JPH01238348A - Control circuit at start of telephone set - Google Patents

Abstract

PURPOSE:To prevent howling at starting by providing a circuit causing a voltage output at mutual connection point between a capacitor and a resistor and a control circuit opening the switch means when the voltage output exceeds a prescribed level and closing it at a prescribed level. CONSTITUTION:With a handset 41 connected, the potential of the input section point A of the control circuit 10 exceeds the threshold level of the control circuit 10 at the start of power supply to a speech circuit and then decreases exponentially. Then the handset 41 is disconnected from the signal transmission circuit during the period in excess of the threshold level and the handset 41 is connected to the signal transmission circuit while the level is below the threshold level. At the point of time when the switching is applied, the charging to the capacitor 16 is finished and since a sufficient current flows to the speech circuit 6, the gain of a transmission amplifier 6 and a reception amplifier 62 is made stable and the gain is sufficiently smaller than the case with no feeding current flowing. Thus, howling due to looping is not caused.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to telephones, and more particularly to a device for preventing howling when the telephone is activated for making or receiving calls.

[Conventional technology]

A conventional circuit of this type is constructed as shown in FIG. The operation of this circuit will be explained as follows.

First, an incoming call signal from the exchange is applied between terminals L1 and L52, passes through a capacitor 1 and a diode bridge 2, and becomes a ringing tone in an incoming call generating section 3. At this time, the handset 4 is on-hook and the hook switch 5 is open. Therefore, the communication circuit section 6 (the part surrounded by the broken line in the figure),
Power is not supplied to the MF dial signal device 7. The MF dial signal device 7 closes the switch 8 only when the MF multiplied signal is generated while being supplied with power, and otherwise keeps the switch open. MF dial generator 7 in the call circuit section 6
The input of the control circuit 10 that controls the selector switch 9 for switching between the transmitter and the transmitter 41 is pulled down by the resistor 11, so when the switch 8 is open, the input is below the threshold value, and the input of the control circuit 10 is pulled down by the resistor 11. Connect to the device 41.

[Problem to be solved by the invention]

Now, let us consider the case where the handset 4 is off-hook (lifted up) from this incoming call state. In this case, when the hook switch 5 is closed, power is supplied to the MF dial generator 7 via the constant current power supply circuit 12, and at the same time, power is supplied to the communication circuit section 6.

At this startup, diode bridge 1 is connected from terminals L1 and L2.
3. Most of the power supply current that has passed through the hook switch 5 flows into the capacitor 16 as a charging current via the parallel-connected resistor 14 and voltage regulator diode 15. Therefore, it takes several tens to several hundreds of times until the capacitor 16 is charged.
For milliseconds, almost no power supply current flows through the transmitting amplifier 61 and the receiving amplifier 62, and the gains of these amplifiers become unstable and high. Therefore, the loop gain of 2 at kl in FIG. 7 becomes high, and howling occurs.

In addition, the balancing network (BN
) 17, resistor 18, 19, and telephone line side impedance ZL until the calling side telephone is connected. The amount of coupling that goes around to the receiver 42 through 20 is increased by 1. Furthermore, when the handset 4 is taken off-hook, the transmitter 41 and the receiver 42 are spatially coupled in relation to the base, and the resulting coupling Wk2 becomes large. These two combine jiikl.

Transceiver 41-transmitting amplifier 61-coupling 1kl by k2
There is a problem in that the gain of the loop consisting of - receiver amplifier 62 - coupling 14 k 2 - transmitter 41 exceeds OdB, causing noise ringing.

The present invention has been made to solve the problems of the prior art described above, and an object of the present invention is to provide a start-up control circuit for a telephone that does not cause 71 ringing when the telephone is started.

[Means to solve the problem]

To achieve the above object, the present invention includes a transmitter, a signal transmission circuit for transmitting the output of the transmitter, a switching means for opening and closing the signal transmission circuit, a capacitor, and a resistor, which are connected in series. , a circuit inserted between the power supply circuit of the telephone and the earth, which produces a voltage output at the interconnection point of the capacitor and the resistor; and when the voltage output exceeds a predetermined level, the switching means is opened, and when the voltage output reaches the predetermined level, The present invention provides a start-up control circuit for a telephone, which is equipped with a close control circuit.

[For production]

When the telephone is started, the power supply circuit of the telephone rises, and this rising voltage is applied to a circuit including a capacitor and a resistor connected in series, which is inserted between the power supply circuit and the ground. Since the capacitor passes a voltage with a sharp rise, the potential at the interconnection point between the capacitor and the resistor in this series-connected circuit is initially equal to the power supply voltage, and then gradually decreases with a time constant. During this voltage change process, the switching means is kept open until the voltage drops to a certain extent, and then the switching means is closed when the voltage drop is detected and the transmitter is connected to the signal transmission circuit.

〔Effect of the invention〕

According to the present invention, the output of the transmitter is not given to the signal transmission circuit during the period when the signal transmission circuit is unstable at the time of starting the telephone, thereby preventing howling caused by unstable operation of the signal transmission circuit. Can be done.

