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Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
I finally found some time to dig out my old Class-E coil! Just thought I would share some of the results of my weekend tinkering while looking at ways to improve the design..
Basically, I stripped off the old primary winding and wound a new primary on a cut-down ring of PVC tubing that fits inside of the secondary coil form. The diameter of the primary ring (with 13 turns of 1.5mm enameled wire) is small enough that I can rotate it by means of a thin fiberglass rod. Rotating the primary ring with respect to the axis of the secondary changes the coupling!
Here are some pictures of the coil setup.
Here we see the primary ring inside of the secondary coil form.
The thin fiberglass rod that adjusts the rotation of the primary coil is seen here.
This is the complete setup.
By using this coupling adjustment along with the shunt variable capacitor, I can find the "sweet spot" that allow nearly 300W of power output with the PA transistor only a little above room temperature.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Changing the coupling does two immediately visible things:
Increasing coupling raises the resonant frequency. This is because the an increase in mutual inductance between the primary and secondary reduces the effective inductance of the coupled system.
More mutual coupling means less shunt capacitance is needed for good power transfer and ZVS operation. (Imaginary part of impedance decreases with coupling almost as 1/sqrt(1-M/L) unless losses are large..in this case, changes in M cause little change in reactive component..).
Also, when using high PA VDD supply voltage operation, use less coupling, higher shunt capacitance (increase feedpoint impedance). Low PA VDD supply voltage operation = more coupling, lower shunt capacitance (decrease feedpoint impedance).
I will try to knock together some more quantitative data with some notes......
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Method for tuning primary/secondary coupling for HF tesla coil
