GB2202714A - A ringing voltage generator - Google Patents

Abstract

A ringing voltage generator circuit comprises a transformer (TR1) the secondary winding (S1) of which delivers an AC output through an opto-coupled triac (161) to a ringing voltage distribution line (1). A control circuit comprising a micro-processor (IC2) and a transistor (T1) switch the opto-coupled triac (IC1) on and off to give the desired ringing voltage on the ringing voltage distribution line (1). A biasing resistor (R1) is connected between the ringing voltage distribution line (1) and a negative biasing voltage of -30 volts DC to return the voltage on the line 1 to -30 volts DC when the opto-coupled triac (IC1) switches off. <IMAGE>

Description

A RINGING VOLTAGE GENERATOR The present invention relates to a ringing voltage generator for delivering a ringing voltage signal onto a telephone line.

In general, to activate the bell on a telephone instrument, a suitable AC signal is connected to the telephone line. This may be connected to the telephone line at the exchange in the case of the telephone line feeding in a single telephone. Where the exchange line is connected into a private automatic branch exchange, which controls a plurality cf extension telephones, it is normal for the ringing voltage signal to be applied to each extension line in the private automatic branch exchange. However, in general, to achieve a suitable AC signal for ringing voltage, relatively heavy AC equipment is required.

Unfortunately, this does not lend itself to miniaturisation. A further problem with known ringing generators is that, in general, when the ringing voltage burst terminates, the voltage on the ringing voltage distribution line remains at the value that the ringing voltage was at at the end of a burst of ringing voltage. In general, the ringing voltage on the ringing voltage distribution line is delivered to the telephone line through a relay. As such relays open and close, because of the voltage on the ringing voltage distribution line, which may be either high or low, electrical disturbances are created, such as, for example, a high voltage surge, arcing and the like.

Such high voltage surges are detrimental to digital circuitry which is normally used in miniaturised equipment, they can cause malfunctioning of the equipment, and in certain cases, may also cause damage to the equipment. Furthermore, arcing at the relays causes the relays to relatively quickly burn out.

There is therefore a need for a ringing voltage generator for delivering a ringing voltage signal onto a telephone line, which is suitable for use with miniaturised equipment.

The present invention provides such a ringing voltage generator.

According to the invention, there is provided a ringing voltage generator comprising a transformer having a primary winding and a secondary winding, the secondary winding delivering an AC voltage output, a gate means through which the AC voltage output from the secondary winding is delivered to a ringing voltage distribution line, and a control circuit to control the gate means, characterised in that the gate means is provided by an opto-coupled triac and a biasing resistor is provided between the ringing distribution line and a biasing voltage. Preferably, the biasing resistor is of value in the range of 5 Kohms to 15 Kohms. Advantageously, the biasing resistor is of 10 Kohms. In one embodiment of the invention, the biasing voltage is negative.

In one embodiment of the invention, the output from the secondary winding of the transformer is connected to the biasing voltage.

The advantages of the invention are many. By virtue of the fact that the gate means is provided by an opto-coupled triac which is suitable for use in miniaturised circuitry, the ringing voltage generator according to the invention is ideally suited for use in miniaturised equipment. Furthermore, because a biasing resistor is provided between the ringing voltage distribution line and a biasing source, the voltage in the ringing voltage distribution line is immediately pulled up or down to the value of the biasing voltage when the gate means switches off the voltage output from the secondary winding of the transformer. Once the biasing voltage is similar to the biasing voltage on the telephone line as the relay between the ringing voltage distribution line and the telephone line opens or closes, electrical disturbances are avoided.Indeed, the use of an opto-coupled triac also facilitates the provision of a relatively low cost ringing voltage generator. A further advantage of the invention is that the generator can readily easily be connected into a private automatic branch exchange at minimal cost.

Additionally, the ringing voltage generator according to the invention has a relatively low power requirement, and accordingly, because of this is particularly suitable for use with miniaturised equipment.

The invention will be more clearly understood from the following description of a preferred embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a ringing voltage generator according to the invention, and Figs. 2(a) to (c) are waveforms of output signals from the ringing voltage generator of Fig. 1.

