[0001] This invention relates to circuit arrangements and more particularly, but not exclusively,
to arrangements including a transformer in which current passed through a primary
winding is used to drive a load, such as a magenetron, connected across a secondary
winding.
[0002] A circuit arrangement which is conventionally used for driving a magnetron is illustrated
in Figure 1. It includes a pulse transformer 1 having a core 2 about which is wound
a primary winding 3 and a secondary winding 4. The primary winding 3 is connected
to a d.c. charging circuit indicated generally at 5 which includes a switch 6 for
controlling transmission of current pulses through the primary winding 3. The secondary
winding 4 is bifilar, that is, it consists of two wires wound together in parallel
such that the adjacent turns carry current in opposite directions. The secondary winding
4 is connected to the supply of the magnetron heater element 7. The proximity of the
two wires produces only a surface effect within the core 2 and magnetic fields generated
by the heater current passing along the secondary winding 4 tend to cancel each other
out. The magnetron cathode is connected to one of the heater element terminals 8,
the other terminal being shown at 9.
[0003] The B-H curve of the core 2, showing the flux density B as a function of magnetic
field strength H is illustrated in Figure 2. The smaller curve, YY¹, is the hysterisis
loop at low magnetising force and the other loop, ZZ¹, is the largest, at which saturation
occurs if H₂ is exceeded. The curves OXY and OXZ show the curve taken on the first
half of the cycle for each loop when the core material is unmagnetised. The part of
the hysteris loop at which operation occurs may be by applying biasing to the core
2. The curves WW¹ and VV¹ are selected by using positive and negative biasing respectively.
When a pulse is passed through the primary winding 3, the transformer core becomes
magnetised. After transmission of the current pulse, and thus removal of the magnetic
field, the remanence R remains, decaying in a time depending on the core material,
the conditions existing in the core and other external conditions. It is desirable
to reset the transformer core between pulses to permit a larger range of the B-H curve
to be used, thus maximising the power which can be handled for a given core volume.
By producing a relatively large change in B as it decreases, the capability of the
core to pass longer pulse widths is enhanced. To reset the transformer core 2, an
additional auxiliary winding 10 is required which is connected to a power supply 11.
After a pulse has been transmitted through the primary winding 3, producing magnetisation,
a pulse is transmitted through the auxiliary winding 10. This causes a magnetic field
to be applied which opposes the effects produced by the pulse through the primary
winding 3, resetting the transformer core.
[0004] The present invention seeks to provide an improved circuit arrangement which includes
a transformer core.
[0005] According to a first aspect of the invention there is provided a circuit arrangement
comprising a transformer having a substantially toroidal core and a winding about
the core which comprises two wires, one being wound in the opposite sense to the other
and at least some of the turns of one wire being wound on a different part of the
core to those of the other. The core is typically, but not necessarily of circular
cross-section. The use of such a winding enhances the operation of the transformer
as it enables a greater change in the B field to be achieved. It is believed that
this is because each of the wires produces a magnetic field within the core material,
rather than the surface effect produced by the conventional bifilar winding, as at
least some of the turns of one wire are spaced apart from those of the other. The
fields produced are equal and opposite and thus completely cancel one another. The
dipole elements of the core material are acted on by the fields which, although their
resultant force is zero, cause the mobility of the dipole elements to be increased
as they are in a state of dynamic equilibrium. Due to microscopic variations in dipole
strength, size and orientation, changes in the applied field produce a faster response.
This enables the core to pass longer pulse widths than is possible using a conventional
arrangement.
[0006] The winding may be arranged such that only part of each of the wires is spaced from
the other, but preferably each wire is wound on a different part, so there is no overlap
between them. The winding may be separate from others on the core but in a particularly
advantageous embodiment, the winding is a secondary winding across which a load is
connected.
[0007] According to a second aspect of the invention there is provided a circuit arrangement
comprising a transformer having a secondary winding across which a load is connected
and means arranged to pass a current through the secondary winding to reset the transformer
core after transmission of current through its primary winding. By the term "resetting"
it is meant that the flux density is reduced from what it would otherwise be, and
not necessarily only that negative saturation or remanence is achieved. By arranging
that the secondary winding is used in resetting the transformer core, the additional
power supply, auxiliary winding and other circuit elements needed in a conventional
arrangement are not required. The invention is particularly applicable to arrangements
in which the load is a magnetron, the current used to reset the core also being the
heater current for the magnetron. This may be achieved by employing a secondary winding
which comprises two windings which are arranged adjacent one another and wound in
opposite senses.
[0008] One way in which the invention may be performed is now described by way of example
with reference to Figure 3 of the accompanying drawings which schematically illustrates
a circuit arrangement in accordance with the invention.
[0009] With reference to Figure 3, a circuit arrangement in accordance with the invention
includes a transformer 12 having a transformer core 13 about which are wound primary
and secondary windings 14 and 15. The primary winding 14 is connected to a d.c. charging
circuit 16 similar to that shown in the circuit arrangement of Figure 1. The secondary
winding 17 is connected to a load, which in this embodiment is a magnetron. The secondary
winding comprises two wires 18 and 19 which are arranged adjacent one another and
wound in opposite senses. The secondary winding 17 includes four terminals 20, 21,
22 and 23. The magnetron heater element is connected across two of the terminals 21
and 22 which are arranged between the two parts 18 and 19 of the secondary winding
17. The terminal 21 is also connected to the magnetron cathode. The terminal 20 is
the input terminal of the heater supply to which the heater current is applied and
the terminal 23 is connected to earth.
1. A circuit arrangement comprising a transformer having a substantially toroidal
core and a winding about the core which comprises two wires, one being wound in the
opposite sense to the other and at least some of the turns of one wire being wound
on a different part of the core to those of the other.
2. An arrangement as claimed in claim 1 wherein one wire is wound on one part of the
core and the other on another part of the core.
3. An arrangement as claimed in claim 1 or 2 and including means for passing a direct
current through the winding.
4. An arrangement as claimed in claim 1, 2 or 3 wherein the winding is a secondary
winding across which a load is connected.
5. A circuit arrangement comprising a transformer having a secondary winding across
which a load is connected and means arranged to pass a current through the secondary
winding to reset the transformer core after transmission of current through its primary
winding.
6. An arrangement as claimed in claim 5 wherein the secondary winding comprises two
windings which are arranged adjacent one another and which are wound in opposite senses.
7. An arrangement as claimed in claim 4, 5 or 6 wherein the load is a magnetron and
current passed through the secondary winding is heater current for the magnetron.
