LINE CIRCUIT FOR DRIVING A RINGER AND A SERVICE INDICATING LAMP IN A TELEPHONE SET

Abstract: A line circuit for driving a ringer and a service indicating lamp in a telephone set, including a high frequency transfor­ mer (T₂) a switching circuit (22) for intermittently discon­ necting a D.C. voltage applied to the primary winding of the transformer (T₂), and a control signal generating circuit (23). The control signal generating circuit generates a first control signal having high frequency and having a first duty ratio when the ringer is to be rung; and generates a second control signal having the same frequency but having a second duty ratio smaller than the first duty ratio when the service indicating lamp is to be lit. The first or the second control signal is applied to the switching circuit so that the high frequency transformer generates a ringing signal or a D.C. voltage to light the lamp. Since the high-frequency transformer is commonly used for generating the ringing signal and the D.C. voltage for the lamp, the line circuit can be made small and light.

Description: The present invention relates to a line circuit connected between a speech path switch and a telephone set, said telephone set having a ringer connected to a subscriber line connected to said line circuit, comprising: a servicing power supply circuit, operatively connected to said telephone set, for generating a ringer driving signal for driving said ringer, said servicing power supply circuit comprising: a transformer having a primary winding and a secondary winding; a direct-current power supply operatively connected to said primary winding, for generating a direct-current voltage; a smoothing circuit connected between said secondary winding and said subscriber line, for smoothing signals at said secondary winding; a switching circuit operatively connected to said first winding for intermittently disconnecting the current path of said direct-current power supply; and a control signal generating circuit operatively connected to said switching circuit, for generating a first control signal having a first frequency and a first duty ratio and said first control signal being applied to said switching circuit when said ringer is to be rung, said direct-current voltage being transformed to said ringer driving signal through said transformer and said smoothing circuit in response to said first control signal. Such a line circuit is known from EP-A-0 106 247. As an example of a service indicating function, the message waiting function is explained. By using a telephone set having the message function, when a called party is absent, a message lamp for message waiting function provided on the telephone set is lit to inform the later arriving party that someone has called him. For example, in a hotel, when a message is left at the front desk during the absence of a guest and should be informed to the guest when he returns to his room, a switch provided on the front desk is operated so that the message lamp of the telephone set in the guest room is lit, informing him that a message has been left at the front desk. By this, the guest can know that there is a message for him. The lamp can be lit not only by the switching operation at the front desk but also by dialing a special number from another telephone set, an exchange in the hotel decoding the special number so as to light the message lamp. To light the message lamp, a direct-current (D.C.) voltage of, for example, 100V must be applied through a subscriber line to the message lamp. Apart from the message lamp, in a two-wire subscriber line, to ring the ringer in the telephone set when a call is received, a low-frequency ringing signal of, for example 16 Hz is sent from a line circuit to the subscriber line. The root mean square value of the ringing signal is, for example, 75V. Conventionally, there are three power supplies, i.e. a first power supply for supplying a voltage e.g. -48V, for communication to the subscriber line, a second power supply for supplying a ringing signal to the ringer, and a third power supply for supplying a message signal to the message lamp. Further, to generate the low-frequency ringing signal having the root square value of 75V as an example, a low-frequency transformer, which is large in size and heavy due to the low frequency, is conventionally employed. To generate the message signal of D.C. voltage, a separate rectifier circuit is conventionally employed by which a commercial alternating current is rectified to obtain a D.C. voltage of e.g. 100V. This increases the number of parts. FR-A-2 419 627 discloses a supply arrangement for a line circuit in which a pulse generator of constant duty ratio is connected to the primary winding of a transformer. The secondary winding of the transformer is connected