Referring to the drawings, and initially to Fig. 1, there is illustrated a circuit diagram of a ringing voltage generator according to the invention for delivering a ringing voltage signal onto a telephone line (not shown). In this case, the ringing voltage is provided on a ringing voltage distribution line indicated by the reference numeral 1 and is applied to the telephone line (not shown) through relays (also not shown) to activate a bell or other alerting device in a telephone (also not shown). The ringing voltage generator circuit comprises a transformer TR1 comprising a primary winding P1 and a second winding S1. The AC output from the secondary winding S1 delivers an AC output of 65 Vrms The output is biased by a -30 volt DC supply.The output from the secondary winding S1 is delivered onto the ringing voltage distribution line through a gate means, in this case a triac of an opto-coupled triac IC1 for switching the supply to the ringing voltage distribution line 1 on and off. First and second line resistors, namely resistors R2 and R3 are provided to give a line impedance of approximately 600 ohms. In this case, the resistor R2 is of 22 ohms, and the resistor R3 is of 550 ohms. A biasing resistor R1 of 10 Kohms biases the ringing voltage distribution line 1 to the -30 volts DC supply, thereby returning the ringing voltage distribution line 1 to -30 volts DC as soon as the opto-coupled triac IC1 turns off, irrespective of value of the ringing voltage output when the opto-coupled triac IC1 is switched off.In this case, the ringing generator circuit is suitable for use in a system in which the biasing voltage on the telephone line or lines is -30 volts DC. A capacitor C1 of 0.1 microfarads is provided across the triac of the opto-coupled triac IC1 to suppress transient voltages when the triac is in a switching state.

A control circuit to control the opto-coupled triac IC1 comprises a micro-processor IC2 and a transistor T1. A control signal from the micro-processor IC2 is applied to the base of the transistor T1 through a current limiting resistor R4 of 3.9 Kohms. When the signal on the base of transistor T1 is in a logic high state, transistor T1 turns on. This provides a current path from a -25 volt supply to a -30 volt supply through the light emitting diode of the opto-coupled triac IC1 and a current limiting resistor R5 of 220 ohms. The output signal from the micro-processor IC2 is derived from the transformer TR1.

In use, a ringing signal is placed on the telephone line 1 when a logic high signal is applied to the base of transistor T1 by the microprocessor IC2. This causes current to be conducted through the light emitting diode of the opto-coupler IC1, which in turn switches on the triac of the opto-coupler IC1. When the triac is in this latched condition, the ringing voltage is delivered onto the ringing voltage distribution line 1 and in turn applied to the telephone line (not shown). When the logic high signal is removed from the base of transistor T1 by the micro-processor IC2, the transistor T1 switches off, thus switching off the current through the light emitting diode of the opto-coupler IC1. When this happens, the triac continues to conduct until the current of AC output from the secondary winding S1 of the transformer TR1 changes direction.At that stage, the triac switches off, thereby switching off the ringing voltage to the ringing voltage distribution line 1 and in turn to the telephone line. In this embodiment of the invention, the triac switches off when the voltage is in its negative half cycle. Since the current lags the voltage, when the triac switches off, the voltage will have just passed its negative peak value, and the biasing resistor R1 pulls up the voltage on the ringing voltage distribution line 1 to -30 volts DC.

One example of a ringing voltage output delivered onto the ringing voltage distribution line 1 to give a ringing cadence is illustrated by the waveform of Fig.

2(c). In this case, the opto-coupled triac IC1 is switched on and off as follows: opto-coupled triac IC1 on 0.4 seconds opto-coupled triac IC1 off 0.2 seconds opto-coupled triac IC1 on 0.4 seconds opto-coupled triac IC1 off 2 seconds.

Figs. 2(a) to (c) illustrate the waveforms of the ringing voltage generator to give this cadence. The waveform of Fig. 2(a) illustrates the output from the secondary winding S1 of the transformer TR1. Fig.

2(b) illustrates the output signal from the micro-processor IC2 which is delivered to the base of the transistor T1. Fig. 2(c) illustrates the ringing voltage on the line 1.

An alternative ringing cadence is achieved as follows: opto-coupled triac IC1 on 1 second opto-coupled triac IC1 off 3 seconds.

Needless to say, while a particular set of waveforms have been illustrated in connection with the ringing voltage generator circuit, any other waveforms could be obtained without departing from the scope of the invention. Needless to say, it will be appreciated that by varying the control signal from the microprocessor IC2 onto the base of the transistor T1, different waveforms will be achieved.

