JPS6162116A - Over current protection circuit - Google Patents

Abstract

PURPOSE:To protect the subscriber's line from sudden overcurrent by providing a digital switch with a means to immediately control the current by detecting the difference of currents supplied to multiple subscribers. CONSTITUTION:In case a sudden excess voltage E is supplied in digital switch system due to, for example, attack of thunder flash, the excess voltage is impressed from constant power source until supply current is controlled. Namely, excess voltage is impressed between base emitters of output transisters GR 5A and 5B by stray capacities 12A and 12B, but the voltage between base emitters is maintained by breakdown voltage of constant voltage diodes 13A and 13B. Further, when the difference of voltage fall down of current E is detected by a current difference detector 10, constant voltage diodes 15A and 15B is parallelly connected to the constant voltage diodes 13A and 13B respectively. As a result, the current supplied to TRs 5A and 5B is always maintained at low level, thus enabling to protect the subscriber's circuit.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a subscriber circuit of a digital exchange, and in particular to overcurrent protection that limits the power supply current when an abnormality is detected in the power supply current supplied to the subscriber line. Regarding circuit improvement.

In a digital exchange, the subscriber circuit is equipped with a so-called BOR3HT function including a current supply function. Such subscriber circuits are subject to ground faults (
Means is provided to detect a so-called ground fault) or cross-wire with a power supply line (so-called electrical cross-circuit), and prevent abnormal excessive current from flowing into the subscriber circuit. The nuclide means needs to work effectively against steep transient currents that occur during lightning strikes and the like.

[Conventional technology]

Figure 2 shows the conventional overcurrent ('
FIG. 2 is a diagram showing an example of a jic protection circuit. In Figure 2,
The A line of the subscriber line is connected to terminal A of the subscriber circuit, and the A line of the subscriber line is connected to terminal A of the subscriber circuit.
The subscriber line B line is connected to.

The subscriber line resistance and the internal resistance of the subscriber terminal are expressed isometrically by a load resistance RL. The power supply control circuit 1 is
A control current for controlling the power supply current is supplied to the distributor 3 through the changeover switch 2, and the distributor 3 transmits the control current supplied from the power supply control circuit 1 to each output transistor 5A through the output amplifiers 4A and 4B. and distribute to the base of 5B. As a result, output transistors 5A and 5B are connected from battery 8 to resistors 6A, 6B and protection diodes 7A and 7B.
A current proportional to the control current is supplied to the load resistor RL via the control current. The power supply control circuit 1 has a feedback path that monitors the power supply current value through a path (not shown) and controls the control current value so that the power supply resistance value including the output transistors 5A and 5B remains approximately constant. However, the feedback path is ineffective for signals in the audio frequency band transmitted through the subscriber line, and the capacitor 9 is provided so that the output transistors 5A and 5B act as a sufficiently high power supply impedance. . - The force difference current detector 10 compares the voltage drop that occurs across the resistors 6A and 6B due to the power supply current, and if the difference between the two types of voltage drops exceeds a predetermined value, a ground fault or a ground fault is detected in the subscriber line. It is determined that a battery has occurred, and the selector switch 2 is activated. As a result, a predetermined control current is supplied from the constant current source 11 to the distributor 3, and the power supply current flowing through the output transistors 5A and 5B is suppressed to a predetermined value (for example, about 30 milliamps), and the output transistors 5A and 5B are Protects against ground fault or battery damage. However, since the control current supplied to the distributor 3 is suppressed from rapid changes due to the influence of the capacitor 9, the control current is suppressed from changing rapidly after the changeover switch 2 is driven until the supply current flowing to the output transistors 5A and 5B is suppressed. There will be a slight delay.

[Problem to be Solved by the Invention 9] As is clear from the above explanation, in conventional overcurrent protection circuits, the difference current detector 10 detects a ground fault or a battery until the supply current is suppressed. There was a slight delay. Therefore, when a steep transient voltage E is induced in the subscriber line due to a lightning strike, for example, the output transistor 5A
An excessive current would flow to each output transistor 5A or 5B through the stray capacitances 12A and 12B of 5B, and there was a risk that the output transistor 5A or 5B would be damaged.

