Michael Ang

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Blue Flower – Charger Hacking and Coil Construction

Electronics, Hacking, Living Art, School April 12th, 2006

Inductive charger hackingInductive charger hackingInductive charger hackingInductive charger hacking

My blue flower will use inductive charging. I took apart an Oral-B CrossAction Power Max toothbrush ($18 at K-Mart) to see how it works and see if it would be useable in my project.

Charger hacking and coil construction pics

The toothbrush side is remarkably simple. There is a coil attached directly to a battery (no rectifier) and a switch which physically connects and disconnects the battery from the motor assembly. The motor appears to be similar to that used in a pager. The attachment on the shaft has an inverted cone offset from the axis of the motor that loosely connects to a metal rod that goes up to the toothbrush head. The design is very elegant, using a minimum of parts and materials.

The base is completely filled with an epoxy or rubber (finding epoxy is the hacker’s worst nightmare!) There is a circuit board that I didn’t free from the epoxy and coils of magnet wire. The magnet wire is formed into stranded wire and then coiled (as opposed to a simple bobbin of magnet wire). The base seems more or less useless for hacking because of all the epoxy. Interesting to see that the shape of the coil is pretty loose.

For my own inductive charger experiment I used 22ga magnet wire on a spool as the primary and 30ga magnet wire coiled around a screw as the secondary (in a transformer, power is transferred from primary to secondary). Using about 0.9 amps at 8V and 5.4kHz I could get an LED to light at 2.6V. There seemed to be a voltage peak at that frequency with this arrangement. The primary had an audible whine, which isn’t too surprising. Now the trick is to figure out the most suitable combination of coil geometries, drive frequency, drive voltage, and core type to get the power I need.

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