■ FIGURE 1. Inductive charger schematic with test points.
A wireless charging system needs to contain the
following circuit elements:
• Any type of oscillator capable of producing the
The schematic shown in Figure 1 is an example
system with test points for troubleshooting possible
problems, plus the meter placement that is necessary to
calculate power efficiency.
Building The Circuit
Before you can fully test the operation of the
transmitter and receiver circuits, you will need to construct
a set of coils.
(also known as "wireless
charging") uses an
electromagnetic field to
transfer energy between
two objects. This is usually
done with a charging
station. Energy is sent
through an inductive
coupling to an electrical
device, which can then use that energy to charge batteries or
run the device.
Induction chargers typically use an induction coil to create
an alternating electromagnetic field from within a charging base
station, and a second induction coil in the portable device takes
power from the electromagnetic field and converts it back into
electrical current to charge the battery. The two induction coils
in proximity combine to form an electrical transformer.
Greater distances between sender and receiver coils can
be achieved when the inductive charging system uses resonant
inductive coupling. Courtesy of Wikipedia.
Creating The Coils
If you are going to design your own coils, try
experimenting with varying diameters of wire, coil
geometries, and different coil sizes. The following is a
description of a coil design technique that is the
culmination and the distillation of many moons of effort in
the application of a single method.
The coil construction can be the most difficult part of
this project. The suggested coils for this project are a flat
pancake style that is reminiscent of the old Tesla primary
coil design. They can be almost impossible to fabricate
without a specific technique. I have tried numerous ways
to create these coils; the method I discuss here provides
the most consistent results.
You will need two acrylic blocks per coil. The blocks
should be of a thickness that makes it difficult to deform
them. I find that about 1/4 ” thick acrylic is fairly rigid
under stress. You can find prefab blocks at most well
stocked craft stores; they are typically used for making
stamping tools. I found the ones I used at Michaels craft
supply, but they can be ordered from various places
The only problem with prefab blocks is the lack of
variety of dimension. The blocks I used are 2. 5” square
which works fine given the dimension of the circuits that I
would like to make wirelessly rechargeable. For the
transmitter and receiver coil, you will need two sets of
block configurations illustrated in Figure 2.
■ FIGURE 2. Winding jigs for transmitter and receiver coils.
August 2013 37