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Re: Questions??



Original poster: "resonance" <resonance@xxxxxxxxxxxx>

I ran your data through a computer program and I think I spotted your problem --- you don't have enough turns on your primary coil to hit the resonace point to match the resonant frequency of your secondary system. I would suggest making up a "scrap wire" primary, flat spiral, of 20 turns of standard #12 AWG house wire scraping a bare point as each turn comes around --- stagger these slightly so your primary tap will fit on without shorting to an adjacent turn.

You can calculate your length of wire on your sec. coil with good old high school math. First, go to your local library and pick up a current copy of the ARRL Radio Amateur's Handbook. They have wire tables that will tell you the dia. of your wire and the number of turns per inch. Or, just take a piece of wire into your high school machine shop and ask a teacher to measure it's diameter for you with a micrometer (mike for short).

Circumference of a circle = pi * dia. This gives you the length of wire for one turn on your sec. coil. In the case of #26 AWG wire, the wire dia. is .0159 inches (159 mils), and with some slightly spacing in winding, this gives you 58 turns per inch. You calculate the length of one turn, then multiply x 58 turns per inch, then multiply this by your 10 inch coil length. Divide by 12 in/ft and you get approx 425 feet of wire on your sec. coil.

With 425 ft., your sec. operating resonant freq is 513 kHz for this coil with your pie-pan capacitance on top. (I estimated the topload at 12 pF). If you have a high school electronics shop they probably have a multimeter than can accurately measured the capacitance of this pie-pan topload for you.

With your .0075 uF primary capacitance, you need 9.4 turns of primary coil to resonate your system. With the 8 turns you have now --- it won't hit resonance point of the secondary coil.

I would suggest going to a full 18 turns and then tapping out one turn at a time from 5 turns until you hit the resonace point --- it should be around 9 or 10 turns but may be more. The scrap wire primary uses 12 ga. scrap house wiring (PVC covered) and you can just use some slices of cardboard on edge --- make slits in the cardboard (4-5 pcs) to hold the wire -- to hold your temporary primary in place.

Stick a short 6 inch long wire off your pie-pans to allow the spark to jump into free air so you can see it's length. Best to let it jump to a ground stand you can slowly back off as it gets longer. Run your variac up to only 40-50% while you are doing this tuning and always unplug the unit before you touch the primary coil. When you find the tap point for the best spark length then you can bring it up to full power.

Your should get 7-8 inch long sparks with this coil.

Later, if you want to improve your performance with this capacitor and transformer, you should use a 4 inch dia. coilform, closewound with #30 AWG wire. The larger inductance (which increases as the square of the radius of the coil and also as the square of the number of turns on the coil) will increase your voltage output substantially. The squaring factor is important to achieve high inductance ---- use large dia. coilforms.

The equation is Vout = -L x dI/dt inductance x the rate change of current (I) with respect to time. Your spark gap controls the time factor, the current I is determined by your capacitor and sparkgap, so your best variable is L. To increase the inductance (L) you use a much larger dia. sec coil.

Have fun!

Dr. Resonance