to a low pass filter via a switching device under the control of a signal generator. The secondary output is adjusted by switching the switching means at a phase controllable relative to the phase of the signal generator. Thus the switching operation has to take place at relatively high voltage in the transformer secondary, so that relatively expensive and bulky components are necessary. Accordingly, an object of the present invention is to provide a line circuit for driving a ringer and a service indicating lamp in a telephone set, in which a power supply circuit is commonly used for driving both the ringer and the service indicating lamp so as to reduce the device size. Another object of the present invention is to provide the above-mentioned line circuit in which, based on the idea of employing a switching regulator for D.C.-D.C. conversion in the power supply circuit, the size and the weight of the line circuit are reduced so as to eliminate the installation problem. According to the invention, the line circuit initially defined is characterized in that said telephone set has a service indicating lamp connected to said subscriber line; said servicing power supply circuit is operable to generate a lamp driving signal for driving said service indicating lamp; said control signal generating circuit is operable to generate a second control signal having said first frequency but having a second duty ratio different from said first duty ratio; and said second control signal is applicable to said switching circuit when said service indicating lamp is to be lit, said direct-current voltage being transformed to said lamp driving signal through said transformer and said smoothing circuit in response to said second control signal; whereby said secondary winding of said transformer is operable to generate different voltages in response to said first control signal and said second control signal, respectively. The above objects and features of the present invention will be more apparent from the following description of the preferred embodiments with reference to the attached drawings, wherein: Fig. 1 is a circuit diagram of a conventional line circuit; Fig. 2 is a circuit diagram of a line circuit according to an embodiment of the present invention; Figs. 3A through 3N are waveform diagrams at various points in the line circuit shown in Fig. 2; and Fig. 4 is a graph of experimental results of the relation between the voltage applied to a message lamp and the pulse width of an output 16 kHz switching signal. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiments, a conventional line circuit will first be described with reference to Fig. 1. In Fig. 1 a telephone set 1 having a message function is connected through a line circuit 2 to a speech-path switch in an exchange. In the line circuit 2, speech-sound signals transferred from the speech-path switch are sent through a transformer T and subscriber lines SL to be applied to terminals L₁ and L₂ of the telephone set 1. A power supply voltage of e.g. -48V, used for communication, is applied to the secondary winding of the transformer T. Between the power supply of -48V and the secondary winding, a switch S₀ is provided. The line circuit 2 is connected to a ringer driving circuit 3 and a lamp driving circuit 4. Since the conventional ringer driving circuit 3 and the lamp driving circuit 4 have large sizes and are heavy, they are usually provided separate from the package of the line circuit 2. The telephone set 1 includes a ringer 5 and a message lamp 6. When the ringer 5 is to be rung, the ringer driving circuit 3 is connected through a relay switch r1₀ and the switch S₀ to the secondary winding of the transformer T. When the message lamp 6 is to be lit, the lamp driving circuit 4 is connected through the relay switch r1₀ and the switch S₀ to the secondary winding of the transformer T. The ringer driving circuit 3 generates a ringing signal having a frequency of 16 Hz and having a root mean square value of 75V, (75 Vrsm), as an example. The lamp driving signal generates a D.C. 100V signal. The ringer driving circuit 3 includes an oscillator for generating an A.C. current of 16 Hz and an additional transformer for generating a 16 Hz signal having 75 Vrms. By switching the internal power supply voltage of -48V, applied to the primary winding of the additional transformer, with the A.C. current of 16 Hz from the oscillator, a 16 Hz signal having 75 Vrms is obtained at the secondary winding of the additional transformer. The lamp driving circuit 4 includes a rectifying circuit for rectifying the commercial A.C. voltage into D.C. 100V to be applied to the message lamp. The disadvantages of the conventional line circuit 2 are as follows. First, since the additional transformer in the ringer driving circuit 3 