GB2120018A - Transformer - Google Patents

Abstract

A transformer comprises a ferromagnetic core with two parallel limbs (1) which are interconnected by means of yokes (3) and on which limbs there is provided a respective transformer winding (7,11) so that the windings are magnetically coupled to the core. A further winding (17) is provided on the core so that it is not magnetically coupled to the transformer windings (7,11) or the core. This further winding (17) may be electrically connected to one of the transformer windings (11) to form the inductor of a filter circuit which includes a capacitor (21) mounted on an insulating plate (13) on said transformer. <IMAGE>

Description

SPECIFICATION Transformer The invention relates to an electrical transformer comprising a ferromagnetic core about which a transformer winding or windings is/ are provided such that the winding or windings is/are magnetically coupled to the core.

A transformer of the kind set forth is known, for example, from DE-OS 21 03 112 and which has limbs on which the winding/windings is/are provided, the limbs being interconnected by yokes. Transformers of this kind are used, for example, as power supply transformers in electronic circuits. In addition to the transformer, often an extra coil or a filter is required, for example, for the elimination of undesirable interference voltages occurring in the mains voltage. The construction and also the cost of a coil or a filter are comparable with those of a transformer, so that the provision thereof has a costincreasing effect.

It is an object of the invention to provide a transformer of the kind set forth which eliminates the expensive, separate manufacture of a coil or filter.

The invention provides a transformer of the type described in the opening paragraph which is characterised in that a further winding is provided about the core in such manner that the effect of any magnetic coupling between said further winding and said winding or windings or said core is substantially zero.

The further winding can operate as a separate coil, independently of the windings of the transformer. Because it requires no core and coil former of it own, it can be added to the transformer for very small cost. A filter can be manufactured in a very simple manner by providing the transformer with at least one capacitor which is electrically connected to the further winding.

The invention also provides an electrical circuit comprising a transformer having a core with a transformer winding wound about said core, and a filter circuit including an inductor and a capacitor, said filter being electrically connected to said transformer winding, characterised in that said inductor is formed by a further winding provided about the core of the transformer in such manner that the effect of any magnetic coupling between said further winding and said transformer winding or said core is substantially zero. The invention will be described hereinafter, by way of example, with reference to the accompanying drawing.

Therein: Figure 1 is a side elevation of an embodiment of a transformer in accordance with the invention, and Figure 2 shows a diagram of a circuit in which the embodiment shown in Fig. 1 is used.

The transformer which is shown in Fig. 1 comprises a core which is made of a ferromagnetic material (for example, iron or ferrite) and which consists of two parallel limbs 1 which are interconnected by yokes 3, thus forming an annular assembly which constitutes a closed magnetic circuit in which an air gap (not shown) may be included, if desirable.

Around one of the limbs 1 (the right-hand limb in Fig. 1) there is provided an insulating coil former 5 with a primary winding 7, and on the other limb there is provided an insulating coil former 9 with a secondary winding 11. An insulating plate 1 3 which supports connection pins 1 5,1 6 is connected to the coil formers 5,9. The primary and secondary windings 7,11 are covered with an insulating foil on which there is provided a further winding 1 7. The further winding 17 consists of turns which extend around both limbs 1 of the core. The connection wires of the primary winding 7 are anchored to the connection pins 1 5 and those of the secondary winding 11 and the further winding 1 7 are anchored to the connection pins 16, for example, by soldering.

When an alternating current is applied to the primary winding 7, a magnetic flux is produced in the core, said flux following a closed path through the core. At a given instant, for example, the flux is oriented as denoted by the arrows 1 9. The flux is surrounded by the turns of the secondary winding 11, so that a voltage is induced in this winding. In other words, the primary and secondary windings are both magnetically coupled to the core and, via the core, magnetically coupled to one another.

The turns of the further winding 17, however, extend around both limbs 1 of the core so that they enclose the equally large fluxes in both limbs which extend in opposite directions. Therefore, the nett flux enclosed by these turns equal zero, so that the further winding 1 7 is not effectively magnetically coupled to the primary or the secondary winding. The same result could be obtained by making the turns of the further winding 1 7 extend around both yokes 3 instead of around both limbs 1. This is because the flux in each of the two yokes are also equal and oppositely directed. When the core comprises more than two limbs (for example, three), the turns of the further winding should enclose all limbs in order to ensure that the nett flux within these turns effectively equates zero.It makes no difference whether the primary and secondary windings 7,11 are each provided on a limb 1 of their own, as in the described embodiment, or together on a single limb or are divided and distributed between the available limbs. If desirable, the primary and the secondary windings may be replaced by a single transformer winding which combines the functions of both windings (autotransformer).

The further winding 1 7 is connected on the one side to one end of the secondary winding 11 and on the other side to a capacitor 21 which is also connected to the other end of the secondary winding. The capacitor 21 is connected to the transformer so that its connection wires are soldered to two of the connection pins 16.

It will be clear that virtually no material other than wire is required for the manufacture of the further winding 1 7. The core with the transformer windings 7,1 1 which are wrapped with foil and which serve to support the further winding 1 7 forms part of the transformer, like the connection plate 13, so that it has no effect on the cost of the further winding.

Fig. 2 shows an example of a diagram of a circuit in which the transformer shown in Fig.

1 is used. Whenever applicabte, the same reference numerals are used in Fig. 2 as in Fig. 1. The primary winding 7 can be connected to the mains via the connection pins 15. In conjunction with the capacitor 21, an inductor comprising the further winding 1 7 forms an LC-filter which eliminates interference from the voltage supplied by the secondary winding 11 before this voltage is applied to an apparatus 23 to be energised via the connection pins 16. If desirable, further components such as resistors can be added to the filter in the same way as the capacitor 21.

Claims (7)

1. An electrical transformer comprising a ferromagnetic core about which a transformer winding or windings is/are provided such that the winding or windings is/are magnetically coupled to the core, characterised in that a further winding is provided about the core in such manner that the effect of any magnetic coupling between said further winding and said winding or windings or said core is substantially zero.

2. A transformer as claimed in Claim 1, in which the core comprises at least two parallel limbs interconnected by yokes with the winding or windings being provided on at least one limb, characterised in that the further winding consists of a plurality of turns which extend around all the limbs of the core.

3. A transformer as claimed in Claim 1, in which the core comprises at least two parallel limbs interconnected by yokes with the winding or windings being provided on at least one limb characterised in that the further winding consists of a plurality of turns which extend around the yokes of the core.

4. A transformer as claimed in any of the preceding Claims 1 to 3, characterised in that a capacitor is mounted on said transformer which is electrically connected to the further winding, said capacitor and said further winding forming an electrical filter.

5. An electrical transformer substantially as herein described with reference to the accompanying drawing.

6. An electrical circuit comprising a transformer having a core with a transformer winding wound about said core, and a filter circuit including an inductor and a capacitor, said filter being electrically connected to said transformer winding, characterised in that said inductor is formed by a further winding provided about the core of the transformer in such manner that the effect of any magnetic coupling between said further winding and said transformer winding or said core is substantially zero.

7. An electrical circuit substantially as herein described with reference to Fig. 2 of the accompanying drawing.