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Re: Question about capacitors for coil



Original poster: "resonance" <resonance@xxxxxxxxxxxx>



Let's answer his question by applying the basic physics that all Tesla coils must follow.

Although spark length is really directly related to power, not potential, it does require a high potential to begin the ionization process, so the basic idea is to apply the equation for potential in an air coil inductor:

The potential difference across an inductor is directly proportional to it's inductance x the rate change of current across it, ie, the dI/dt when I is current in Amperes and t is time in seconds.

From physics, V = - L dI/dt (dI/dt is calculus meaning rate of change of current per unit time)

The minus sign just indicates the polarity is opposite to the inducing voltage (not a critical factor here so we can ignore it)

The rate change of current, dI/dt really depends on a number of efficiency factors such as spark gap resistance, primary connection resistance, etc. We use 4 AWG welding cable with copper battery lugs for our primary connections (inside tygon tubing) and use 3/4 - 1 inch dia. copper pipes for our sparkgap thus keeping the switching (dI/dt) value high as possible in current and fast as possible in quenching time of the sparkgap). High current (the dI) and fast speed (the dt) is important.

This leaves our best "tinker" factor, the inductance (L) of the secondary coil.

Inductance increases as the square of the radius of the sec coil, so you want a large dia. to promote a large inductance. That's one reason Tesla used a 52 foot dia. sec coil in Colorado Springs. I assume your garage wouldn't allow such a large coil, so it's best to use a 4 inch dia. as compared to a 2 inch dia. sec coilform. You have 4 times more inductance for the same length of sec coil. Also, inductance varies as the square of the number of turns, so you want a lot of turns on the sec. Too many turns and you run into excessive resistance above which more turns will provide a decrease not increase in performance.

Assuming the average coil's efficiency around 70%, then the sec voltage equation becomes:

Vsec  =  Vpri x 70% x SQR (Lsec/Lpri)

I've used antenna current x field measuring systems developed by Terry Fritz (our super-duper moderator) and this equation is very close --- usually within a few percent so it really does work.

Later, after your coil is all tuned up and running great, then you can measure your pri inductance and sec inductance with an inductance meter (local Universities or tech colleges all have them and could do this for you), and then calculate the actual output voltage of your coil. These measurements are taken on an electronic inductance meter with no power applied to the coil.

So, it's best to use a 4 inch dia. coil form as compared to a 2 inch dia. form as you will have a coil with much better performance. The cost of PVC --- perhaps a "cutoff" at a local large plumber's shop, will be nearly the same for either size and your performance gain is 400% with the 4 inch size as compared to the 2 inch size, so why not start with the best design possible? Sometimes the plumbing shop will sell you a cutoff very cheap as they usually just end up throwing them away. If you are a high school student you might even ask for a "donation" to your project --- they might say yes and then it's free!

With a 30 mA sign xmfr usually the 26 inch winding length with #30 AWG wire will give great performance. With larger coils it's best to stick with 1,000 turns of wire for optimum performance. Use a flat spiral on the primary with 25 turns for the test using scrap wire --- when you find the correct tuning point at 40% variac power, then you can replace these scrap wire turns primary coil with some nice looking 1/8 inch dia. copper tubing or #6 AWG solid wire (Home Depot #6 AWG ground wire).

Happy coiling,

Dr. Resonance
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, October 01, 2006 3:29 PM
Subject: Re: Question about capacitors for coil


Original poster: John <guipenguin@xxxxxxxxx>

Thanks Dr. Resonance.

I have been debiting what size diameter secondary I should use with my coil before I goto the hardware store and grab some pvc. 4" by 26" seems a bit large for where I wanted to start. What would be a good size diameter for this transformer? I have read a lot about different people's opinions on hight/diameter ratios...but I'm not sure what diameter to start. I was thinking 2" diameter at less then 12 in hight...but this may be way to small for decent spark length....