is used at such a low frequency of 16 Hz, the size and weight of the additional transformer are very large so that the line circuit 2 become very large and heavy if it includes the ringer driving circuit 3 and the lamp driving circuit 4, causing an installation problem. Second, since the lamp driving circuit employs the rectifying circuit for rectifying the commercial A.C. voltage, the size and weight of the device are further increased. Third, since the power supply circuit for driving the ringer and the power supply circuit for driving the lamp are separately provided, the size and weight of the device are still further increased. Now, preferred embodiments of the present invention will be described. Figure 2 is a circuit diagram of a line circuit according to an embodiment of the present invention. In Fig. 2, a line circuit 2a is connected between a speech path switch and a telephone set 1. The telephone set 1 in Fig. 2 is the same as the telephone set 1 in Fig. 1 and includes the ringer 5 and the message lamp 6. The line circuit 2a includes a speech current supplying circuit 2b and a servicing power supply circuit 2c. The speech current supplying circuit 2b includes a first transformer T₁ having a primary winding connected to the speech path switch and having a secondary winding connected, through relay switches r1₁ and r1₂ and the subscriber lines SL, to the terminals L₁ and L₂ of the telephone. The direct current through the secondary winding is disconnected by a capacitor C₁. One end of the capacitor C₁ is connected to a power supply of -48V. The other end of the capacitor C₁ is connected to the ground. The servicing power supply circuit 2C includes a second transformer T₂ having a primary winding and a secondary winding, a D.C. power supply of +24V connected through an inductor L₁ of 2.7 mH to the primary winding of the transformer T₂ , a smoothing circuit 21 connected between the secondary winding of the transformer T₂ and the relay switches r1₁ and r1₂ , a switching circuit 22 connected to the primary winding of the transformer T₂ , a control signal generating circuit 23 connected to the switching circuit 22, and a relay driving circuit 24. The smoothing circuit 21 includes diodes D₁ and D₂ , an inductor L₂ , and a capacitor C₂. The switching circuit 22 includes an NPN transistor TR₁ having a collector connected to the primary winding of the transformer T₂ , a base operatively receiving, through an amplifier A and a resistor R₁ of 100 Ω, a control signal H or I which is output from the control signal generating circuit 23, and an emitter connected to the ground. The base and the emitter of the transistor TR₁ are connected through a biasing resistor R₂ of 100 Ω. The control signal generating circuit 23 includes a control circuit 25, a counter 26, NAND gates 27 through 31, and inverter gates 32 through 34. The control circuit 25 generates at its output O₁ , O₂ , and O₃ , a message lamp control signal, a ringer control signal, and a relay control signal, respectively. The counter 26 receives an internal clock signal having a frequency of, for example, 2.048 MHz, and divides the frequency of the clock signal into sub-frequencies of 2.048 MHz/2⁴, 2.048 MHz/2⁵, 2.048 MHz/2⁶, and 2.048 MHz/2⁷ which are output at terminals Q₄ , Q₅ , Q₆ , and Q₇ , respectively. When the relay R is not activated so that the relay switches r1₁ and r1₂ are OFF as illustrated, a direct current is supplied from the D.C. power supply of -48V through subscriber line SL to the telephone set 1 having the message waiting function. In response to the ringer control signal output from the terminal O₂ of the control circuit 25, a first control signal H is generated from the control signal generating circuit 23. In response to the message lamp control signal output from the terminal O₁ of the control circuit 25, a second control signal I is generated from the control signal generating circuit 23. The generation of the first or the second control signal is described in more detail with reference to Figs. 3A through 3M. Figures 3A through 3C show the waveforms of the divided signals A, B, and C of 2.048 MHz/2⁴=128 kHz, 2.048 MHz/2⁵=64 kHz, and 2.048 MHz/2⁶=32 kHz output from the terminals Q₄ , Q₅ , and Q₆ of the counter 26, respectively. Figure 3D shows the waveform of the inverted signal of the divided signal having the frequency of 2.048 MHz/2⁷. That is, the signal shown in Fig. 3D is the inverted signal of the signal output from the terminal Q₇ inverted by the inverter 32. In the following, the signal in Fig. 3D is referred to as a first control signal D. The first control signal D has the frequency of 16 kHz and has a duty ratio of 50%. The signals A, B, C, and D are gated through the NAND gate 27, resulting, at its output, in a signal E as shown in Fig. 3E. The signal E is