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.
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.
Receiver Circuit Construction and Testing
With the exciter circuit working, I then shifted my focus to the power collection coil. I again pounded nails into a wood board. I spaced the four nails 1.75 inches by 3 inches. I wound 10 turns of 24 ga wire around the nail. When done, I measured the inductance at 20uH. I then connected a 0.082uF capacitor across the coil and also installed a 47 ohm 1 watt load resistor. I then connected the scope probe across the 20uH coil leads and placed the coil at the center of the exciter coil. I connected two ends of a wire to my signal generator and held the wire close to the coil while I varied the frequency. The resonant peak was quite close to 125KHz. I reconnected my signal generator to the driver circuit I fired up the exciter circuit. I adjusted the exciter frequency slightly, for a peak across the receiver coil. I measured 12 volts peak to peak across the 47 ohm resistor. This corresponded to 0.4 watts into the resistor. If I moved the small coil, so its corner matched the corner of the exciter coil, the voltage increased by 20% to about 15 volts peak to peak. That shifted the power to 0.6 watts. So, with the receiver coil in the center of the exciter coil, the +5v supply measured 0.34 amps (1.7 watts) while delivery 0.4 watts to the receiver coil. This corresponded to an efficiency of about 24%. If I use a better FET driver circuit and wind the exciter coil from 22 gage wire, I should be able to get fairly close to the 1 watt power transfer I was shooting for. I think with a few changes, the overall efficiency might get closer to 50%.
The next step would be to connect the receiver coil to a bridge rectifier circuit, with a filter capacitor and a load resistor to see what kind of DC voltage I can produce. Then I’ll figure out the best way to regulate the DC output to about 5v, to charge the cell phone’s lithium ion battery.
I might also consider ways to manage the power to the exciter coil. With no receiver coil nearby, the Q of the exciter turned network will be high and the average power will also be high. Paradoxically, more power is drawn from the DC supply when there is no receiver coil in the center of the exciter coil. I have a patent on a technique to detect a nearby tuned network. Perhaps I could use a circuit similar to it to keep the average power to the exciter circuit low, when there is no battery to be charged.