Inductive chargers are becoming more popular
for phones, watches, headphones, and other
devices — they’re everywhere! I look forward
to imagining a completely wireless future, where
power moves through the air like Wi-Fi and radio.
But for now, I’ll show you how to use induction to
make some fashionable wireless accessories!
HOW DOES IT WORK?
At its core, wireless charging uses the magnetic
field created by every electrical wire to transfer
power between coils. This is possible because
of Faraday’s law of electromagnetic induction,
which tells us that an electrical force is produced
across an electrical conductor (a wire) in a
changing magnetic field. Faraday’s theory
is also responsible for important things like
transformers, electric motors, and generators.
Every wire with an electrical force produces a
magnetic field with a north and south pole, which
can be further described with Lenz’s law, which
tells us how we can find the magnetic polarity of
every electrical field. As shown in Figure
A
, you
can predict the flow of the magnetic field by
pointing your right thumb in the direction of the
current; your fingers will then follow the flow of
the magnetic field! It sounds wild, but I literally
mean that every wire with electricity in it
produces a magnetic field, which can be used to
make things like motors, solenoids, and loads of
other tech.
Because every individual wire’s field is very
small, adding many wires in a coil multiplies
the strength of the force. The shape of the coil,
as we’ll explore, affects the shape of the field.
Figure
B
shows a spring coil, but they can be flat,
circular, helical, and many other shapes.
There are two coils called inductors in our
wireless power circuit, one in the transmitter
(the device providing power) and the other in
the receiver (the device being powered). When a
current passes through a coil in the transmitter
— the charging station, for example — the moving
electric charge creates a magnetic field, which
passes through the opposite coil in the receiver,
inducing a current in the receiver coil which in
turn passes through a rectifier and goes on to
charge your battery or power your device.
Inductors are typically made of enamel-coated
A
copper wire, aka magnet wire, because copper has
low resistance and the enamel insulation ensures
the current passes through the entire spiral in the
coil, and doesn’t short-circuit across it.
COUPLING INDUCTORS
There are a few factors that govern how far you
can hold your transmitter coil from your receiver
coil, as well as the power capacity for your coil.
Inductors transfer power by coupling, meaning
the two electromagnetic fields mesh together.
Playing with the parameters below can help you
achieve this more efficiently. Electromagnetics
and inductive circuitry are a whole field of study,
so I’ll just give you the basics here that can help
you get started.
• Inductor size and shapehave a direct impact
on coupling. For the best results, as many
lines of the magnetic field produced by the
transmitter coil must intersect directly with the
receiver coil. Pairing coils of the same shape
will ensure the best coupling (Figure
C
).
• Distance is also an important factor in power
transfer. As the coils move apart, the inductive
coupling is rapidly reduced; efficiency of90%
or more can only be achieved if the distance-
to-coil-diameter ratio is less than about 0.1,
otherwise the efficiency falls rapidly. However,
this can produce some interesting effects if
you want your lights to fade!
• LC circuits, named for their components
— an inductor marked L, and a capacitor
marked C — can be used to create a more
efficient coupling circuit (Figure
D
). Tuning
the frequency of an LC circuit can create
C D
L
v
+
-
i
C
B
Lee Wilkins, HyperPhysics, Takehiro Imura/Springer Nature
71
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