It will also be appreciated that while resistors of specific values have been described, resistors of other suitable values could be used without departing from the scope of the invention. In certain cases, it is envisaged that the line resistors may be dispensed with. Further, it will be appreciated that a capacitor of capacitance other than that described may also be used without departing from the scope of the invention. In certain cases, the capacitor C1 may be dispensed with. While the biasing resistor in this particular embodiment of the invention has been described as being of 10 Kohms, any other suitable value could be used. It has, however, been found in practice that a biasing resistor of 5 to 15 Kohms is preferable, and a particularly advantageous value is 10 Kohms.Further, it is envisaged that in certain cases the biasing resistor may act to pull down the voltage on the ringing voltage distribution line to the biasing voltage in cases where the triac switches off at the positive peak or in the positive half of the cycle. It will, of course, be appreciated that the seconday winding S1 may develop voltages of value other than 65 Vrms Needless to say, other biasing voltages may also be used besides a -30 volt DC supply. The biasing voltage may be selected to match the biasing voltage of the equipment with which the ringing voltage generator is to be used. In certain cases, it is envisaged that the biasing resistor may be biased with a positive DC voltage. While it is advantageous that the output signal from the micro-processor IC2 is derived from the transformer TR1, it may be derived from any suitable tap on the transformer, either on the primary or secondary, or indeed other secondary windings of the transformer.

It will also, of course, be appreciated that it is not essential that the signal should be derived from the transformer.

While the control circuit has been described as comprising a micro-processor and a transistor, any other suitable control circuit means could be used.

Claims (19)

1. A ringing voltage generator comprising a transformer (TR1) having a primary winding (P1) and a secondary winding (S1), the secondary winding (S1) delivering an AC voltage output, a gate means (IC1) through which the AC voltage output from the secondary winding (S1) is delivered to a ringing voltage distribution line (1), and a control circuit (IC2) to control the gate means (IC1), characterised in that the gate means (IC1) is provided by an opto-coupled triac (IC1) and a biasing resistor (R1) is provided between the ringing voltage distribution line (1) and a biasing voltage.

2. A ringing voltage generator as claimed in Claim 1 characterised in that the biasing resistor (R1) is of resistance in the range 5 Kohms to 15 Kohms.

3. A ringing voltage generator as claimed in Claim 2 characterised in that the biasing resistor (R1) is of 10 Kohms.

4. A ringing voltage generator as claimed in any preceding claim characterised in that the output from the secondary winding (S1) of the transformer (TR1) is connected to the biasing voltage.

5. A ringing voltage generator as claimed in any preceding claim characterised in that the biasing voltage is similar to the biasing voltage on a telephone line to which the ringing voltage distribution line (1) is connected.

6. A ringing voltage generator as claimed in any preceding claim characterised in that the biasing voltage is a negative biasing voltage.

7. A ringing voltage generator as claimed in Claim 6 characterised in that the biasing voltage is a -30 volt DC supply.

8. A ringing voltage generator as claimed in any preceding claim characterised in that the output of the secondary winding (S1) of the transformer (TR1) is 65 volts root mean square.

9. A ringing voltage generator as claimed in any preceding claim characterised in that a first line resistor (R2) is provided in series with the optocoupled triac (IC1) and the biasing resistor (R1).

10. A ringing voltage generator as claimed in Claim 8 characterised in that the ringing voltage is fed to the ringing voltage distribution line (1) from a connection between the biasing resistor (R1) and the first line resistor (R2).

11. A ringing voltage generator as claimed in Claim 10 characterised in that a second line resistor (R3) is provided on the ringing voltage distribution line (1).

12. A ringing voltage generator as claimed in Claim 11 characterised in that the sum of the values of the first and second line resistors (R2,R3) is 600 ohms approximately.

13. A ringing voltage generator as claimed in Claim 12 characterised in that the first line resistor (R2) is of 22 ohms value.

14. A ringing voltage generator as claimed in any preceding claim characterised in that a capacitor (C1) is provided across the triac of the opto-coupled triac (IC1) for the suppression of transient voltages when the triac is in a switching state.

15. A ringing voltage generator as claimed in any preceding claim characterised in that the control circuit (IC2) comprises a micro-processor (IC2) which delivers a signal to switch on and off a transistor (T1), the transistor being provided in series in a circuit with the light emitting diode of the opto-coupled triac (IC1).

16. A ringing voltage generator as claimed in Claim 15 characterised in that an output signal from the micro-processor (IC2) to the transistor (T1) is derived from the transformer (TR1).

17. A ringing voltage generator as claimed in any preceding claim characterised in that the opto-coupled triac (IC1) is held by the control circuit (IC2) in a conducting mode for 0.4 seconds, in a non-conducting mode for 0.2 seconds, in a conducting mode for 0.4 seconds, and in a non-conducting mode for 2 seconds, to obtain a ringing voltage cadence.

18. A ringing voltage generator as claimed in any of Claims 1 to 16 characterised in that the opto-coupled triac (IC1) is held by the control circuit (IC2) in a conducting mode for 1 second and in a non-conducting mode for 3 seconds to obtain a ringing voltage cadence.

19. A ringing voltage generator substantially as described herein with reference to and as illustrated in the accompanying drawings.