Bifilar coil
Encyclopedia
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
, the word bifilar describes wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
which is made of two filaments or strands. It is commonly used to denote special types of winding wire
Enameled wire
Enamelled wire is wire coated with a very thin insulating layer. It is used in applications such as winding electric motor coils, speakers and transformers. It is also used in the construction of electromagnets and inductors....
for transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s. Wire can be purchased in bifilar form, usually as different colored enameled wire bonded together. For three strands, the term trifilar coil is used.
Description and applications
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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
Parallel (geometry)
Parallelism 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. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...
direction). 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 bifilar coil (more often called the bifilar winding) is used in modern electrical engineering as a means of constructing wire-wound resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s with negligible parasitic self-inductance.
A different type of bifilar coil is used in some relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
windings and transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s used for a switched-mode power supply
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
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
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
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
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
) that drives the relay. The main disadvantage of this method is that it greatly increases the switching time of the relay.
When used in a switching transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, one winding of the bifilar coil is used as a means of removing the energy stored in the stray magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
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
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
usually by a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
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
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
is in a high resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
state, the high voltage which would appear on the semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
switching transistor would exceed its electrical breakdown
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
or even damage it.
Bifilar coils are claimed by advocates of "scalar waves" to generate said waves.
History
An early example of the bifilar coil can be seen in Nikola TeslaNikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...
'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 capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s. 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.
Patents
- Nikola Tesla - - Coil for Electro-Magnets - 1893, July 7 - Bifilar coil winding technique