- A bifilar coil contains two closely spaced, parallel windings that can be used to either magnify or neutralize magnetic fields. When the currents flow in the same direction, the magnetic fields add up, but when they flow in opposite directions the fields cancel out. - Bifilar coils are used in some relay and transformer applications to suppress back-emf by intercepting magnetic energy through an isolated secondary coil. However, this increases switching time. - Bifilar coils impose inductance in common mode but not differential mode, making them useful for eliminating common mode signals in electronic circuits like Ethernet cables.

1. Coils:
• A wire coil can be used to concentrate magnetic flux in a controlled
area. The most common form of coil, known as a solenoid, can be a
single loop of wire or multiple turns in series. The diagram on the left
represents the magnetic field in a typical solenoid.
• By winding multiple turns in the same direction its possible to
increase the total magnetic field strength. A typical solenoid posses a
property known as inductance which creates a resistance to the
change of flow of current in the wire.

3. Bifilar coil
• 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.

5. Description and applications:
• 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). 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.

6. • 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.

7. • Bifilar coils impose an inductance in the common mode, but impose
no inductance in the differential mode. Coils in such a combination
are widely used to eliminate ingress or egress of common mode
signals from electronic signalling circuits. This arrangement is used in
transmission and reception magnetics of Ethernet cables[2] and
conspicuously in the form of a ferrite bead clamped to the outside of
USB, laptop power supply and HDMI cables.

8. Generation of voltage in Coil:
• When a magnetic field penetrates a coil, the induced rotating electric
field generates additive elementary voltages along the coil. Every turn
in the coil receives the same voltage, we extract the power along the
length of the coil, but we miss the power available in the area inside
the coil !! To understand this, let’s examine the induced electric field:

11. • If we examine above Fig, which represents a single turn of the coil
shown in Fig.1, we see that the magnetic field penetrates all of the
area inside that ring. This magnetic field will be transformed to a
voltage because it will drive a rotating electric field, this field is able to
induce a voltage and this induced voltage will cause the current to
flow due to the different of voltage between the two points A and B
in Fig.1

12. The important, generally forgotten thing about the induced electric field,
is its availability inside the coil as shown in following Fig:

13. • The induced electric field exists independent of the conducting loop.
In other words, an induced electric field permeates all of the space
within the region of the changing magnetic field, as indicated by the
red field lines in above Fig. What about this field? It is wasted power.
It is wasted power at point A, as well as all of the distance between
the two points A and B. If we want to achieve power amplification we
have to combine the magnetic field with the induced electric field in
such a way as to conserve the non-conservative electric field! The
induced rotating electric field will remain non-conservative but we
could play with the induced voltage created by that field using a Tesla
Bifilar Coil (“TBC”).

14. We need to extend the capacitive side of a Tesla bi-filar coil in order to
benefit from the induced E field to a reasonable degree.