JPH0137893B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to an interface circuit between a transmitting/receiving circuit and a central office line, that is, a telephone interface circuit.

Technical Background of the Invention In the old days, in order to insulate the central office line side and the transmitting/receiving circuit, there were interface circuits for telephones that were coupled only by a transformer. In order to obtain a current of several hundred mA), the transformer would have to be large, so it is necessary to make the transformer smaller by passing only the alternating current component through the transformer, and the transmitter/receiver circuit receives power from an AC adapter to amplify the signal. Conventional telephone interface circuits such as those described below have come into use.

A conventional telephone interface circuit will be explained with reference to FIGS. 1 to 3. The telephone 1 was roughly constructed as shown in FIG. In other words, the office line terminals 10 and 11 connected to the office lines L ₁ and L ₂ , respectively.
In between, an incoming signal or a predetermined voltage appears. The station line terminal 10 is connected to a capacitor circuit 12 which is a series circuit of a capacitor and a resistor. A hook switch 19 is disposed in parallel with the capacitor circuit 12 and is closed when the handset 23 is lifted. Then, the incoming signal that has passed through the capacitor circuit 12 reaches the diode bridge 13, passes through the positive side line 14 of the diode bridge 13, and is input to the incoming call detection circuit 15. Connected to the incoming call detection circuit 15 is a relay 16 that is tilted toward the incoming call detection adjustment circuit 17 side when waiting for an incoming signal. The incoming call detection adjustment circuit 17 is connected to the negative line 18 of the diode bridge 13, and the remaining connection points of the diode bridge 13 are connected to the office line terminal 11.

In the above circuit, when a call is made, the relay 16 falls toward the DC separation circuit 20 side. A transformer 21 coupled to a transmitting/receiving circuit 22 is connected to the DC separation circuit 20, and a handset 23 is connected to the transmitting/receiving circuit 22.

Specifically, as shown in FIG.
It consists of a series circuit with 3. In the incoming call detection circuit 15, the signal corresponding to the incoming call signal outputted from the diode bridge 13 is bypassed by the varistor 151, the current is limited by the resistor 152, and the presence or absence of an incoming call is detected by the photocoupler 153. . Furthermore, even during a call, the photocoupler 153 monitors the state of the DC current sent from an exchange (not shown). This monitoring result is used, for example, to automatically disconnect the line when the call is on hold.

On the other hand, the DC separation circuit 20 specifically includes the third
It is configured as shown in the figure. That is, a circuit consisting of an npn transistor 201 and a resistor 202 connected to the emitter of this transistor 201 is connected between one contact of the relay 16 and the negative side line 18 of the diode bridge 13. The direct current obtained from the transistor 201
It is connected to pass from the collector to the emitter. Furthermore, the base of the transistor 201
A circuit including a capacitor 203 on the primary side of the transformer 21, a resistor 204, and a capacitor 205 is connected between the emitters. Therefore, the audio signal that has passed through the diode bridge 13 is passed through a low-pass filter consisting of a transformer 21 and a capacitor 203, a resistor 204, and a capacitor 205.
A low-pass filter consisting of the following converts the signal into a signal containing only sufficiently low frequency components compared to the audio band, and adds the signal to the base of the transistor 201.
In other words, a DC component is applied to the base of the transistor 201, and the transistor 201 passes only the DC component output from the diode bridge 13. Therefore, all of the audio component signals do not flow through this transistor 201. is transmitted from the transformer 21 to the transmitting/receiving circuit 22 and the handset 2 without attenuation.
3.

Problems with the Background Art However, in the conventional interface circuit, there are individual differences in the h _FE of the transistors 201 used in the DC isolation circuit 20, and when a transistor with a large h _FE is used, the transistor 20
The amplitude exceeding the collector-base voltage of 1 (P-
When an audio component signal with P) arrives, the transistor 201 is cut off when the collector-base voltage exceeds the positive side, and the transformer 21 is eventually cut off.
The peak side portion of the signal applied to the signal will be clipped, making it impossible to obtain the dynamic range necessary for voice communication.

