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High Inductance Primary

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LAN_403

M.A.D.

Sun Sept 11 2011, 05:48PM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

Hello, I need some help with a theory of mine.

The way I understand it is that an ignition coil is a highly coupled resonance transformer. The primary has very high inductance and when a DC current is aplied across it, it holds the charge. When the supply is cut by the points, in the distributor, the transformer resonates with a cap across the pimary leads.

If any of this is not correct please let me know.

So, here is my idea. If I could build a tesla coil with a high inductance primary then I could use some low frequency tubes as the switch.

What would be the best way to build a high inductance primary that is small and does not force me to have to use an impossibly small capacitor? As always any help is apreciated.

Dr. ISOTOP

Sun Sept 11 2011, 06:53PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

An ignition coil is not resonant. When the DC current is cut, L*dI/dt creates a voltage spike across the primary, which is further stepped by the turns ratio.

M.A.D.

Sun Sept 11 2011, 09:05PM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

Thank you for the info.

I thought I had read somewhere the ignition coil were resonant, but if they are not what is the capacitor (condenser) in the distributor do?

Also I do not know a lot about inductors, but wouldn't this still work for a tesla coil with a high inductance primary and a cap across the primary leads? Also how much would the voltage rise if the primary was charged with 2000V 53mA?

Also I was planning on putting the coil in a cage connected as a RF ground how can I figure out how much this would cause the secondary frequency to drop?

magnet18

Sun Sept 11 2011, 09:11PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

I believe that the capacitor is there to store energy for dumping into the spark plugs, but I'm not 100% on that.
It might be a smoothing capacitor

I"m not sure if it would work for a tesla coil, but if it did it would be fairly low power. A MOT would be a better option.

I'm not sure what you mean by the voltage rising, if you mean resonant rise then it can get very, very high if the energy has nowhere to go.

It depends on how far the cage is form the secondary, I don't know of any programs that can do that, someone else on here might know. One option is to build a circuit that can find the resonant frequency and just measure it.

Dr. Dark Current

Sun Sept 11 2011, 09:37PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

well the capacitor produces a kind of resonant action with the primary.
If you built a low frequency primary tank, you would also have to build a secondary with several thousand turns, at which point you can also use a ferrite core transformer.

Ash Small

Mon Sept 12 2011, 01:04AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The condenser prevents the points from arcing as they open, thus reducing points erosion.

This is a link to an abstract from a 1912 paper that says it resonates with the coil.

It serves the same purpose as putting a snubber capacitor across a transistor in a simple flyback driver circuit.

(I was trying to find a link to a different article, but have not yet found it)

EDIT: Wikipedia says this:

"Opposite potentials are induced in the secondary when the interrupter 'breaks' the circuit and 'closes' the circuit. However, the current change in the primary is much more abrupt when the interrupter 'breaks'. When the contacts close, the current builds up slowly in the primary because the supply voltage has a limited ability to force current through the coil's inductance. In contrast, when the interrupter contacts open, the current falls to zero suddenly. So the pulse of voltage induced in the secondary at 'break' is much larger than the pulse induced at 'close', it is the 'break' that generates the coil's high voltage output. A "snubber" capacitor of 0.5 to 15 Î¼F is used across the contacts to quench the arc on the 'break', which causes much faster switching and higher voltages. So the output waveform of an induction coil is a series of alternating positive and negative pulses, but with one polarity much larger than the other."

M.A.D.

Mon Sept 12 2011, 02:24AM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

Thank you for the info on ignition coils. I have been reading some similar stuff on inductors, and how they can hold current even when voltage hits zero. Quite interesting, but a little bit cofussing.

My plan is to build a tesla coil driven by a MOT (no doubler) and a low frequency tube. The tube will force 2000V at 53mA DC through the tesla coil primary for the first half of the sine wave then (if my control circuit works) the tube will be shut off.

If the tesla coil's primary has high enough indutance then, the primary will have a large amount of power still in it and the only place for it to go is the tuned capacitor across the primary's leads. This should cause it to resonate with the secondary and produce arcs.

I plan for this tesla coil to be rather small, the secondary will have about 1200 turns of 38awg (about 6'' long). The RF ground will be a cage arount the coil, about 3'' from the outside of the topload so this should help lower the frequency.

What size of wire and how many turns should I use on the tesla coil's primary to yeild enough inductance? Could I use the same 38awg that is going to be on the secondary? Also how can I possibly figure out how to tune this thing, or am I going to have to build one of those tesla coil tuner things, to even have a chance?

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