JPH0984258A - Protective circuit for communication - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protective circuit for communication which can surely protect communication equipment such as a switchboard from overcurrent which is caused by lighting, a cross contact of power lines, etc., and which can be used repeatedly. SOLUTION: Between terminals 'a' and 'c' and 'b' and 'd', series combinations of positive temperature coefficient termistors 1, 1 and thick-film resistors 2, 2 are serially connected. Connections 'e' and 'f' between the positive temperature coefficient thermistors and the thick-film resistors are connected to an earth and a power supply Vcc through a diode bridge 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a communication protection circuit, and more particularly to a communication protection circuit for protecting a communication device such as an exchange from an overvoltage or an overcurrent generated due to a lightning strike or a contact of power lines.

[0002]

2. Description of the Related Art Conventionally, as a protection circuit of this kind, FIG.
A protection circuit as shown in Fig. 3 has been proposed (Japanese Patent Publication No. 3-5).
No. 135). The protection circuit 20 shown in the same figure has gas tube arresters 21 and 21 connected in series and grounded at the connection point between the A and B lines of the communication line side input terminal, and is connected to a protected device such as an exchange. The gas tube arresters 21 and 21 are the varistor 22 and 22 which are connected in series between the line A and the line B of the side output terminal and whose connection point is grounded.
Are connected in parallel with each other, and between them, the positive temperature coefficient thermistors 23 and 23 and the thick film resistor 2 are connected to the A line and the B line, respectively.
4 and 24 are connected in series and are connected in series.

The positive temperature coefficient thermistors 23, 23 do not normally operate at the maximum signal current of the exchange (about 100 mA), but the current capacity of the subscriber circuit of the exchange (about 200 m).
It has the characteristic of operating when it exceeds A). The thick film resistors 24, 24 are set to have a rated power (about 1 W) that can withstand the current withstand of the subscriber circuit of the exchange. Further, the thick film resistors 24, 24 are
In order to reduce the amount of impedance imbalance between the B line and the B line, the positive temperature coefficient thermistors 23, 23 and the thick film resistors 24, 2
The series resistance value of No. 4 is trimmed to have an accuracy of about ± 1%.

In such a protection circuit 20, when a surge current induced in a communication line by a lightning strike enters from the input terminal on the communication line side, first, the gas tube arresters 21, 2 are inserted.
1 absorbs surge current, gas tube arrester 21,
The surge current that could not be absorbed by 21 is varistor 22,
22 absorbs and protects the exchange. Further, when an overcurrent enters from the input terminal on the communication line side due to the contact of power lines, the positive temperature coefficient thermistors 23 and 23 self-heat and abruptly increase the resistance value, thereby suppressing the overcurrent and protecting the exchange. .

[0005]

However, in such a conventional protection circuit 20, the positive temperature coefficient thermistors 23, 23 do not operate due to a short-time surge current due to lightning strike.
The surge current that cannot be absorbed by the gas tube arresters 21, 21 passes through the positive temperature coefficient thermistors 23, 23 and flows to the thick film resistors 24, 24. Even if the power lines are touched, the overcurrent flows through the thick film resistors 24, 24 in a short time until the positive temperature coefficient thermistors 23, 23 operate and suppress the overcurrent. In this way, though over a short period of time, an overcurrent flows through the thick film resistors 24, 24, and power exceeding the rated power is applied.
The characteristics of the positive temperature coefficient thermistors 23 and 23 deteriorate
And the series resistance values of the thick film resistors 24, 24 differ from each other. As a result, the amount of impedance imbalance between the A line and the B line increases, and the communication quality of the exchange deteriorates.
There was a problem that 0 could not be used repeatedly.

Therefore, a main object of the present invention is to solve the above-mentioned problems, and it is possible to reliably protect a communication device such as an exchange from an overcurrent generated due to a lightning strike or a touch of a power line, and to enable repeated use. It is to provide a protection circuit for communication.

[0007]

In order to achieve the above object, the present invention has a positive temperature coefficient thermistor and a resistor connected in series between a communication line side input terminal and a protected equipment side output terminal. In the communication protection circuit provided as above, an overcurrent protection unit is provided between the positive temperature coefficient thermistor and the resistor.

According to the above construction, by providing the overcurrent protection means between the positive temperature coefficient thermistor and the resistor, the surge current induced in the communication line by the lightning strike or the overcurrent generated by the contact of the power lines. Protects the resistor from the overcurrent that has passed through the positive temperature coefficient thermistor when is input from the input terminal on the communication line side.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of a communication protection circuit according to the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a communication protection circuit of the present invention. In FIG. 1, a and b are communication line side input terminals, and c and d are protected equipment side output terminals. Between the terminals a and c and between the terminals b and d, a positive temperature coefficient thermistor 1 and a RuO ₂ system thick film resistor 2 are connected in series, respectively. A diode bridge 3 serving as an overcurrent protection means is connected between the positive temperature coefficient thermistors 1 and 1 and the thick film resistors 2 and 2. The diode bridge 3 connects the cathode of the diode 3a and the anode of the diode 3b at a connection point g, connects the cathode of the diode 3c and the anode of the diode 3d at a connection point h, and connects the anode of the diode 3a and the anode of the diode 3c to the connection point. i and the cathode of the diode 3b and the cathode of the diode 3d are connected at the connection point j. Then, the connection points e and f of the positive temperature coefficient thermistors 1 and 1 and the thick film resistors 2 and 2 are connected to the connection points g and h of the diode bridge 3, respectively, and the connection point j of the diode bridge 3 is grounded. The communication protection circuit 4 is configured by connecting i to the power supply Vcc. Here, the communication protection circuit 4
Trims the thick film resistors 2 and 2 to equalize the series resistance values of the positive temperature coefficient thermistors 1 and 1 and the thick film resistors 2 and 2 so that the terminals a and c and the terminals b and d are connected. The amount of impedance imbalance is reduced.