[Means for solving problems]

The above problem can be solved by providing means in the subscriber circuit of the digital exchange to detect the difference between the currents supplied to the A line and the B line of the subscriber line, and to immediately limit each of the currents. Effect] That is, according to the present invention, after a difference between the currents flowing in the A line and the B line of the subscriber line is detected, each of the currents is immediately limited to a predetermined value or less, so that there is no steep transient in the subscriber line. Even if a voltage is induced, it is possible to protect the subscriber circuit from damage.

[Example 3 Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing an overcurrent protection circuit according to an embodiment of the present invention. Note that the same reference numerals indicate the same objects throughout the plan. In FIG. 2, a voltage diode 13A and a constant voltage diode 15A are connected in parallel between the base of the output transistor 5A and the negative electrode of the battery 8 via a switch 14A. A constant voltage diode 13B1 and a switch 1 are connected between the base of the output transistor 5B and the ground.
A constant voltage diode 15B is connected in parallel via 4B. The switches 14A and 14B are driven by a differential current detector 10'.

In FIG. 1, the power supply control circuit 1 and the constant current source 11 control the power supply currents of the output transistors 5A and 5B at the second TI! The process is similar to that in J. On the other hand, if a sudden transient voltage E is induced in the subscriber line due to a lightning strike or the like, the output transistors 5A and 5B
An excessive voltage is applied between the base emitters of the output transistors 112A and 12B due to the stray capacitances 112A and 12B, but since the voltage between the base emitters is maintained at the breakdown voltage of the constant voltage diodes 13A and 13B, the current flowing through the output transistors 5A and 5B is Predetermined value (for example, 150 to 200 milliamps)
This will be prevented from happening.

Furthermore, when the differential current detector 101 detects the difference in the voltage drop across the resistors 6A and 6B due to the transient voltage, it immediately drives the switches 14A and 14B and changes the voltage regulator diodes 15A and 15B to the voltage regulator diodes 13A and 13B, respectively. Connect in parallel. As a result, the current flowing through output transistors 5A and 5B is maintained at a lower value (eg, 50 to 80 milliamps) by the breakdown voltage of voltage regulator diodes 15A and 15B. Note that the constant voltage diodes 13A and 13B are always connected between the pace emitters of the output transistors 5A and 5B, and the constant voltage diodes 15A and 15B are outputted immediately when the differential current detector 1,0゛ drives the switches 14A and 14B. Since tJ[ is applied between the pace emitters of transistors 5A and 5B, output transistors 5A and 5B can be sufficiently protected even against steep transient voltages.

As is clear from the above description, according to this embodiment, the output transistors 5A and 5B are connected to the constant voltage diode 13.
Since the output transistors 5A, 13B, 15A and 15B are protected without delay, there is no possibility that the output transistors 5A and 5B will be damaged even if a steep transient voltage is induced in the subscriber line.

Note that FIG. 1 is only one embodiment of the present invention, and the configuration of the subscriber circuit, for example, is not limited to that shown in the figure, and many other modifications may be considered, but any In this case, the effects of the present invention remain the same.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to sufficiently protect the subscriber circuit even when a steep transient voltage is induced in the subscriber line due to a lightning strike, etc. Improved reliability.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an overcurrent protection circuit according to an embodiment of the present invention, and FIG. 2 is a diagram showing an example of a conventional overcurrent protection circuit. In the figure, ■ is a power supply control circuit, 2 is a changeover switch, and 3 is a power supply control circuit.
is a distributor, 4A and 4B are output amplifiers, 5A and 5
B is an output transistor, 6A and 6B are resistors, 7A and 7B are protection diodes, 8 is a battery, 9 is a capacitor, 10 and 10' are differential current detectors, 11 is a constant current source,
12A and 12B are stray capacities, 13A, 13B, 15
A and 15B are constant voltage diodes, 14A and 14
B is the switch, A and B are the terminals, E is the transient voltage, RL
indicates the load resistance.

Claims (1)

[Claims] In the subscriber circuit of a digital exchange, the A of the subscriber line
An overcurrent protection circuit comprising means for detecting the difference between the currents supplied to the line and the B line and immediately limiting each of the currents.