Also, while there is no problem when making a call to the central office lines L ₁ and L ₂ using one telephone, if you connect multiple telephones to the central office lines L ₁ and L ₂ and are unable to make a call, the problem will not occur. Otherwise, you may not be able to make a call. First, although it is desirable that the potential of the central office line terminals 10 and 11 be equal to the potential nearest to the central office exchange, a voltage drop occurs due to the impedance of the communication path from the central office exchange to the central office line terminals 10 and 11. The potential of 11 is lower than the potential of the nearest central office exchange. In addition to this, when a plurality of interface circuits (on the right side of the figure from the office line terminals 10 and 11) are connected, the potential between the office line terminals 10 and 11 decreases.

OBJECT OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a telephone interface circuit that can ensure an appropriate call state under any situation during a call. That is, the first purpose is to ensure an appropriate call state without being affected by the constant _hFE of the transistor, and the second purpose is to ensure that a suitable communication condition is achieved without being affected by the constant hFE of the transistor. Also,
The goal is to ensure proper call conditions.

SUMMARY OF THE INVENTION Therefore, in the present invention, for a transistor used in a DC isolation circuit, in order to obtain the necessary collector-base voltage, a constant voltage element is used to obtain the above-mentioned required voltage in a loop including a transformer. Connected to position.

Furthermore, in the present invention, the configuration of the incoming call detection circuit is configured by a parallel circuit of a resistor and a photocoupler, and a relay having the following configuration is also arranged. That is, when waiting for an incoming signal, the signal obtained from the diode bridge is applied to the incoming call detection circuit to bypass the DC separation circuit, and during a call, the signal obtained from the diode bridge is applied to the DC separation circuit to bypass the incoming call detection circuit. I connected a relay to bypass it.

Embodiments of the Invention Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 4 is a circuit diagram of an embodiment of a DC isolation circuit 42 according to the present invention. In the figure, the same elements as in FIG. 3 are given the same numbers and their explanations will be omitted. In this embodiment, a circuit in which diodes 421 and 422 are connected in series is used as a constant voltage element. Specifically, the cathode of the diode 421 and the resistor 204 are connected, and the capacitor 2
The positive side of 03 and the anode of diode 422 are connected. Furthermore, a series circuit of a resistor 423 and a photocoupler 424 is connected between the transformer 21 and the negative line 18 of the diode bridge 13, and a resistor 425 is connected in parallel with this series circuit. The telephone according to this embodiment has the same configuration as that shown in FIG. 1, with the only difference being the DC separation circuit.

With this configuration, the transistor 201
Even if h _FE is large, the transistor 20
Since diodes 421 and 422 as constant voltage elements are arranged between the collector and base of transistor 201, the forward voltage generated by these diodes 421 and 422 compensates for the voltage between the collector and base of transistor 201. As a result, it is possible to obtain as much dynamic range as necessary for a proper call.

Furthermore, in this embodiment, the transistor 2
Even when the transistor 201 is turned off due to a voltage drop between the collector of 01 and the negative line 18 of the diode bridge 13, a small amount of current can flow through the diode bridge 13 via the resistor 425, so that other telephones cannot Calls can be guaranteed even when DC is consumed.

Furthermore, the photocoupler 424 can detect direct current during a call. This photocoupler 424 has a more important function in the embodiment shown in FIG. 6, which will be described below.
This is not particularly necessary in this embodiment.

Next, an embodiment will be described in which an appropriate call can be ensured even when a plurality of telephones are connected to one central office line.

FIG. 5 shows a circuit diagram of the incoming call detection circuit 40 according to this embodiment. In the figure, 401 is a current limiting resistor. This resistor 401 has a resistor 40
2 and a photocoupler 403 are connected in parallel. This photocoupler 403 is provided to detect an incoming signal.

FIG. 6 is a block diagram for explaining the interface circuit according to the present invention. The incoming call detection circuit 40 in the figure is the circuit shown in FIG. 5, and the DC separation circuit 42 is the circuit shown in FIG. 4.

The feature of the circuit shown in Fig. 6 is that the relay 4
It is in 1. When the relay 41 is waiting for an incoming signal,
The signal obtained from the diode bridge 13 is applied to the incoming call detection circuit 40 to bypass the DC separation circuit 42, and during a call, the diode bridge 13
The signal obtained from the incoming call detection circuit 40 is supplied to the DC separation circuit 42 to bypass the incoming call detection circuit 40.