inverted through the NAND gate 28 so that a signal F as shown in Fig. 3F is obtained. The signal F shown in Fig. 3F is hereinafter referred to as a second control signal. The second control signal F has a frequency of 16 kHz, which is the same as the first control signal D, but has a duty ratio of 6.25%. This will be understood as because of the gating of the signals A, B, C, and D through the NAND gate 27. Operation to Ring the Ringer When the ringer 5 is to be rung, the control circuit 25 outputs a ringer control signal having a low frequency of, for example, 16 Hz. The ringer control signal is inverted by the inverter 33. The inverted ringer control signal is shown in Fig. 3G. The inverted ringer control signal shown in Fig. 3G and the first control signal D shown in Fig. 3D are gated through the NAND gate 29. Thus, at the output of the NAND gate 29, an intermittent signal H shown in Fig. 3H is obtained. The intermittent signal H is a superimposed signal of the first control signal D and the ringer control signal G. The superimposed signal has the high frequency of 16 kHz and has a duty ratio of 50% but is intermittent together with the low frequency of 16 Hz. That is, the high level of the low-frequency signal, i.e., 16 Hz, consists of the high-frequency signal, i.e., 16 kHz having the duty ratio of 50%. The superimposed signal H is applied through the NAND gate 31 and through the amplifier A and the resistor R₁ in the switching circuit to the base of the switching transistor TR₁. The transistor TR₁ then conducts an intermittent current from the power supply of +24V through the inductor L₁ , the primary winding of the transformer T₂ , and the transistor TR₁ to the ground. The intermittent current is determined by the superimposed signal H. That is, the current passing through the transistor TR₁ has a rectangular waveform of low frequency of 16 Hz, in which each high level of 16 Hz consists of high-frequency signals of 16 kHz. Since the current supplied to the primary winding of the transformer T₂ includes the high frequency of 16 kHz, the transformer T₂ can be made a small high-frequency transformer. This is a main advantage of the present invention. The transformer T₂ converts the D.C. voltage of 24V into a high voltage of, for example, 200V peak to peak, which is sufficient to ring the ringer. This conversion is effected by the function of a switching regulator utilizing the high frequency of 16 kHz. The high frequency component of 16 kHz of the induced signal at the secondary winding of the transformer T₂ is smoothed by the smoothing circuit 21. Thus, at the output of the smoothing circuit 21, a 16 Hz signal J having a peak-to-peak voltage of 200V is obtained, as shown in Fig. 3J. The 16 Hz signal J is the continuous ringing signal CR which is supplied to the relay switches r1₁ and r1₂. Simultaneous with the output of the ringer control signal, the control circuit 25 outputs a relay driving signal from the output terminal O₃. The relay driving signal is applied to the base of the transistor TR₂ , so that the relay R is driven. The relay driving signal is an intermittent signal consisting of a high level of, for example, one second and a subsequent low level of, for example, two seconds. Accordingly, the relay switches r1₁ and r1₂ are turned ON for one second and then are turned OFF for two seconds. Thus, a ringing signal K shown in Fig. 3K is supplied through the relay switches r1₁ and r1₂ and the transformer T₁ to the telephone set 1 to intermittently ring the ringer 5. The ringing signal K has a 16 Hz signal for one second and a subsequent low level for two seconds. In a practical example, there are another seven telephone sets (not shown) connected through seven relay switches (not shown) to the output of the smoothing circuit. Also, there are another seven relay driving circuits (not shown) corresponding to the above-mentioned seven relay switches. The control circuit 25 determines which of these relay driving circuits should be activated. That is, a total of eight telephone sets are connected to one line circuit. In the ringing signal shown in Fig. 3K, there are three phases ₁ , ₂ , and ₃ in one cycle. The first phase ₁ is used for ringing the ringer in one of four telephone sets among the above-mentioned eight telephone sets. The second phase ₂ is used for ringing the ringer in one of the remaining four telephone sets in the eight telephone sets. Figure 3L shows the ringing signal formed by the second phase ₂. The third phase ₃ is commonly used for lighting the message lamp in one of the eight telephone sets, as shown in Fig. 3N. Operation to Light the Lamp When the message lamp 6, such as a neon lamp, is to be lit, the control circuit 25 outputs the lamp control signal from the output terminal O₁. The lamp control signal is a