■ FIGURE 7.
project that I converted to be wirelessly
rechargeable. I took an existing project
that is an 8 x 8 LED matrix Pong game that
is powered by a lithium-polymer battery
source. The game has a footprint of
3” x 2” with a battery supply on the back
of the board. I mounted the receiver coil
on a board with the same dimension as
the game, leaving plenty of room for the
electronics in the receiver.
I wanted to keep the receiver card as
thin as possible to not add much more
depth to the existing project. Figure 7 is a
photo of the charging receiver attached to
this project design that I want to charge
The entire receiver board adds only an
additional 1/4” to the depth of the project.
A single IC battery charging manager
shown in Figure 8 is connected to the
output of the 5V regulator. This chip
(manufactured by Maxim Integrated)
requires just a few external components,
and will manage the charging for a single
cell lithium battery. The MAX1811 has an
LED that indicates when charging is
I have been able to get a nominal
service life of approximately 400 charges
with this device. I even use it to charge my super
CHARACTERIZING POWER OUTPUT & EFFICIENCY
FDH055N15A — N-Channel Power Trench MOSFET 150V, 167A, 5. 9 mΩ
COIL DIAMETER = 2. 5 | APERATURE = 1” | SEPARATION = 0.25”
FREQUENCY = 12. 9 kHz DUTY CYCLE = 50%
Input Voltage = 12V Output Voltage 5.06V (31V unregulated)
Voltage Drop across 10 ohm load shorted = 0.710V ( I = E / R ) 710 mA
Input = 900 mA Output = 710 mA Efficiency = 710 mA / 900 mA 100 = 78%
sure to use an adequate wattage resistor for the dummy
load. By generating current values close to one amp, you
will need a resistor rated at 5W.
Testing Your Circuit
When breadboarding certain power transistors, it may
be necessary to attach smaller diameter wires to the leads
in order to plug into your breadboard. You will also need
a way to intercept the (+) lead from your power supply to
attach the ammeter.
Connect the coils to your breadboard circuits and
attach the meters as shown back in Figure 1. Place the
receiver coil over the transmitter coil, separating them
with one of the acrylic blocks to act as an insulator. Apply
power to the transmitter circuit and record the values from
both meters. Close SW1 to short the dummy load across
the output of the regulator.
You should notice the input current value rise due to
the short being reflected back to the primary. You may
need to heatsink your power transistor. If it becomes
excessively hot at resonance, you need to check your
work. Try the suggestions given in the Troubleshooting
It’s very simple to add a recharging receiver to your
projects. The following is an example of a battery powered
40 August 2013
This circuit was intentionally designed to be simple, so
troubleshooting should be correspondingly easy. The
following are voltages that should be present at the
various test points shown in the schematic from Figure 1.
1) There should be 5V at TEST POINT B (check the
12V supply voltage if not 5V).
2) There should be approximately 2.5V at TEST POINT
A (check the 08M2 power supply or the code).
3) There should be a minimum of 6V at TEST POINT
C (check the rectifier or AC across the coil). Test the
regulator by connecting the 12V supply to the input
4) You should have 5V at TEST POINT D (check the
5) There should be 12V AC or greater at TEST POINT
E (check the coil connection if the test points in listings 1
and 2 are okay).
6) You should have an AC value at TEST POINT F
(check the secondary coil connection if the test point in
listing 5 is okay).