〔Example〕

FIG. 1 shows an embodiment of the present invention, in which a switch 8
The circuit shown in FIG. 6 differs from the circuit shown in FIG. 6 in that a capacitor 21 is connected in parallel with the circuit shown in FIG. Therefore, to avoid duplication of explanation, the points related to the present invention will be explained focusing on the operation.

Now consider the case when the handset 4 is taken off the hook (lifted up) and the communication circuit 6 is activated. When the hook switch 5 is off-hook, the hook switch 5 is closed, and the capacitor 16 is connected to the telephone line through a parallel circuit of a resistor 14 and a constant voltage diode 15.
will be held. At this time, the switch 8 is open, but since the capacitor 21 is in a discharged state, it can be seen as a transient conductive circuit. Therefore, the resistors 14 connected in parallel
, the power supply current that has passed through the constant voltage diode 15 flows through the capacitor 21 and into the resistor 11. As a result, the input level of the control circuit 10 of the changeover switch 9, that is, the potential at point A, once rises to the line potential as shown in FIG. Pass the threshold level inside. When the threshold level is exceeded, the control circuit 10 generates an output, and the selector switch 9 is switched in response to this output, thereby disconnecting the microphone 41 from the transmitting amplifier 61 inserted in the signal transmission circuit of the telephone circuit. Changeover switch 9 is set to M instead.
Connect the F dial signal device 7.

After this, the capacitor 21 is charged with a time constant (several 100 milliseconds) determined by the capacitance of the capacitor 21 and the resistance value of the resistor 11, and the input level of the control circuit 10 falls below the threshold, and the transmitter 41 is connected by the changeover switch 9. be done.

That is, when the transmitter 41 is connected, as shown in the circuit configuration of FIG. 2 and the voltage change characteristics of FIG.
The potential at the input point A of the control circuit 10 rises above the threshold vth of the control circuit 10 at the start of power supply to the communication circuit, and then decreases exponentially. The transmitter 41 is disconnected from the signal transmission circuit during a period in which the threshold vth is exceeded, and the transmitter 41 is connected to the signal transmission circuit in a period below the threshold vth. At the time when this switching is performed, the power supply to the capacitor 16 is finished and sufficient current is also supplied to the telephone circuit 6, so the transmitting amplifier 6
1. The gain of the receiving amplifier 62 is also stable, and the gain is sufficiently smaller than when the feeding current does not flow. Therefore, howling due to the loop shown in FIG. 7 does not occur.

Next, the operation at the time of generation and termination of the MF multiplied signal which is switched by the changeover switch 9 and given to the signal transmission circuit will be explained.

When the MF signal is generated, the switch 8 is closed, but at this time the capacitor 21 is charged, so if the resistance value of the resistor 22 is sufficiently smaller than the resistance value of the resistor 11, the equivalent circuit shown in Fig. 4 is obtained, and the potential at point A is rises slowly with the time constant of the parallel circuit of capacitor 21 and resistor 22, as shown in FIG.

Further, when the MF signal ends, the switch 8 opens, but at this time the capacitor 21 is discharged, so the potential at point A rises smoothly with the time constant of the capacitor 21 and the resistor 22, as shown in FIGS. 2 and 3. Therefore, when an MF signal is generated,
There is no buzz from the handset 42 at the end.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is an explanatory diagram of a time constant circuit including the capacitor 21 in the circuit of Fig. 1 when a transmitter is connected, and Fig. 3 is an illustration of the time constant circuit in that case. A characteristic diagram showing the potential change at point A. Fig. 4 is an explanatory diagram of the time constant circuit including the capacitor 21 in the circuit of Fig. 1 at the time of MF signal generation and termination, and Fig. 5 shows the potential change at point A in that case. 6 is a circuit diagram of a conventional telephone, and FIG. 7 is an explanatory diagram of the cause of howling in a conventional telephone circuit.
FIG. 8 is a diagram showing a side sound prevention circuit in the circuit of FIG. 6. 1.16, 20.21... Capacitor, 2.13...
・Diode bridge, 4...Handset, 5...
・Hook switch, 6... Call circuit, 8... Switch, 9... Changeover switch, 10... Control circuit, 1
1゜14.18.19...Resistor, 12... Constant current circuit, 15... Constant voltage diode. Applicant's agent Mr. Sato Figure 1 Figure 6 Figure 7 Figure 8

Claims (1)

[Claims] A transmitter, a signal transmission circuit that transmits the output of the transmitter, a switching means that opens and closes this signal transmission circuit, a capacitor, and a resistor are connected in series, and the circuit is connected between the power supply circuit of the telephone and the ground. Activation of a telephone, comprising: a circuit inserted into a circuit that produces a voltage output at the interconnection point of a capacitor and a resistor; and a control circuit that opens the opening/closing means when the voltage output exceeds a predetermined level and closes the switching means when the voltage output reaches a predetermined level. Time control circuit.