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Wireless Battery Charger
By: Dave Johnson    July 24, 2007

Exciter Coil and Driver Circuit Construction

I pounded a few nails in a wood board, spaced 6.5 inches x 7.5 inches.  I then wound 20 turns of some good quality 24ga insulated wire around the nails. When I got done, the inductance measured 220uH.  I found a nice set of high voltage mica capacitors to form a value of 0.007uF and soldered them in series with the coil.  I then built the driver circuit shown below.  I attached a small copper strip to the two power FETs, to act as a heat sink.  Since the drain of both devices connects to the same point, I could solder their tabs directly to the copper plate. Note that I also installed a high-voltage voltage divider network, so I could safely measure the voltage across the coil.  If you build this circuit, don’t connect your scope probe directly to the coil.  The voltage is high enough; that it could destroy your probe and might even damage your scope.  Also be careful not to touch the coil when the circuit is running. You could suffer a nasty RF burn.  I made this mistake a while back, when I built a 75 watt 125KHz exciter.  Those RF burns hurt like hell.


                    Exciter Coil with Receiver Coil

Exciter Coil Driver Circuit

Exciter Coil Driver Schematic

With everything wired up, I setup my square wave signal generator for a nice 12v peak signal and connected it to the circuit’s input.  When all was ready, I turned on the +5v supply and adjusted the signal generator frequency up and down, until I saw a peak on the coil voltage test point with my scope.  The frequency was pretty close to 125KHz.  The voltage measured about 400 volts peak to peak.  The meter on my +5v supply said the average current was about 0.5 Amps.  I was hoping for more but this was not bad for the first test.  Although the DC resistance of the some 50 feet of the 24 gage wire making the coil was only about 1 ohm, the actual impedance of the wire due to “skin effect” was much higher.  There might be some tables somewhere on the Internet, which could tell me what the high frequency resistance of the wire might be, but I prefer to find out by experiment.  I’m confident that by going to 22 ga wire, I could gain some more power.  Right now, this was an acceptable first attempt. 

To test the efficiency of the driver circuit, I moved the frequency well off the resonant point and measured the +5v current draw.  It measured 0.08 Amps. This meant that about 0.4 watts was being dissipated in my push-pull transistor driver circuit.  I can later use a more efficient driver and get this figure down to a lower value.

125KHz Driver Circuit


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