As a result, when waiting for an incoming signal, the relay 41 is tilted down as shown by the solid line in FIG. As a result, the photocoupler 403 is turned on, indicating that an incoming call has been received. This signal reaches the negative line 18 of the diode bridge 13 via the relay 41. Therefore, when waiting for an incoming signal, reliable incoming call detection is performed.

When it is time to make a call, the switch 19 and the relay 41 fall down as indicated by the broken lines in FIG. As a result, the signal output from the diode bridge 13 bypasses the incoming call detection circuit 40 and passes through the relay 41.
It reaches the transformer 21 and the DC separation circuit 42 via.

Therefore, the incoming call detection circuit 40 is a circuit that is not involved during a call, so even if multiple telephones are connected to one office line, there is little voltage drop, so even if the line is long, , can make enough calls.

Furthermore, since the DC separation circuit 42 has a configuration as shown in FIG. 4, the photocoupler 424 can detect DC current during a call. Therefore, even if the incoming call detection circuit 40 is bypassed during a call, the relay 4 is activated when waiting for an incoming call signal.
1 is tilted as shown by the solid line in FIG. 6, and the incoming call detection function can be maintained as in the conventional case.

Furthermore, in the interface circuit configured as described above, if the values of the resistors 401 and 402 of the incoming call detection circuit 40 are appropriately selected, the incoming call detection circuit 40 will be activated by the relay 4 when switching from waiting for an incoming signal to talking.
It can act as a spark quenching circuit by lowering the voltage generated across the terminal 1.

Furthermore, the resistors 423 and 425 of the DC separation circuit 42
If the value of is selected appropriately, the DC separation circuit 42 will act to lower the voltage generated across the relay 41 when switching from talking to waiting for an incoming signal.
It can act as a spark quenching circuit.

Effects of the Invention As explained above, according to the present invention, an appropriate communication state can be ensured without being influenced by h _FE of the transistor of the DC separation circuit. Therefore, a uniform telephone interface circuit can be provided without worrying about the constant h _FE of the transistor in the DC separation circuit.

Furthermore, even when a plurality of telephones are connected to the same station line, an appropriate communication condition can be ensured and the communication range can be expanded.

[Brief explanation of drawings]

FIG. 1 is a block diagram for explaining a conventional example.
Figures 2 and 3 are circuit diagrams of the main parts of the conventional example;
The figure is a circuit diagram for explaining an embodiment of the present invention.
The figure is a circuit diagram for explaining another embodiment, and FIG. 6 is a block diagram of the embodiment. 13...Diode bridge, 21...Transformer,
40... Incoming call detection circuit, 41... Relay, 42... DC separation circuit, 421, 422... Diode (constant voltage element).

Claims (1)

[Claims] 1. In a telephone interface circuit that supplies a signal obtained from a central office line to an incoming call detection circuit and a DC separation circuit via a diode bridge, a transformer is provided between the collector and the base to reach the transmitting/receiving circuit. a low-pass filter placed between the base and emitter of the transistor; and a constant voltage element placed in a loop including the transformer so as to obtain a required collector-base voltage for the transistor. A telephone interface circuit characterized by having a DC separation circuit consisting of. 2. In the interface circuit of a telephone that provides a signal obtained from the office line to the incoming call detection circuit and the DC separation circuit via a diode bridge, the incoming call detection circuit consisting of a parallel circuit of a photocoupler and a resistor, and the collector-base a transistor with a transformer connected to the transmitting/receiving circuit, a low-pass filter placed between the base and emitter of this transistor, and a loop including the transformer so as to obtain the required collector-base voltage for the transistor. a DC isolation circuit consisting of a constant voltage element arranged in the incoming call detection circuit; when waiting for an incoming signal, the signal obtained from the diode bridge is applied to the incoming call detection circuit to bypass the DC isolation circuit, and when a call is being made, the diode bridge is A relay for supplying a signal obtained from the DC separation circuit to the DC separation circuit to bypass the incoming call detection circuit.