A bifilar coil is an electromagnetic coil that contains two closely spaced, parallel windings. In engineering, the word bifilar describes wire which is made of two filaments or strands. It is commonly used to denote special types of winding wire for transformers. Wire can be purchased in bifilar form, usually as different colored enamelled wire bonded together. For three strands, the term trifilar coil is used. Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... A coil is a series of loops. ... Engineering is the design, analysis, and/or construction of works for practical purposes. ... A wire is a single, usually cylindrical, elongated strand of drawn metal. ... Filaments surrounding a solar flare, caused by the interaction of the plasma in the Suns atmopshere with its magnetic field. ... Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. ... Three-phase pole-mounted step-down transformer. ...

Description and applications

There are four types of bifilar-wound coils:

1. parallel-wound, series connected
2. parallel-wound, parallel connected
3. counter-wound caduceus coil, series connected
4. counter-wound caduceus coil, parallel connected

Some bifilars have adjacent coils in which the convolutions are arranged so that the potential difference is magnified (i.e., the current flows in same parallel direction). The magnetic field created by one winding is multiplied with that created by the other, resulting in a greater net magnetic field. Others are wound so that the current flows in opposite directions. The magnetic field created by one winding is therefore equal and opposite to that created by the other, resulting in a net magnetic field of zero (i.e., neutralizing any negative effects in the coil). In electrical terms, this means that the self-inductance of the coil is zero. The Caduceus Two caduceuses without wings as decoration of door portal in Ztracená street in Olomouc (Czech Republic). ... Potential difference is a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. ... Parallel is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. ... To meet Wikipedias quality standards, this article may require cleanup. ... Inductance is a physical characteristic of an inductor, which is an electrical device that produces at any time a voltage proportional to the instantaneous rate of change in current flowing through it. ...

The bifilar coil (more often called the bifilar winding) is used in modern electrical engineering as a means of constructing wire-wound resistors with negligible parasitic self-inductance. Resistor symbols (US and Japan) Resistor symbols (Europe, IEC) A pack of resistors A resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohms law. ...

A different type of bifilar coil is used in some relay windings and transformers used for a switched-mode power supply to suppress back-emf. In this case, the two wire coils are closely spaced and wound in parallel but are electrically isolated from each other. The primary coil is driven to operate the relay, and the secondary coil is short-circuited inside the case. When the current through the primary is interrupted, as happens when the relay is switched off, most of the magnetic energy is intercepted by the secondary coil which converts it to heat in its internal resistance. This is only one of several methods of absorbing the energy from the primary coil before it can damage the device (usually a vulnerable semiconductor) that drives the relay. The main disadvantage of this method is that it greatly increases the switching time of the relay. Automotive style miniature relay A relay is an electrical switch that opens and closes under control of another electrical circuit. ... Three-phase pole-mounted step-down transformer. ... A switched-mode power supply, switch-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches power transistors (such as MOSFETs) rapidly on and off in order to stabilize the output voltage or current. ... In physics, heat, symbolized by Q, is defined as energy in transit. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ...

When used in a switching transformer, one winding of the bifilar coil is used as a means of removing the energy stored in the stray magnetic flux which fails to link the primary coil to the secondary coil of the transformer. Because of their proximity, the wires of the bifilar coil both "see" the same stray magnetic flux. One wire is clamped to ground usually by a diode so that when the other "primary" wire of the bifilar coil no longer has a voltage applied across it by the switching transistor, the stray magnetic flux generates a current in the clamping coil with the primary side voltage appearing across it, causing an equal voltage to appear across the primary winding. If this clamping coil was not used, the stray magnetic flux would attempt to force a current to flow through the primary wire. Since the primary wire is switched off and the switching transistor is in a high resistance state, the high voltage which would appear on the semiconductor switching transistor would exceed its electrical breakdown or even damage it. Three-phase pole-mounted step-down transformer. ... Magnetic flux, is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. ... The word ground has several meanings: The surface of the Earth Soil, a mixture of sand and organic material present on the surface of the Earth Ground (electricity), in electrical engineering, something that is connected to the Earth or at the voltage defined as zero (in the US, called ground... Types of diodes closeup, showing silicon crystal In electronics, a diode is a component that restricts the direction of movement of charge carriers. ... Assorted transistors A transistor is a semiconductor device that uses a small amount of voltage or electrical current to control a larger change in voltage or current. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ... Breakdown may refer to: Electrical breakdown Chemical breakdown Biological decomposition Nervous breakdown Societal breakdown A breakdown is a statement explaining the details of something such as a bill or cost of a plan A script breakdown for a play, film, or comic book Breakdown (album), by Melissa Etheridge. ...

History

An early example of the bifilar coil can be seen in Nikola Tesla's United States patent 512,340 of 1894. Tesla explains that in some applications (which he does not specify) the self-inductance of a conventional coil is undesired and has to be neutralised by adding external capacitors. The bifilar coil in this configuration has increased self-capacitance, thereby saving the cost of the capacitors. It is notable that this is not the kind of bifilar winding used in non-inductive wirewound resistors where the windings are wired anti-series to null out self-inductance. Nikola Tesla (1856-1943)[1] was a world-renowned Serbian inventor, physicist, mechanical engineer and electrical engineer. ... 1894 (MDCCCXCIV) was a common year starting on Monday (see link for calendar). ... Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. ...

See also

Trifilar coil

References

• Tesla's patent
• Bifilar relay coils

Patents

• Nikola Tesla - U.S. Patent 512340  - Coil for Electro-Magnets - 1893, July 7 - Bifilar coil winding technique