About this blog

I presented my project in May and will not be adding new posts. The information contained in this blog provides an overview of how a wireless power system may be built. There are posts describing some of the different ways I approached the problem and the results of those attempts. Interested readers may also find the relevant links I have posted useful for further information.

I did not make much of an attempt to clarify my posts as I was writing them because I primarily used this blog to organize my research for my own use. That being the case, please feel free to email if you have any questions.

Enjoy!

Final Implementation

The bulk of my time on this project was spent trying to build a transmitter. My assumption was that if I could simply generate a large enough signal, harnessing it would be the easy part. Unfortunately, I was not able to build an adequate transmitter and with the semester’s end rapidly approaching I chose to concentrate on building a working proof of concept model. In the end, my power source was a Sonicare toothbrush base. I built a demonstration model that consists of:

Receiving Coil
Rectification State
Power Storage Circuit
Mechanical Stage (demonstration)

The receiving coil contains two levels of windings of 24 AWG magnet wire. The lower level (closer to transmitter) has 50 windings and the upper has 10 windings. This coil has an alternating current induced in it that must be rectified. I used a classic four diode H-Bridge configuration to achieve rectification. The power level at this point is quite low, hence the necessity of the power storage circuit.

The circuit I used is a modified version of a circuit designed by Mark Tilden for use in low power robotics. It is called a Type 1 Solar Engine because it was intended for use in solar powered applications. The basic operation of this circuit is to charge a 4700uF capacitor from a low power source. When the voltage across the terminals of the capacitors exceeds a predetermined threshold set by zener diodes the circuit switches from charging to discharging.

In this application, the capacitor charges on the secondary coil and discharges into a small micro-cassette recorder motor. The cycle takes approximately one second creating a clock-like movement as the motor is pulsed. Finally, through a series of gears and a rubber pulley belt, a small image is rotated.

Variations on Final Build

My first challenge was to build an oscillator. The oscillator is an essential feature of the power source. I intended to duplicate the Sonicare system and then make modifications to scale up the power output. Of course, there are numerous options when choosing an oscillator. My first choice was a Colpitts oscillator.

I chose the Colpitts because it easy to assemble from discrete components, inexpensive and simple to troubleshoot should problems arise once the circuit became more complex. A final benefit of the Colpitts is that it can maintain oscillations into the megahertz if necessary.

I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental.
At times it was difficult to initiate oscillation. Professor Joseph Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a soldered circuit board. When the Colpitts worked, the oscillations were stable and I measured 400mVPP on the output.

The next step was be to amplify this signal and drive a transmission coil. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely. I attempted to use multiple amplification stages of different designs. Unfortunately, none of them were effective. It seemed that any loading whatsoever on the output of the Colpitts made it collapse. I decided to abandon the Colpitts.

For the sake of brevity I will not go into as much detail about the subsequent design alterations. My next permutation was to use a 555 timer IC. The out put was very stable but not sinusoidal. I considered adding a low pass filter to the output to correct this. For the sake of simplicity, I replaced the 555 with a Wein-Bridge oscillator

Now that I had a stable oscillator, I focused my attention on an amplification stage. There were many variations. Ideally there would be two stages: voltage gain and subsequent current gain. I attempted this with FETs, BJTs, a combination of both and finally with Op-Amps. I was never able to generate enough gain to drive a significant amount of current into my primary coil.

Rectification & Voltage Regulation

Rectification Overview: Will implement a full wave rectifier with Shottky Diodes (0.3V drop instead of 0.7v with standard diode). Since output will not be constant over time will need some sort of voltage regulator. Cannot use a transistorized regulator, maybe a shunt regulator?

Basically, when the current through the Zener is sufficient to take it into the breakdown region, it tends to regulate the voltage across it’s terminals.

Colpitts & Series Resonance

With the help of Prof. Pozzi I was able to observe clear indication of series resonance using my .003mH coil. At 9.1Mhz this coil and a 1pF capacitor were used. The resonant bandwidth of +-5% Vmax was very narrow with Vmax falling at +- 200kHz. Colpitts Oscillator – I would like to be able to build an oscillator for this project. I chose the Colpitts because it seemed fairly simple.

I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental. At times it was difficult to initiate oscillation. Prof. Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a circuit board.

When the Colpitts worked, the oscillations were stable and I measured 400mVPP. The next step would be to amplify this signal. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely at which point I gave up for the night.

3/17/08

Colpitts testing again - Was able build stable, functioning oscillator. I must’ve misplaced a capacitor during the last trial because the Colpitts worked perfectly this time. I attempted to slow down the oscillator using larger capacitors but was unable to attain kHz oscillations. Not sure why. Also, I added a capacitor in parallel with C3 (see above) with resulted in approximately 75% increase in amplitude at the same frequency. The observed oscillation amplitude was 2Vpp-3.5Vpp with a source of 10VDC-30VDC.

Colpitts Oscillator with loosely wound air-core coil – I connected one end of the transmission coil to the node between L1 and C2. The other end was free. With fo of 8.54MHz and Afo of 2Vpp I observed 1Vpp on the secondary coil. I was unable to observe resonance at this frequency since the capacitance required to match the 0.004mH inductor was 9E-11F which was unavailable to me (and might not exist).

Conclusion – I need to slow the Colpitts to a frequency that is more useful.

Disassembly of Sonicare Electric Toothbrush

Secondary coil of toothbrush has two levels of coils, the second having a larger diameter than the first. One possibility for this coil design is to have the coil widen as the magnetic field widens - since this device operates in a fixed position when charging. The voltage measured across the the coil when in the charging position was in the hundreds of millivolts. The voltage measured across the battery charging terminals was approximately 2.21V. This would indicate that the secondary coil is not self resonant and requires support reactances for the circuit to achieve resonance. I was unable to disassemble the base, charging unit (transmitter) because it is encased in epoxy. I determined that the base unit oscillates at 80kHz.

Charging cell phone with Sonicare toothbrush –

measurements: V_term=2.21V, I_term =0.165mA

Resonance Review 11/9/07

XL = XC, fres=(2*pi*sqrt(LC))^-1, resonance effected by resistance (shifts fres). Skin effect in inductor needs to be considered. Should use series RLC circuit to reduce impact of resistance. Not sure if a high Q circuit would be beneficial.



At resonance the series resonant circuit appears purely resistive. Below resonance it looks capacitive. Above resonance it appears inductive.



Impedance is at a minimum at resonance in a series resonant circuit.

Source: www.allaboutcircuits.com - 11.13.2007