CH626487A5 - Circuit for connecting ringing signals to a telephone set - Google Patents

Description

The present invention relates to an electronic circuit for connecting call signals to a telephone set, which circuit is connected to this apparatus via two first connection terminals and receives the call alternating voltage superimposed on the line supply borrowing voltage via two further connection terminals, with two identical ones, each between a first one and a second terminal connected to triacs and having a control circuit which receives ringing signal transmission pulses and controls the triacs.

Such a circuit is already known from French patent application No. 7,629,418. The call signal can be forwarded to a subscriber set in a purely electronic manner in this conventional connection circuit. The circuit contains two main triacs (one per line wire), which are switched to their conductive state during the presence of call signal transmission pulses and through which the line supply direct current and the call alternating current then flow on the one hand. The two triacs are controlled by applying an AC signal to their respective gates. The control alternating current for each of the two main triacs is generated by an auxiliary triac, which in turn is controlled by a transistor circuit which receives the call signal transmission pulses. A capacitor connected in series with each of the auxiliary triacs causes a phase shift between the control alternating current of the main triacs and the alternating current flowing through these main triacs, such that the instantaneous value of the control current is high at the zero crossing of the controlled current; consequently, the main triacs conduct throughout the sine wave period.

The use of a relatively complex control device with auxiliary triacs forms a major disadvantage of the conventional arrangement described. In addition, both the ringing circuits of the telephone sets and the lines have different electrical properties, so that the phase of the ringing alternating current varies from line to line. For some lines, the instantaneous value of the control alternating current for the main triacs when the controlled alternating current passes zero is therefore too low to be able to trigger the triacs.

It is therefore an object of the present invention to provide a circuit for connecting ring signals to a telephone, which circuit requires a simple control circuit and which ensures in all cases that the triacs are conductive throughout the sine wave period.

The characteristic features of such a circuit are specified in claim 1.

An embodiment of the invention will now be explained in more detail with reference to the drawing. Show it:

Fig. 1 shows an inventive circuit for connecting call signals to a telephone, and

2a and 2b waveforms of signals in the circuit according to the invention.

The circuit according to the invention in FIG. 1 contains two connecting terminals L and L 'connected to the two wires of the subscriber line and two further connecting terminals A and A \ to which two superimposed voltages are continuously applied:

- The line feed voltage -V (-48 V) and

- The call alternating voltage with an effective value u and a frequency f (usually 80 V, 50 Hz). This AC voltage is generated symmetrically with respect to the DC supply voltage, that is to say an AC voltage u / 2 is generated in each arm of the DC supply voltage source.

These two superimposed voltages can be generated by a transformer TR with three windings: one primary winding and two identical secondary windings. An AC voltage is applied to the primary winding, which induces an AC voltage u / 2 in each of the secondary windings, these induced voltages being of opposite phase.

Two identical triacs 1 and 1 ', which are connected between the connection terminals L and A or L' and A ', allow the DC supply voltage -V and the AC call voltage u to be applied to the subscriber line in their conductive state.

The design of the switching circuit described so far corresponds to the prior art.

The two triacs 1 and 1 'are controlled according to the invention by a control circuit C, which will be described below. It contains an NPN transistor 2, the emitter of which is connected to the voltage source -V and the collector of which is connected to the gate of the triac 1 via a resistor 3, a diode 4 and a further resistor 5. The forward direction is the same for both diode 4 and transistor 2. A capacitor 6 is connected between the anode of the diode 4 and the reference electrode of the triac 1. The same applies to the triac 1 ': a PNP transistor 2', the emitter of which is connected to earth and the collector of which is connected to the gate of the triac 1 'via a resistor 3', a diode 4, a further resistor 5 'and a capacitor 6 'is connected. Both transistors 2 and 2 'are controlled by a third PNP transistor 7, the collector of which is connected directly to the base of transistor 2, the emitter of which is connected to the base, of transistor 2' via a resistor 8, and the base of which is the call signal transmission pulse Ti receives. The base of transistor 2 is biased via a resistor 9 connected between its base and its emitter. The base of the transistor 2 'is over a

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biased the same resistance value as the resistance 9 'having resistance 9'.

The operation of the control circuit C will now be described. The presence of call signal transmission pulses Ti puts transistor 7 into its conductive state, 5 which consequently also transistors 2 and 2 'are brought into their conductive state. The call AC voltage,

whose waveform can be seen from FIG. 2a, charges the capacitor 6 via the transistor 2, the resistor 3 and the diode 4 during part of the first half cycle. During the same time, the capacitor 6 'is charged via the transistor 2', the resistor 3 'and the diode 4'. During the remaining time of the sine wave cycle, the capacitors 6 and 6 'then discharge slowly, specifically via the resistor 5 and the gate of the triac 1 or via the resistor 5' and the gate of the triac 1 '. The waveform of the voltage across the terminals of the capacitors is shown in Fig. 2b. The discharge time constant is chosen so that the voltage at the capacitor connections never falls below a certain value

626 4871

Vmin drops so that the gate current generated in triacs 1 and 1 'is higher than their trigger current. The gate current will always be higher than the trigger current. The triacs therefore conduct throughout the sine wave cycle regardless of the phase of the AC ringing current flowing in the line.

The diode 4 prevents the discharge of the capacitor 6 via the collector-base section of the transistor 2. The same function is performed by the diode 4 'for the transistor 2'. The resistors 3 and 3 'protect the transistors 2 and 2' during the transition to the conductive state, the capacitors 6 and 6 'acting as a short-circuit line at this moment. The resistor 8 fixes the base-emitter voltage of the transistors 2 and 2 'to a suitable value.

While the call signal is “at rest”, which means the time interval between two call signal transmission pulses, the line is supplied with direct current via a circuit 10. During these “rest periods”, the circuit receives 10 control pulses T2.

1 sheet of drawings

Claims (2)

626487 2 PATENT CLAIMS 1.Electronic circuit for connecting call signals to a telephone set, which circuit is connected to this device via two first connection terminals (L, L ') and via two further connection terminals (A, A') the ringing alternating voltage superimposed on the line supply direct voltage (-'V) receives, with two identical triacs (1; 1 ') connected between a first (L; L') and a second (A; A ') connection terminal and with a control circuit (C) which receives call signal transmission pulses (Ti) and which Triacs controls, characterized in that the control circuit (C) contains means which supply the gates of the triacs a rectified current which is always greater than the trigger current of the triacs, this gate current being caused by the alternating call voltage. 2. Circuit according to claim 1, characterized in that the said means consist of the following elements: - each from a capacitor (6; 6 ') assigned to each of the triacs (1; t'), the first connection of which is connected to a reference electrode and the second connection of which is connected to the gate of the assigned triacs via a resistor (5; 5 ') connected is; - each from an NPN (2) or PNP (2 ') transistor assigned to the triacs (1; 1'), the emitters of which are connected to the voltage terminal (-V) and the earth terminal (X) of the supply DC voltage source are connected and their collectors are each connected to the second connection of the capacitor (6; 6 ') via a respective resistor (3; 3') and a respective diode (4; 4 ') conducting in the same direction as the transistors, whereby the transistors are brought into their conductive state by the presence of call signal transmission pulses (Ti).