A current limiter of this type is known e. g. from the French Patent Application No. 2 714 230.

In this known current limiter the superconducting mate- rial forms part of a secondary winding in a transformer, as the secondary winding is serially connected to the su- perconducting material. The superconducting material is formed as a conductor which is presumably manufactured in that a superconducting material is enclosed by an ordi- nary electrical conductor, e. g. of silver.

A current limiter of the above-mentioned known type oper- ates in the manner that when a current runs in the pri- mary winding, a current is induced in the secondary wind- ing and thereby also in the superconducting material. The superconducting material has the property that when the current exceeds a certain value, the superconducting ma- terial will assume a state in which it will have a high resistance, and the current in the secondary winding as well as in the primary winding will thereby be limited.

However, a current will still be able to run in the elec- trical conductor enclosing the superconducting material, which means that the characteristic of the current lim- iter will not be quite as steep as is desirable.

Accordingly, an object of the invention is to provide a current limiter of the above-mentioned type, which has a steep characteristic, and which is moreover easy to cool and has a compact structure.

The object of the invention is achieved in that the sec- ondary winding is formed by a substrate on whose one side or both sides the superconducting material is deposited.

Expediently, as stated in claim 2, the superconducting material is grown epitaxially.

A film having a very high superconducting current density is formed hereby.

Since, as stated in claim 3, the substrate may e. g. be monocrystals of magnesium oxide (MgO), strontium titanate (SrTiO3), sapphire (Al203), etc., which are insulating ma- terials, they will give a very steep characteristic to- gether with the superconducting material, as the secon- dary winding will almost be an insulator in case of great currents.

Particularly in connection with the recent superconduct- ing materials, such as crystalline oxides of yt- trium/barium/copper, bismuth/strontium/calcium/copper or thallium/barium/calcium/copper, a current limiter having a very steep characteristic may be provided when the su- perconducting material assumes its so-called normal state.

When, as stated in claim 4, the secondary winding con- sists of several substrates with deposited superconduct- ing materials, it is ensured that the value of the maxi- mum current to be allowed to run in a circuit may be ad- justed.

For a practical structure it is an advantage in terms of production when, as stated in claim 5, the secondary winding is formed as a disc with a central through hole which has a geometrical shape allowing the ferromagnetic core to pass through it.

The secondary winding may hereby be constructed quite simply by placing discs on an iron core which can be separated into two yokes.

When, as stated in claim 7, a third winding connected to a load is positioned close to the secondary winding, ex- cess energy may be dissipated in the load, when the su- perconducting material is in its insulating state which, as mentioned, occurs when the current reaches the maximum value.

The invention will now be explained more fully with ref- erence to an embodiment shown in the drawing, in which fig. 1 shows an iron core with a superconducting current limiting element according to the invention, fig. 2 schematically shows the secondary winding made of the superconducting material, and fig. 3 schematically shows a third winding arranged close to the secondary winding.

Fig. 1 shows a ferromagnetic iron core 3 which has a pri- mary winding 2 and a secondary winding 5. The primary winding 2 is connected to a circuit 1, which may e. g. be the supply mains, which is to be protected against over- loading so that a current ImaX is maximally allowed to run in the circuit.

A current in the primary winding will induce a current in the superconducting and short-circuited secondary wind- ing. As long as the induced current is smaller than the critical current in the superconductor, the inductance of the primary winding is very low.

When the current in the secondary winding reaches the critical current intensity, the inductance of the primary winding increases very strongly.

Fig. 2 shows the secondary winding on an enlarged scale.

It consists of a substrate 7, which is insulating and on whose upper side a superconducting material, here indi- cated by the reference numeral 6, is deposited. This sec- ondary winding may e. g. be made as a disc by thin-film technique. As shown by the reference numeral 8, the disc may be provided with a hole which has dimensions corre- sponding to the external dimensions of the ferromagnetic core 3 in fig. 1.

As will be seen in fig. 3, a third winding 6 connected to a load 7 is arranged close to the secondary winding 5.

When the current in the secondary winding 5 is below the maximum value, the inductance will be very low in the primary winding, which means that no current of impor- tance can be induced in the third winding.

When, on the other hand, the current exceeds its maximum value, the inductance will be very high in the primary winding, which means that a current which can be dissi- pated in the load 7 will be induced in the third winding.

It should moreover be mentioned that the actual cooling of the superconducting material, which may e. g. be per- formed with liquid nitrogen (77k), will be relatively easy to carry out because of the simple structure of the secondary winding.