JPH0332131Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an improvement in a line interface circuit that generates a voltage from a line current of a subscriber line or the like and supplies this voltage to a telephone set as a driving voltage.

[Technical background of the invention and its problems]

Conventionally, as this type of circuit, for example, the one shown in FIG. 1 is known. That is, this circuit connects a telephone 3 to subscriber line terminals 1a and 1b via a transformer 2 in an alternating current manner, rectifies the line current of the subscriber line with a diode bridge circuit 4, and outputs the rectified output. It is led to a voltage generating circuit 8 consisting of a smoothing capacitor 5, a current limiting resistor 6, and a constant voltage diode 7 to generate a voltage, and this voltage is supplied to the telephone 3 as a drive voltage (power supply voltage) V _CC . . Therefore, the voltage between the subscriber line terminals 1a and 1b (between the line terminals) in such a circuit is due to the voltage drop of the diode bridge circuit 4, the voltage drop due to the primary winding of the transformer 2, and the voltage drop due to the current limiting resistor 6. It is determined by the voltage drop and the Zener voltage of the constant voltage diode 7.

Incidentally, the DC resistance R between the line terminals in this type of circuit is generally standardized so that 220Ω>R>50Ω. For this reason, when only a relatively small line current is supplied, for example to a telephone located far from the exchange, it is difficult to obtain sufficient voltage to drive the telephone. For example, when the line current is 20 mA as shown in Figure 2, even if the DC resistance between the line ends 1a and 1b is maximum (R _nax = 220Ω), the voltage between the terminals is only 4.4V at maximum. It will not occur.

In this way, when the voltage generated at the line end 1a and one frame is small, in the conventional circuit, the voltage drop in the primary winding of the transformer 2 is large, so that the drive voltage V _CC is actually applied to the telephone 3. For example, the voltage applied to the line end becomes significantly smaller.
In the case of 4.4V, the voltage is about 1.8V, which is lower than the voltage value required to drive the telephone 3, so the range of application had to be limited.

In addition, in the conventional circuit, since the drive voltage V _CC is approximately determined by the Zener voltage of the voltage regulator diode 7, the drive voltage V _CC is determined depending on the magnitude of the line current.
It was difficult to set it to an appropriate value.

[Purpose of invention]

This invention was made with attention to the above circumstances,
It is possible to generate sufficient driving voltage for the telephone to operate even when the line current is small, while satisfying the DC resistance standard between the line ends, and to generate as large a driving voltage as possible according to the line current. It is an object of the present invention to provide a line interface circuit that can generate a driving voltage that is stable even with load fluctuations of a telephone set.

[Summary of the idea]

In order to achieve the above object, the present invention provides a rectifier circuit in a line interface circuit that generates a driving voltage for a telephone based on a line current supplied from a station power supply via a balanced two-wire line and supplies the telephone to the telephone. A transistor shunt circuit is connected in parallel to the AC connection circuit connected between the output terminals, and this transistor shunt circuit shunts the main current of the rectified line current output to reduce the voltage drop in the transformer of the AC connection circuit. In this way, a sufficiently large driving voltage can be obtained, and a part of the rectified output flowing through the transformer is passed through the base resistor to the base current of the transistor shunt circuit, thereby directing the shunt current flowing through the transistor shunt circuit to the line. The current value is varied according to the line current to obtain the largest possible driving voltage according to the line current.Furthermore, the remainder of the rectified output flowing through the transformer is supplied to the telephone as a load current, and the change in this load current is A change in the base current is detected through the base resistor, and the change in the base current is suppressed by the feedback action of the transistor shunt circuit, thereby reducing a change in the drive voltage due to a change in the load current. It is something.

[Example of idea]

FIG. 3 is a circuit diagram of a line interface circuit according to an embodiment of the present invention, in which the same parts as in FIG. 1 are given the same reference numerals.

In the same figure, subscriber line terminal (line terminal) 1
An input terminal of a diode bridge circuit 4 is connected between a and 1b, and an AC connection circuit 10 consisting of a series circuit of a transformer 2 and a capacitor 9 is connected between the positive and negative output terminals of the diode bridge circuit 4. There is. A telephone 3 as a line connecting device is connected to the transformer 2 of this AC connection circuit 10.

