STEP 6: BUILDING THE PRIMARY COIL AND MAKING IT RESONATE
The Primary coil is the second inductor we are building. It guides the voltage from the transformer and creates pulses of power into strong electromagnetic energy that the secondary coil then amplifies. This can be a flat coil wound around the base of the secondary coil or spiralling upwards in an inverted cone style. I chose to use the flat spiral coil method because it seemed simplest to build.
The parameters of this flat coil depend on the resonance and self-inductance of the secondary circuit (the coil and top load together). I built my primary from copper tubing of 3/18” diameter but ¼” would work just as well.
You begin finding these parameters by calculating the resonant frequency of your secondary circuit using the sum of the inductance of the secondary coil and the top load.
Once you know what frequency it should resonate at (mine was about 585kHz), you can use this LC calculator to find out what inductance that frequency corresponds to (use the capacitance of your bank for this calculation). I got an inductance of 0.0238mH as my needed value for my primary coil.
Once you have this inductance, you can alter the number of turns in the following equation until your value matches using this relation:
As you can see from the equation above, your inductance increases proportionally to the number of turns you set. Of course you can just use the Flat Spiral Coil calculator for this as well. Playing around with this equation gave me around 10.5 turns. As a rule of thumb, add a few more turns onto that just to be sure.
Once you know the number of windings you would like to use, lay it all out with the innermost winding at twice the diameter of your PVC pipe and every winding after that 1/4” back. It doesn’t matter if you continue winding clockwise or counter clockwise.
Finally, you connect the inner most end of the primary coil to ground while the other end will be tapped into with an alligator clip at many different positions throughout the testing.
Inductance of a Spiral Coil(in H) :
Why is resonance so important?
Resonance happens when the primary coil shoots the current into the secondary coil at just the right time to maximize the energy transferred into the secondary coil.
It’s like timing when to push someone on a swing in order to make it go as high as possible. While resonance doesn’t increase the amount of energy transferred from the primary to the secondary coil, it does facilitate its transfer.
In our LC circuits, what happens is that when the capacitor discharges, it sends current into the inductor where it is stored as energy in the magnetic field. As the current flow into the inductor diminishes, the field collapses and creates an opposing voltage that flows back into the capacitors to charge them up again. This happens over and over again. The resonant frequency of a circuit is the number of times this cycle happens per second. In order to get the most efficient Tesla Coil, the resonant frequency of the primary and secondary circuits must match.
It is almost guaranteed that even with the correct calculations; you will not get resonance right away. This is where a lot of the testing comes in. You must tap in to the primary coil at different points until you find its sweet spot. You'll know when you've found this spot because you'll see sparks!