Step 3: Coils and Oscillator
Ok, so what we will do here is build the oscillator first and then the coil. Using the oscilloscope, we will tune the primary coil until it resonates at the desired frequency.
The oscillator is pretty simple and was tested both in simulations and in a practical circuit. I derived it from the one here on wikipedia, however values had to be changed and more BJTs (transistors) were added after I discovered that max current transfer improved. Rough schematic and pictures are below. Note to add more BJTs just connect them in ‘parallel’ to the one in the schematic, pin-to-pin.
The primary coil started as ~18 turns at ~15cm diameter, then I removed turns based on the final shape/diameter. If you look at the picture of the jig you see how I easily made the coils to a certain size. Just cut out some holes in a cardboard box and use pens on an angle to wrap the wire around.
In the other picture of the completed circuit/coil, I forced the coil into a roughly rectangular shape, so the inductance changed. Simply make the coil with a few extra turns and then connect that up to the oscillator (directly, solder it without cables or any other wires). Place the ‘scope probes across the coil and check the wave period. Remove coils until the period matches what you want (in my case, it was 12.5us). By remove coils I mean physically remove a turn by cutting the wire and re-soldering the end. Excess wire will lead to more inductance and you won’t get the right value.
After you are finished with the primary coil you’ll do essentially the same thing for the secondary coil. Just unsolder the primary and repeat. However when making the coil you are welcome to change the diameter and number of turns. I used my hand for the second one and started with ~30 turns, to make it smaller and easier to fit in things.
Once completed you can wrap the coils, although this is risky as you will change the inductance significantly so you’ll have to do a considerable amount of trial and error to get it right. The inductance changes because you are forcing the wires closer together.
Now onto the slave pickup and CW generator!