JPS5853289A - Overcurrent protecting circuit - Google Patents

Abstract

PURPOSE:To ensure the protection of an overcurrent with an extremely low cost, by setting a resistor which is fused with its own generation of heat in series to a semiconductor circuit that supplies a call current to a subscriber. CONSTITUTION:A resistor 2 and a semiconductor element 3 are connected in series to both ends of a power supply 1, and a call current is supplied to a subscriber line from a terminal 4. The voltage generated across the resistor 2 is led to a control circuit 5. The circuit 5 controls the equivalent resistance of the element 3 to keep the call current at a prescribed level. The resistor 2 is formed with a thin or thick film on the ceramic substrate of a large heat resistance and then fused with its own generation of heat if the call current increases by some reason.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a circuit connected to a telephone exchange and included in a telephone current supply circuit that supplies telephone current to subscribers, and which automatically controls the telephone current supply circuit when the telephone current increases abnormally. Regarding the circuit to be stopped.

A semiconductor is introduced into the communication current supply circuit, and a semiconductor element, such as a transistor, is placed in the flow path of the supplied communication current.
In recent years, devices have come into wide use in which a converter-emitter circuit is inserted to control the equivalent resistance value of the semiconductor element so that the communication current reaches a predetermined value. Since it is necessary to install a large number of communication current supply circuits, it is necessary that the circuits be simple and inexpensive, and it is desirable that the mounting space be small. Circuits using semiconductor elements meet this demand well, and have the excellent feature of stably controlling the communication current at any set value.

However, if the control function malfunctions for some reason and a short circuit occurs on the output side, an excessive current may flow and become dangerous. For this reason, circuits of this type are inserted on both the positive and negative sides of the communication current supply circuit, but in many cases, the circuits on both sides often fail, causing excessive current to flow. Although it is technically possible to further add a current limiting circuit to prevent this, it is not desirable from the standpoint of making the device economical.

Is this invention a call like this? ``The purpose of this invention is to provide an overcurrent protection circuit that can automatically cut off the current when an excessive current occurs in a current supply circuit, and that has an extremely inexpensive configuration.

The present invention provides a resistor that is inserted in series with a semiconductor element and used to detect the communication current flowing through the semiconductor element.
It is characterized in that it is constructed of a conductor film formed on a ceramic substrate, and that the conductor film is configured to self-heat and melt when a current is lower than the allowable maximum current of the above-mentioned communication current.

An embodiment will be explained with reference to drawings.

The figure is a circuit diagram of a device according to an embodiment of the present invention. A resistor 2 and a semiconductor element 3 are connected to both ends of the power supply 1, one end of which is grounded.
are connected in series to supply speech current from terminal 4 to the subscriber line within the exchange. The voltage generated across this resistor 2 is guided to a control circuit 5, and this control circuit 5 controls the semiconductor element 3 so that this voltage, that is, the value of the communication current is constant.
configured to control the equivalent resistance of.

Here, the feature of the present invention is that this resistor 2 is
A conductor film is formed of a thin film or a thick film on a ceramic substrate with high thermal resistance, and the conductor film is fused by self-heating at a current value below the maximum allowable value of the communication current. It is located in a place where it is configured so that

In a more specific embodiment, the resistor 2 is made of a conductive film formed of a tantalum nitride thin film on a ceramic substrate, and its size is 5 mm x 10 fi. Its resistance value is 510. Talking current value ld, 19.3mA ~ 1
It is possible to set it to a predetermined value between 20 mA.

When a current of about 270 mA continues to flow through this conductive film, a power dissipation of about 3.7 W occurs and the film melts in a few seconds. The semiconductor element 31d used is one that will not be destroyed even by a current of about 500 mA.

In the device having such a configuration, when the circuit is operating normally, the control circuit 5 operates so that the voltage across the resistor 2 caused by the communication current is always within a predetermined value range. Even if the output terminal 4 becomes almost short-circuited, the equivalent resistance of the semiconductor element 3 is controlled to be high so that an abnormally large current will not be supplied. However, if a failure occurs in the control circuit 5 and its surroundings for some reason, or if the output terminal 4 is short-circuited with an unexpectedly low impedance, an abnormally large current will be generated in the semiconductor element 3.

At this time, the self-heating of the resistor 2 melts the conductive film constituting the resistor 2 at a current value smaller than the current value at which the semiconductor element 3 is damaged. If this occurs, the abnormal current will be automatically stopped and maintenance will be required.

According to such a configuration, (1) the resistor for current detection is originally necessary and there is no need to add it specially; (2) fusing due to self-heating is not self-recoverable, but its operation is It is extremely reliable and suitable for preventing dangers that rarely occur. (3) Formed on a ceramic substrate, which reduces the variation in melting current. (4) Formed on a ceramic substrate. (5) By forming on a ceramic substrate, the reliability of the resistor can be improved; (6) By forming on a ceramic substrate, other electronic components can be attached to this ceramic substrate. (7) It is extremely inexpensive compared to other methods of providing additional circuits.

[Brief explanation of drawings]

The figure is a circuit configuration diagram of a device according to an embodiment of the present invention. 1...Power supply, 2...Resistor, 3-...Semiconductor element,
4...terminal, 5...control circuit.

Claims (1)

[Claims] (1) A resistor connected to a circuit that supplies communication current to subscriber lines in a telephone exchange and inserted into a communication current flow path, and a semiconductor element connected in series with this resistor in this flow path. and a control circuit which inputs the voltage generated across the resistor and controls the resistance value of the semiconductor element to be increased when the voltage exceeds a predetermined value, respectively on the positive and negative lines. , the resistor is composed of a conductor film formed on a ceramic substrate, and when the communication current increases abnormally, the conductor film is caused to generate heat by itself at a current value below the maximum allowable value of the communication current. An overcurrent protection circuit characterized in that it is set to melt.