continuously high level signal. In response to the message signal, the second control signal F shown in Fig. 3F having the high frequency of 16 kHz but having a small duty ratio of 6.25% is gated through the NAND gate 30. The second control signal F is then gated through the NAND gate 31 so that the second control signal is obtained as a signal I at the output of the NAND gate 31, as shown in Fig. 3I. The signal I is the same as the second control signal F. Thus, the second control signal I is supplied to the base of the transistor TR₁ so that the transformer T₂ converts, by the function of the switching regulator, the D.C. voltage of +24V into a high voltage of, for example, 100V, necessary to light the lamp 6 but insufficient to ring the ringer 5. The message lamp 6 is lit by a D.C. voltage between 90V and 130V. Since the duty ratio of the second control signal F is 6.25%, which is smaller than the duty ratio of 50% in the first control signal D, the appropriate voltage of 100V, which is lower than the 200V peak-to-peak voltage for ringing the ringer 5, can be obtained. The high voltage of 100V is then smoothed by the smoothing circuit 21 so that the A.C. components are eliminated, as shown in Fig. 3M. The relay switches r1₁ and r1₂ operate in the same way as in the operation to ring the ringer 5. Thus, a message lamp signal N is supplied to the message lamp 6 to light it. Since no separate rectifier circuit is necessary to obtain the D.C. voltage for driving the lamp, the line circuit can be made compact. The duty ratio of the second control frequency F may not always be 6.25% but may be changed within a certain range. This will be clearer from the following description with reference to Fig. 4, which shows an experimental result of a relation between the voltage applied to the message lamp and the pulse width of the second control signal. In Fig. 4, the vertical axis represents a voltage applied to the terminals L₁ and L₂ of the telephone set 1; and the horizontal axis represents the pulse width WID of the second control signal F. The second control signal has the frequency of 16 kHz as mentioned before. Therefore, one cycle of the second control signal F is 62.5 µs. When the pulse width WID is 4 µs, the duty ratio is 6.4%. The experimental result shows that the voltage applied to the terminals L₁ and L₂ is 120V when the pulse width WID is 4 µs. The message waiting lamp 6 can be lit by the voltage between 90V and 130V. Therefore, the pulse width WID may be selected within a range between 4 µs+α and 4 µs-β, where α and β are experimentally determined in such a way that the maximum voltage and the minimum voltage applied to the lamp 6 are determined as 130V and 90V, respectively. When the pulse width WID is increased, the voltage is also increased to approach 200V as by the first control signal D of duty 50%. The present invention is not restricted to the above-described embodiment. Various changes and modifications are possible without departing from the scope of the invention. For example, the frequency of the first control signal and the second control signal may be any frequency as long as it is sufficiently high to operate the transformer T₂ as a switching regulator. Further, the frequency of the ringer control signal may also be any frequency as long as it can ring the ringer 5. From the foregoing description, it will be apparent that, according to the present invention, by employing a high-frequency transformer as a switching regulator for obtaining a ringing signal, and since the high-frequency transformer is small in size and light in weight in comparison with a conventionally employed low-frequency transformer, the size and weight of a line circuit having a function to drive a ringer and a service indicating lamp can be made small. In addition, since the same high-frequency transformer is also used for obtaining a lamp driving signal, the conventionally employed separate rectifier can be eliminated. This also leads to a smaller size and smaller weight of the line circuit. Recently, four-wire multifunctioning telephone sets such as key telephone sets which can provide various services other than the speech service have been developed. Such a multifunctioning telephone set includes an oscillator as a ringer. Therefore, to such a multifunctioning telephone set, it is not necessary to send the 16 Hz ringing signal. When such four-wire telephone sets and the two-wire telephone sets as in the present invention have to be incorporated into a hydrid exchange system, the line circuit package for the two-wire system must be compact so as to be easily replaced by the line circuit package for the four-wire system. Thus, the line circuit of small size and small weight according to the present invention is particularly advantageous in the hybrid system.

