Circuits of this type are used, for example, but certainly not exclusively, in telecommunications devices such as telephones, fax machines, modems, telephone installations etc. and are used to protect these from damage which can be caused by the occurrence of excessive voltages and/or currents on the telephone lines. Excessive voltages and/or currents on the telephone lines can be caused, for example by lightning striking, high-voltage lines contacting the telephone lines, induced voltages and short-circuits.

Protective circuits of this type currently consist of an element with positive temperature coefficients such as thermistors (e. g. PTC). A thermistor of this type has the characteristic that its resistance increases with increasing temperature. If a high current flows through a thermistor due to an excessive voltage or excessive current on a telephone line, the thermistor heats up and thus increases its resistance which leads to a reduction of the current flowing through it.

However, the use of a thermistor has various disadvantages. For example, if it becomes only high- resistance in the event of high currents and voltages, the circuit to be protected does not disconnect from the source, so the circuit to be protected can be damaged despite the protective circuit. Also, thermistors have capacitative properties which can lead to problems, in particular in high-frequency applications.

The object of the present invention is therefore to develop a device which in all circumstances and without undesired side effects, offers reliable protection for the circuit to be protected.

This object and other objects are achieved by a protective circuit having a resistor through which there passes current supplied to the circuit to be protected or output by the circuit to be protected or a current dependent thereon. Also, the protective circuit contains a switching element which is heated by the thermal energy output by the resistor and which carries out a switching process when a specific temperature is reached.

As the protective circuit contains a switching element, it can ensure that the protective circuit is completely separated from the source in dangerous situations. Damage to the circuit to be protected can thus be completely eliminated.

As the protective circuit only has one resistor apart from the switching element, it cannot interfere with the signals which are supplied to the circuit to be protected or which are output therefrom.

The protective circuit of this invention therefore offers reliable protection for the circuit to be protected in all circumstances and without undesired side effects.

The invention is described in more detail hereinafter with the aid of an embodiment and with reference to the drawings, in which: Figure 1 is a plan view of an arrangement containing a plurality of protective circuits; and Figure 2 is a lateral view of the arrangement shown in Figure 1.

The protective circuit is a hybrid circuit arrangement which is soldered or inserted in the circuit

to be protected. However, the invention is not restricted to this embodiment and the protective circuit can also be constructed and arranged in any other way.

For example, the protective circuit may be utilized in a telecommunications device such as, a telephone, a fax machine, a modem or a telephone installation. The circuit to be protected can also be any other circuit.

The protective circuit protects against damage caused by an excessive voltage and/or current on a line.

The protective circuit can also protect against damage which is caused by excessive voltages and/or currents on other lines, for example on the lines supplying with energy the circuit to be protected.

The protective circuit is arranged so that current supplied by the circuit to be protected, or output by the circuit to be protected, or a current dependent thereon passes through it, or more precisely, at least through a resistor contained therein.

The hybrid circuit arrangement forming the protective circuit, preferably consists of a substrate with good thermal conductivity, preferably glass or ceramic, on which a resistor, a switching element and terminals for connection to the circuit to be protected are provided.

The resistor is formed directly on the substrate, and may be produced by thick-film or thin-film methods.

The switching element, preferably consists of a bimetallic thermostat which is preferably applied by gluing or soldering to the substrate directly adjacent to the resistor.

A current flowing through the resistor causes it to heat up proportionally so that as the current increases so does the heat. The thermal energy produced in the resistor is transferred to the switching element via the thermally conductive substrate.

The resistor and the switching element are calibrated so that the switching element does not switch as long as the heat generated by the resistor indicates that it is operating below voltage and/or the current limits. The switching element switches when heat from the resistor rises to a level which can only be reached when the voltage and/or the current exceed safe limits so as to become dangerous to the circuit to be protected.

By switching the switching element, the circuit is protected from excessive voltage and/or current. For example, the connection between the circuit to be protected and the line, which supplies or draws an excessive voltage and/or current, can be broken by the switch. In this case, the resistor and the switching element can be connected in series. However, it would also be conceivable that another switch or relay is actuated by the switching element or that other actions are triggered.

Once the switching element has been switched as a result of the excessive voltage and/or current, current flow through the resistor is interrupted. The resistor, substrate and the switching element therefore cool down until the switching element automatically switches back into the original state and the system can again revert to normal operation. If the voltage and/or the current are still excessive, the switching element is actuated again. If the voltage and/or the current have reverted to normal values in the meantime, the switching element retains its originated operational state.

A plurality of protective circuits of the above- described type can also be provided on a substrate. An arrangement comprising four protective circuits is shown in Figures 1 (plan view) and 2 (lateral view). The substrates are here denoted by the reference numeral

l, the switching elements by the reference numeral 2, and the terminals for connecting the arrangement to the protective circuit are denoted by the reference numeral 3. The resistors are not shown in the drawings; the course and dimensions of the thick-film or thin-films forming this are freely selectable, as long as the desired switching behavior of the switching element can thus be achieved. Excessive voltages or currents on four different lines can be detected by an arrangement of this type.

The above-described protective circuit provides reliable protection for the circuit to be protected, regardless of the details of the practical implementation, in all conditions and without undesired side effects.