BUILD IT YOURSELF
By Matthew Bates
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As an avid hobbyist, I would like to have a convenient way to recharge my battery powered
projects without having to tie up the USB ports on my computer. Borrowing from the concept
of the wireless chargers on the market, I decided to build my own. So if you like the idea of
having a wireless replacement for your USB port, open up your surplus parts drawer and let's
start the induction process.
How Does Inductive
Wikipedia defines resonant inductive coupling as “the
near field wireless transmission of energy between two
coils that are tuned to resonate at the same frequency.”
The formula for calculating the resonant frequency is:
ƒr = 1/( 2*pi*√(LC))
You can use a meter to determine inductance, but not
for the distributed capacitance that accumulates between
windings. You can use the following formula to determine
the self-capacitance or the interwinding capacitance:
36 August 2013
C = .29L + 0.41R + 1.94[ √ (R^3/L) ]
C = Self capacitance in picofarads
R = Radius of coil in inches
L = Length of coil in inches
The prototype coil for this project was wound using
some surplus wire I had left over from a previous project.
The size of the coil was based on a dimension that was
indicative of most of my average size projects. The coil
was a flat, single layer spiral coil created with 26 AWG
enameled magnet wire that had a 1” inner diameter and
an outer diameter of 2. 5”.