Now, between the output terminals of the diode bridge circuit 4, a shunt circuit 11 having a transistor is provided in parallel to the AC connection circuit 10. This shunt circuit 11 connects the collector of the NPN transistor 12 to the positive output terminal of the diode bridge circuit 4, and connects the emitter to the emitter resistor 1.
3 to the negative output end of the diode bridge circuit 4, and its base is connected to the connection point between the transformer 2 and capacitor 9 of the AC connection circuit 10 via the resistor 15 of the voltage generation circuit 14. It is. The voltage generating circuit 14 also includes a resistor 15 connected to the base of the transistor 12, a capacitor 16 connected between the base of the transistor 12 and the diode bridge circuit 4 connection side of the emitter resistor 13, and the AC connection. The circuit 10 includes a constant voltage diode 17 connected to both ends of the capacitor 9. This constant voltage diode 17
defines the upper limit of the drive voltage V _CC supplied to the telephone 3.

Because of this configuration, the capacitor 9 is connected in series to the transformer 2, and the capacitor 1 is connected between the base and emitter of the transistor 12.
Since the transformer 6 has a high impedance in terms of alternating current, the alternating current signal sent from the exchange (not shown) via the subscriber line flows through the transformer 2. Therefore, the telephone 3 is connected to the subscriber line terminal 1a,
1b.

On the other hand, since the transistor shunt circuit 11 is provided, most of the DC line current supplied from the central power source of the exchange via the subscriber line flows into the collector of the transistor 11 after passing through the diode bridge circuit 4. , a portion flows into the base of the transistor 12 via the primary winding of the transformer 2. Therefore, the voltage drop occurring in the primary winding of the transformer 4 is extremely small.

Here, the voltage between the subscriber line terminals 1a and 1b, that is, the voltage between the line terminals, is determined by the voltage drop of the diode bridge circuit 4, the voltage drop due to the primary winding of the transformer 4, the emitter resistance 13 of the transistor 12, and the base resistance 15 of the transistor 12. and the voltage drop between the base and emitter of the transistor 12, _VBE , and the sum of the voltage drop due to the emitter resistor 13 and the base resistor 15 and the base-emitter voltage of the transistor 12 becomes the drive voltage _VCC . . Therefore, as mentioned above, in the circuit of this embodiment in which the voltage drop due to the primary winding of the transformer 2 is small, most of the voltage drops other than the voltage drop of the diode bridge circuit 4 are used as the driving voltage V _CC of the telephone. 3 will be supplied with power.

Therefore, even when connected to a subscriber line with a small line current, a sufficiently large driving voltage necessary to drive the telephone 3 can be generated without increasing the DC resistance between the subscriber line terminals 1a and 1b. becomes possible. For example, a drive voltage of approximately 2.7V can be generated even when the line current is at least 20mA. In other words, while maintaining the DC resistance value R between the subscriber line terminals 1a and 1b within the standard value of 220Ω>R>50Ω, even if the telephone 3 is connected to the subscriber line with any line current value. It is possible to generate a drive voltage V _CC that is large enough to drive .

Furthermore, the circuit of this embodiment does not simply shunt the line current, but also configures the shunt circuit 11 using transistors 12 to change the shunt current according to the magnitude of the line current. . Therefore, it is possible to generate the largest possible drive voltage V _CC according to the current value of the line.

Note that when connected to a subscriber line with a very large line current, a large drive voltage V _CC is generated according to the line current value, but its upper limit is limited to, for example, 5V by the voltage regulator diode 17. . Therefore, the problem that an excessive drive voltage V _CC is supplied to the telephone 3 does not occur. Furthermore, in the circuit of this embodiment, drive voltages V _CC of various values are generated for each subscriber line depending on the line current value, but all voltages are within the operable voltage value of the telephone 3. There is no problem as long as it is within the range.

Furthermore, in the circuit of this embodiment, when a load change or a temperature change in circuit characteristics occurs in the telephone 3,
The value of the current flowing into the telephone 3 from the output terminal of the drive voltage V _CC changes, and as a result, the drive voltage V _CC also tends to change. However, in the circuit of this embodiment, the above change is suppressed and the drive voltage V _CC is stabilized by the following action.

That is, for example, suppose that the impedance of the telephone 3 decreases and the current flowing into the telephone 3 from the output terminal of the drive voltage V _CC increases. Then,
The base current of transistor 12 decreases, and the drive voltage V _CC temporarily decreases accordingly. However, when the base current decreases, transistor 12
Since the impedance of transistor 12 increases, the collector current of transistor 12 decreases. When the collector current decreases, the emitter resistance 1 of transistor 12
Since the voltage drop across transistor 12 is reduced,
The base current becomes easy to flow, the base current increases, and returns to a state close to the value before the decrease. When the base current returns, the voltage drop across the base resistor 15 almost returns to the value before the drop. That is, the increase in the load current is compensated for by the decrease in the shunt current of the transistor shunt circuit 11, and as a result, at least the voltage drop across the base resistor 15 of the drive voltage V _CC maintains the value before the drop. It turns out. Therefore, the drop in the drive voltage V _CC is compensated for, and thereby the drive voltage V _CC is kept more stable than in the case where there is no feedback function.

