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Step 3Coils 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!
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!
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That being said, remember that square waves contain a lot of extra energy in the high frequencies because of the rough edges. Sine waves are much nicer for this application, so as long as the output is sine-ish, you should be good. Using square waves would just mean lots of wasted power in the primary side.
1. Switching transistors wastes less energy, since they have to act like a resistor at half-on instead of a full-on or full-off switch.
2. The input impedance of the resonant coil looks like a short circuit at resonance, and an open circuit at all other frequencies, so if the fundamental frequency of the square wave is at resonance, it will act just like the sine wave case, but the high frequency components of the square wave will see no load. It will bandpass filter the energy by blocking high frequencies, not by shunting them.
I'm not sure if I agree with the second point. My circuit is causing a current oscillation primarily, the voltage oscillations are caused by the current. Or to put it more correctly, the current is being forced in the circuit, voltage does whatever it wants. When you switch to a square voltage oscillation I'm not sure it works as easily as described, since the load on the secondary is causing all sorts of complicated 3rd order effects :).
Meh, doesn't matter, both the method you describe and the method I've worked out achieve the same end, and both will be surprisingly inefficient when implemented, haha.
A transistor can't "force" current through a high resistance. It's not a current source.
"The resonant network filters the higher harmonic currents. Thus, essentially only sinusoidal current is allowed to flow through the resonant network even though a square wave voltage (Vd) is applied to the resonant network."
Yes, both will be inefficient, I'm just trying to learn the most efficient way to do it.
If i were to change to value of inductor and capacitor. would i need to adjust the resistor values around the BJT to compensate it?
Thanks :)
cause I'm using a 18AWG enamelled copper wire.. However, i could not obtain any form of sin wave off the oscilloscope.
Having said that, the transistors I used are generic ones that have a 500mA current rating and those are the ones I recomend you use. You can get them anywhere you buy parts from.
The ratings themselves will effect the oscillator but I'm not sure how. All I know is that the more current you can sink through the coil the better, so putting more transistors in parallel or using transistors with more current sink ability will give better results. You'll have to experiment to see what gives you the best results. If you do, let me know how it goes!
And the 'coil' in the diagram is exactly what you think it is, the primary coil :).