The communication protection circuit 4 is installed between the communication line and the exchange which is the protected device. In this case, the terminal a is connected to the Tip line side and the terminal b is connected to the Ring line side. The power supply Vcc connected to the connection point i of the diode bridge 3 shares the power supply of the exchange (the power supply voltage is approximately -48V). Also, the positive temperature coefficient thermistors 1, 1
The characteristics of are selected according to the withstand voltage and withstand current required when touching, but the withstand voltage is approximately 70 to 250 V, maximum current 1 to
2A is used. The diode of the diode bridge 3 has the same maximum current of 1 to 2 A as that of the positive temperature coefficient thermistor 1 and has a small forward voltage of about 1 to 2 V.

Next, the protection operation of the communication protection circuit 4 in the above embodiment will be described. First, Tip line and R
When the power line touches the ing line, the positive temperature coefficient thermistor 1,
1 causes an overcurrent to flow, and the positive temperature coefficient thermistor 1,
No. 1 self-heats and the resistance value suddenly increases to suppress overcurrent, and protects the thick film resistors 2 and 2 and the exchange. further,
If the positive temperature coefficient thermistors 1 and 1 do not operate, surge current induced in the communication line due to lightning strike, or the overcurrent that passes during a short time until the positive temperature coefficient thermistors 1 and 1 operate,
Ground and power supply Vcc by diode bridge 3
The thick film resistors 2 and 2 and the exchange are protected by being clamped at and flowing to the ground or the power supply Vcc.

With such a protection operation, the thick film resistors 2 and 2 are always protected from overvoltage and overcurrent together with the exchange, so that the characteristics are not deteriorated. Therefore, the unbalanced amount of impedance between the terminals a and c and between the terminals b and d does not change, and the communication quality of the exchange does not deteriorate, so that the communication protection circuit 4 can be repeatedly used.

As shown in FIG. 2, the communication protection circuit 4 of this embodiment has terminals a and b and positive temperature coefficient thermistors 1 and 1.
A surge current may be temporarily absorbed by connecting arresters 5 and 5 which are connected in series and whose connection point is grounded between and. However, even without the arresters 5 and 5, if the surge waveform (10 × 1000) μsec, the peak value voltage of 1 KV, and the surge of about 100 A of current can protect the thick film resistors 2 and 2 and the switching device sufficiently, the arrester 5 , 5 can use an arrester having a smaller size and a smaller capacity than the conventional one, and it is not necessary to provide a varistor after the thick film resistors 2 and 2. Furthermore, as shown in FIG. 3, varistors 6 and 6 can be used instead of arresters 5 and 5.

Thus, the communication protection circuit 4 of this embodiment
In order to prevent the deterioration of the characteristics of the thick film resistors 2 and 2, the diode bridge 3 is provided, so that the surge absorber portion in the front stage, which is provided in front of the positive temperature coefficient thermistors 1 and 1 and which includes, for example, an arrester or a varistor, is small. The thick film resistors 2 and 2 can be formed or deleted, and the subsequent surge absorber portion formed of, for example, a varistor can be deleted. Since the diode of the diode bridge 3 having a small forward voltage of about 1 to 2 V is used as described above, the entire circuit can be downsized as compared with the conventional one.

In the above-mentioned embodiment, the positive characteristic thermistors 1 and 1 and the thick film resistors 2 and 2 are connected to the ground and the power source Vcc through the diode bridge 3 from the connection points e and f. The overcurrent protection means provided between the thermistors 1 and 1 and the thick film resistors 2 and 2 is not limited to this, and any overcurrent protection means may be used. For example, as shown in FIG. 4, a bidirectional silicon surge protection device 7,7 which operates at a voltage exceeding the voltage of the power supply Vcc and has a low terminal voltage during operation of 1 to 2 V is provided with a positive temperature coefficient thermistor 1,1 and a thick layer. It may be connected in series between the membrane resistors 2 and 2 and the connection point may be grounded. As with the case of using the diode bridge 3, this can also be protected from a larger overcurrent with a small shape.

[0017]

As described above, according to the communication protection circuit of the present invention, the surge current induced in the communication line by lightning strike or the overcurrent generated by the contact of the power line intrudes from the input terminal on the communication line side. In this case, the overcurrent protection device provided between the PTC thermistor and the resistor protects the resistor from the overcurrent that has passed through the PTC thermistor, which prevents the deterioration of the resistor characteristics. It is possible to use the protection circuit repeatedly.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a communication protection circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a modified example of the communication protection circuit of FIG.

FIG. 3 is a circuit diagram showing another modification of the communication protection circuit of FIG.

FIG. 4 is a circuit diagram showing still another modified example of the communication protection circuit of FIG.

FIG. 5 is a circuit diagram showing a conventional communication protection circuit.

[Explanation of symbols]

 1 Positive Characteristic Thermistor 2 Thick Film Resistor 3 Diode Bridge 3a Diode 3b Diode 3c Diode 3d Diode 4 Communication Protection Circuit

Claims (1)

[Claims] 1. A communication protection circuit comprising a positive characteristic thermistor and a resistor connected in series between a communication line side input terminal and a protected equipment side output terminal, wherein a positive characteristic thermistor and a resistor are provided. A communication protection circuit characterized by being provided with an overcurrent protection means in between.