As described above, with the circuit of this embodiment, the following effects can be obtained.

By providing a shunt circuit 11 consisting of a transistor 12 and its emitter resistor 13 in parallel to the transformer 2 for AC connection, the DC resistance R between the line ends can be reduced even when the line current is small. Large drive voltage without increasing V _CC
As a result, the telephone 3 can be reliably driven.

In addition, since the shunt current of the shunt circuit 11 is changed according to the line current, and thereby the drive voltage V _CC is varied, it is possible to obtain the largest possible drive voltage V _CC according to the line current at that time. I can do it. Note that by providing the constant voltage diode 17, the upper limit of the drive voltage V _CC can be suppressed, and thereby a highly reliable drive voltage V _CC can be obtained.

Furthermore, even if the load on the telephone 3 fluctuates, which causes the current flowing into the telephone 3 from the drive voltage output terminal to change and the drive voltage V _CC changes, the feedback effect of the transistor 12 and its emitter resistor 13 will cause the above-mentioned Changes in the drive voltage V _CC can be suppressed to a minimum, thereby making it possible to generate a stable drive voltage V _CC against additional fluctuations.

Each of the above effects can be obtained with an extremely simple circuit configuration consisting of one transistor 12, its emitter resistor 13, base resistor 15, and the like.

Note that the present invention is not limited to the above embodiments. For example, a circuit using a field effect transistor (FET) may be applied as a transistor shunt circuit, or a transistor may be an NPN
In addition to the PNP type, the PNP type may also be used. In addition, the configuration of the voltage generation circuit, the configuration of the rectifier circuit, the configuration of the transistor shunt circuit, etc. can be modified in various ways without departing from the gist of the present invention.

[Effect of idea]

As described in detail above, the present invention connects a transistor shunt circuit in parallel to the AC connection circuit connected between the output terminals of the rectifier circuit, and uses this transistor shunt circuit to divide the main current out of the rectified output of the line current. A part of the rectified output flowing through the transformer is passed through the base resistor to the base of the transistor shunt circuit to reduce the voltage drop in the transformer of the AC connection circuit, thereby obtaining a sufficiently large driving voltage. The shunt current flowing through the transistor shunt circuit is thereby varied in accordance with the line current value, so that the largest possible drive voltage according to the line current is obtained, and the remainder of the rectified output flowing through the transformer is The load current is supplied to the telephone as a load current, and the change in this load current is detected as a change in the base current via the base resistor, and the change in the base current is suppressed by the feedback action of the transistor shunt circuit, thereby reducing the load current. This is designed to reduce changes in drive voltage due to changes in .

Therefore, according to the present invention, it is possible to generate sufficient driving voltage for the telephone to operate even when the line current is small, while satisfying the DC resistance standard between the line ends, and furthermore, the line current can be can generate the largest possible driving voltage according to the
Moreover, it is possible to provide a line interface circuit that can generate a stable drive voltage even when the load on the telephone varies.

[Brief explanation of the drawing]

Figure 1 is a circuit configuration diagram of a conventional line interface circuit, Figure 2 is a diagram for explaining the DC characteristics of the circuit, and Figure 3 is a circuit configuration diagram of a line interface circuit in an embodiment of the present invention. be. 1a, 1b...Subscriber line terminal (line terminal), 2
...Transformer, 3...Telephone, 4...Diode bridge circuit, 10...AC connection circuit, 11...
Shunt circuit, 12...transistor, 13...emitter resistor, 14...voltage generation circuit.

Claims (1)

[Claims for Utility Model Registration] In a line interface circuit that generates a driving voltage for a telephone based on a line current supplied from a station power supply via a balanced two-wire line, and supplies this driving voltage to the telephone, A rectifier circuit for rectifying the line current of a two-wire line and outputting the rectified output from a pair of output terminals, a transformer and a bypass capacitor, and the telephone set is connected to the output terminal of the rectifier circuit via the transformer. an AC connection circuit for AC connection of the AC connection circuit; It has at least a base resistor for connecting a connection point between the transformer and the bypass capacitor of the AC connection circuit and the base of the transistor, and a part of the DC current flowing through the transformer out of the rectified output is transferred to the transistor. A voltage determined by at least a voltage drop due to the base resistance and a voltage drop due to the emitter resistance of the transistor is used as the drive voltage. A line interface circuit comprising: a voltage generating circuit that supplies voltage to the telephone from a connection point between the transformer and the bypass capacitor.