Claims: A line circuit connected between a speech path switch and a telephone set, said telephone set having a ringer (5) connected to a subscriber line connected to said line circuit, comprising: a servicing power supply circuit (2C), operatively connected to said telephone set, for generating a ringer driving signal (K) for driving said ringer, said servicing power supply circuit comprising: a transformer (T₂) having a primary winding and a secondary winding; a direct-current power supply (+24V) operatively connected to said primary winding, for generating a direct-current voltage; a smoothing circuit (21) connected between said secondary winding and said subscriber line, for smoothing signals at said secondary winding; a switching circuit (22) operatively connected to said first winding for intermittently disconnecting the current path of said direct-current power supply; and a control signal generating circuit (23) operatively connected to said switching circuit, for generating a first control signal (D) having a first frequency and a first duty ratio (50%); and said first control signal (D) being applied to said switching circuit (22) when said ringer is to be rung, said direct-current voltage (+24V) being transformed to said ringer driving signal (K) through said transformer and said smoothing circuit in response to said first control signal; characterized in that: said telephone set has a service indicating lamp (6) connected to said subscriber line; said servicing power supply circuit is operable to generate a lamp driving signal (N) for driving said service indicating lamp; said control signal generating circuit (23) is operable to generate a second control signal (F) having said first frequency but having a second duty ratio (6.25%) different from said first duty ratio; and said second control signal (F) is applicable to said switching circuit (22) when said service indicating lamp is to be lit, said direct-current voltage being transformed to said lamp driving signal (N) through said transformer and said smoothing circuit in response to said second control signal; whereby said secondary winding of said transformer is operable to generate different voltages in response to said first control signal and said second control signal, respectively. A line circuit as set forth in claim 1, wherein said first control signal (D) having said first frequency (16 kHz) and having said first duty ratio is made discontinuous intermittently together with a second frequency (16 Hz) lower than said first frequency (16 kHz) and is applied to said switching circuit, whereby said smoothing circuit outputs an alternating signal having said second frequency (16 Hz), used as said ringer driving signal; and said second control signal (F), having said first frequency (16 kHz) but having said second duty ratio (6.25%), is continuously applied to said switching circuit (22), whereby said smoothing circuit outputs a direct-current voltage used as said lamp driving signal. A line circuit as set forth in claim 1, wherein said control signal generating circuit (23) comprises: a control circuit (25) for generating a ringer control signal (G) and a lamp control signal; a first gate circuit (29) having inputs operatively connected to receive said first control signal (D) and said ringer control signal (G) and having an output connected to said switching circuit; and a second gate circuit (30) having inputs operatively connected to receive said second control signal (F) and said lamp control signal and having an output connected to said switching circuit; said ringer control signal (G) being generated when said ringer (5) is to be rung, said ringer control signal having a second frequency (16 Hz) lower than said first frequency (16 kHz), said lamp control signal being generated when said service indicating lamp (6) is to be lit, said lamp control signal being a direct-current signal, whereby, said first gate circuit (29) operatively outputting a first superimposed signal of said first control signal (D) and said ringer control signal (G), said first superimposed signal having said first frequency (16 kHz) but being intermittent together with said second frequency (16 Hz), and said second gate circuit (30) operatively outputting a second superimposed signal of said second control signal (F) and said lamp control signal, said second superimposed signal being continuous and having said first frequency (16 kHz) and said second duty ratio (6.25%). A line circuit as set forth in claim 3, wherein said switching circuit (22) comprises a transistor (TR₁) having a collector connected to said primary winding of said transformer (T₂), a base connected to receive said first superimposed signal and said second superimposed signal, and a collector being connected to the ground. A line circuit as set forth in claim 3, wherein said control signal generating circuit (23) comprises a counter (26) for dividing an internal clock signal (2.048 MHz) into a plurality of subfrequencies; a third gate circuit (32) operatively connected to one output of said counter (26) for outputting said first control signal (D); and a fourth gate circuit (27) operatively connected to the other outputs of said counter (26) for outputting said second control signal (F).

Inventors: ITOH, MASAKI, KOIKE, JUNICHI, TANAKA, YOSHISHIGE
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: H04M19
IPC codes: H04M3, H04M19