Sunday was a big day! Jamison and I started with another trip to Fry’s. We needed more magnet wire, a better power supply to clean up the noise we saw on the cheap one we already had, and some perfboard. On the way home, we stopped by Home Depot for 6″ pipe and Fred Meyer’s for reading glasses. Never needed those before… 8-)
Upon our return home, we set up our lab and pulled out the new power supply. After deciding to use a different scope probe and calibrating it (we missed that step last time), we got a nice, clean 5V voltage. Because we wanted to see the contrast, we then connected the probe to the cheap “wall wart” power supply. Look at that — a nice, clean 5V voltage! Huh. I guess the new power supply can go back and I’ll try to be more understanding when a customer calls for tool assistance and hasn’t calibrated his scope probes.
Jamison wrapped a second coil and we hooked it up to a green LED. Connect the power, move the coils close together. First light! Yes!!
Now it was time to improve Jamison’s project. We tried a full-wave rectifier to increase the LED’s brightness, with not a large (if any) effect. We built a two-stage Cockcroft-Walton generator, with much better success.
Next we decided to improve the coils. We cut the 6″ pipe into short lengths, an inch and a half or so. We wrapped the magnet wire around them and used the oscilloscope to tune the number of wraps to get the desired frequency. It seemed like we were having to remove too many coils, so we found a formula for the inductance of a short air-core cylindrical coil. Plotting inductance as a function of wraps showed we could get by with much fewer than the 18 wraps we thought we needed for the desired 53 uH.
Emboldened with new information, we continued experimenting and found twelve wraps got us nearest to the desired 80 kHz frequency. We built a second, matching coil and we found we could transfer power much farther. The first set of coils had to be nearly touching. The second set could be more than four inches apart.
Unfortunately, we hadn’t left long enough leads on the coils, so we decided to wrap another set using all the information gained along the way. The forms were only an inch “long.” We sawed deeper groves for the wire’s start and end so the coils wouldn’t need to spread and we spaced tape around the coil to hold them tight. I haven’t measured, but I think we can have six inches between the coils and still light the LED.
We want to try making a smaller secondary coil, maybe an inch in diameter. That will be for another day.
But wait — there’s more
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