is connected between the secondary DC voltage and the base of the third transistor.

9. A circuit as claimed in Claim 8 in which the second impedance means is connected between the emitter of the third transistor and ground of the DC line voltage supply.

10. A circuit as claimed in Claim 9 in which the control means is connected between the second impedance means and the emitter of the transistor for monitoring the voltage across the second impedance means.

11. A circuit as claimed in any of Claims 5 to 10 in which the secondary DC voltage is derived from a voltage of logic circuitry of the control means.

12. A circuit as claimed in any of Claims 5 to 11 in which the secondary voltage is approximately 5 volts.

13. A circuit as claimed in any of Claims 5 to 12 in which the predetermined voltage is similar to the secondary DC voltage.

14. A circuit as claimed in any preceding claim in which the circuit is adapted for applying the DC line voltage to the output terminals under the control of the control means.

15. A circuit as claimed in Claim 14 in which the circuit is adapted for reversing the polarity of the DC line voltage applied to the output terminals under the control of the control means. I5

16. A circuit as claimed in Claim 14 or 15 in which the DC line voltage is connected to one of the input terminals of the bridge circuit through a high AC and low DC impedance input circuit.

17. A circuit as claimed in Claim 16 in which a third switch means is provided in the input circuit for switching out the impedance in the input circuit for increasing the AC drive capability of the circuit.

18. A circuit as claimed in any preceding claim in which the DC line voltage is applied to one of the input terminals of the bridge circuit through a high impedance constant current output circuit.

19. A circuit as claimed in Claim 18 in which the second impedance means forms part of the high impedance constant current output circuit.

20. A circuit as claimed in Claim 18 or 19 in which the third transistor forms part of the high impedance constant current output circuit.

21. A circuit as claimed in any preceding claim in which the control means comprises an electronic control circuit.

22. A circuit as claimed in any preceding claim in which the control means comprises a microprocessor. 16

23. A circuit as claimed in any preceding claim in which the frequency at which the respective pairs of first switch means are switched for alternately reversing the voltage on the output terminals of the bridge circuit is determined by the control means.

24. A circuit for developing an AC ringing voltage from a DC line voltage supply, the circuit being substantially as described herein with reference to and as illustrated in the accompanying drawing.

25. A private branch exchange comprising a circuit as claimed in any preceding claim for developing an AC ringing voltage from a DC line voltage supply for applying to an extension line from the private branch exchange. F.F. GORMAN & CO.