Tuesday, November 6, 2012

Wireless Power Transmission

After I got a hold of an Braun Electric toothbrush, the concept of inductive power coupling has intrigued me. The charger connection is completely insulated. The toothbrush has a hole on the bottom that mates with a small cylinder sticking out of the charger. (PICTURE)

To check out the effect I took a coil of wire wrapped around a plastic spool and set it on top of the charger. I obtained approximately 1.5V open circuit and 500mA short circuit. I couldn't get too near the cylinder so the coupling wasn't the best. (PICTURE)

Next, I cut a 120V to 12V E shaped core "filament transformer" about the size of my fist in half, to produce two separated E shaped iron cores. Cutting through all the copper windings, paper and iron core made a real mess, took a lot of time to cut and dulled my hack saw. I used a vice to keep the core slices from separating.

I next wound the core with about 200 wraps of transformer wire I got at Radio Shack (I used the entire spool however many wraps that was). Clear plastic tape was used to keep the windings in place. I used a small piece of 220 grit sandpaper to take the enamel off the ends of the wire. I checked the inductance with my wavetek multimeter and it showed .3 mH. The resistance showed 3.3 ohm. The reason I am worried about this is that I don't have extra 20A fuses for my meter and I want to get an estimate of the quiescent current of my transformer without blowing up my circuit breaker, my meter and myself.

A little math (determine impedance from resistance and inductance): V = I Z => I = V / Z

Z =R + jwL =R + j * 2 * pi * f * L =R + j * 6.28 * 60 * .0003 =3.3 + .11304j

I = 120 / (3.3 + .11304j) = 40A

This doesn't look good for my meter fuse or my breaker!

After further thought about the way a transformer works and electric versus magnetic circuits I determined the following.

The small quiescent current of a power transformer is a function of the reverse magnetic field produced by the voltage impressed on the secondary windings and not on the inherent impedance of the coil. When the magnetic field is broken (with an air gap) in a transformer it behaves like a shorted secondary and draws potentially lots and lots of current.

For this to really work well, a higher frequency should be used. I'm curious what my toothbrush uses.

Breakthrough discovery! I just hooked up my handy dandy yet old as dirt Freq Counter to that inductive coil and viola! 21 kHz and change.

I need a higher frequency source than 60Hz. I have a function generator, but it tops out under a single Watt power output and has a 50 ohm output impedance. Higher impedance means more coil wraps... no good. I discovered that an old audio amp I have laying around will put out 50W up to 70kHz at 8 ohm output impedance. I think I will choose 20 kHz however. Higher frequencies may cause extra loss in the rectification process later. The coil inductance that I need to produce more than 8 ohms of impedance at 20 kHz. More Math: Z = j*2*pi*f*L Z = 8j ohms L = Z/(2*pi*f) f = frequency = 20,000 Hz pi = 3.14 L = 64uH This is definitely more reasonable. In fact I may try and wind this around a plastic core.

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