Search this site:

In action at the Nottingham Gaussfest several years ago. Power was from a Pole Distribution Transformer (PDT) , and the coil was running a 200bps synchronous rotary with phase control.

Capacitance was 115nF provided by a pulse cap (65nF) and Cornell-Dubilier's 942C20P15K-F capacitors (50nF).

The coil has been updated since, with increased power (lower value ballast), more capacitance (125nF), and the use of a bigger (borrowed) 50 x 11 inch toroid.

This has all added up to increased performance. Unfortunately the venues I currently attend all suffer from low ceilings, so bigger coils can never reach their true performance.

In an effort to overcome this, the coil was lowered in 2015 and at the same time was given the ability to run with offset SRSG electrodes.

This involves swapping my 'normal' rotor disc for a new 'off-set' version. To aid changing discs back and forth for comparison tests, the base unit layout was also altered to give quicker and easier access.

The practical work on this is now all done, with testing due in Oct 2015. In the meantime though I have added a new **Web Page** explaining the details.

Soft Iron core output voltage:

Vs = (Vp x (Sec turn / Pri turn)

Air Cored output voltage:

Vs = Vp*sqrt(Cp/Cs)

Or

Vs = Vp*sqrt(Ls/(2 x Lp))

sqrt = Square Root

Cp = Primary capacitance value

Lp = Primary inductance

Cs = Secondary capacitance value

Ls = Secondary inductance

Vs = Secondary output volts

The first part of my tesla coil is powered by a conventional 12Kv pole distribution transformer (*aka* 'Pigs').*(Click the above picture for details of the transformer)*

This then feeds into an **air-cored** resonant transformer, or as they are simply called..... a Tesla Coil.

**To understand how a Tesla coil works you first need to understand
a couple of basic points about the components and terms used:**

**1) Inductors:** *(component)*

The Tesla coil's primary and secondary coils are both inductors in electrical terms. When the current flowing through an inductor changes, it will create an opposing or reverse voltage.
**Wikipedia Article**

**2) Spark Gaps:** *(component)*

A sparking plug in a car is a basic spark gap, its break-down voltage dependant on the electrode gap size. Once it conducts, hot ionized air in the gap gives it the ability to carry on, so long as a current is flowing.

**3) Capacitor:** *(component)*

A good analogy for a capacitor is to think of it as a sponge, placed on spilt water and left to slowly soak it up. If left for a minute and then given a quick, hard squeeze, one minute's worth of soaking-up is instantly released in a fraction of a second.
In a Tesla coil circuit this so called 'soaking-up' stage lasts only a few milliseconds, while the 'squeezing-out' can be a thousand times quicker in a few micro (millionths) of a second.
**Wikipedia Article**

**4) Resonance:** *(terminology)*

The property of resonance is fundamental to the operation of Tesla coils.
A good analogy is a garden swing. If left to swing on its own it will do so at its resonant frequency, only slowing down due to friction and gravity.
If you stand behind the swing and push it just as it swings away from you each time, it will get higher with each subsequent push. This is because you are adding power at, and only at, the correct time-point in the swing's cycle.
You are therefore adding momentum at the same time interval as the swing's resonant frequency, this means the push you gave is in resonance with the swing.

Resonance does not magically increase the amount of energy, it only facilitates its transfer.
So if you're looking for tesla related, so-called 'free energy, or 'zero point' energy information, which some people seem to associate with tesla coils, this is not the site for you!

**5) Resonant Circuit** *(terminology)*

If a capacitor is placed across an inductor and voltage applied, you will have a resonant circuit. As the capacitor discharges, it sends current into the inductor that stores this as energy in its magnetic field. But as the capacitor discharges, the current into the inductor also diminishes.
This causes its magnetic field to collapse and generate an opposing voltage back into the capacitor, allowing the cycle to start all over again. The number of times that this 'back and forwards' cycle happens per second, is the circuit's resonant frequency, expressed in Hertz (Hz).

Using different capacitance and inductance gives different frequencies.

**Note: **Because of resistive losses the current reduces every cycle down to zero.

There is NO such thing as free energy!

**In** the circuit of Fig 1 above, the capacitor (**'C'**) is charged up by a high voltage source, like my example of the sponge soaking up water.

**Once** the capacitor attains a high enough voltage the spark gap fires and conducts (Fig 2 below). The spark gap is now a short-circuit that completes the resonant circuit (shown in red) of the primary inductor and capacitor.

**The** spark gap firing is virtually an instantaneous discharge of the capacitor energy into the inductor and is like my earlier example of the sponge being instantaneously squeezed out.

The inductor (the primary) stores this energy in its magnetic field with its lines of force cutting into the secondary coil (another inductor) and induces a voltage into it. Once the capacitor is empty, the current flow into the inductor stops, and its magnetic field collapses causing a reverse current (now fairly reduced) to flow back into the capacitor again.

This back and forth ** diminishing** cycle (called the 'Primary Ringdown') of capacitor to inductor and back, continues until there is insufficient current flowing to keep the spark gap conducting. The point to remember is that every time this primary cycle occurs, more energy has also been transferred away to the secondary, so the primary inductor's magnetic field stores less and less energy on each cycle.

Unfortunately every time the spark gap conducts, losses also occur in the form of heat and light, so you want the minimum number of cycles that are consistent with getting all of the available energy transferred to the secondary.

Usually after two, three, or possibly four cycles the majority of the energy has been transferred and the primary current has dropped enough to allow the spark gap to stop conducting (called quenching).

Once the spark gap has quenched it allows the capacitor to get a fresh charge and the whole affair can start again.

You can see here that doubling the value of C (provided your power source is robust enough) will give you twice the power. But doubling the voltage that the capacitor is charged up to will give 4 times the power, because the voltage value is squared, that's why if you want spark length its best to go for a higher voltage power source.

While the primary circuit is resonating and transferring its energy, the following is also occurring in the secondary circuit

The toroid on top of the coil acts like a capacitor with respect to the surrounding ground. This is easier to see in the diagrams below.

Fig

This is because at just the right point in time of its cycle (like you pushing that swing in the example) another magnetic field from the primary circuit, which remember is

Remember the primary and the secondary need to have the same resonant frequencies for them to interact successfully (in reality there is a deliberate slight mis-match, explained elsewhere). Typically this is in the hundreds of Kilo-Hertz.

Eventually the voltage on the surface of the toroid at the top, rises so high that the toroid's curved surface cannot retain the charge anymore, and breakout occurs. This will either be a misty purple corona discharge or, if all components are suitably balanced to one another, a whitish solid streamer down to earth or into the air.

And secondly no new energy from the power source can be added to the circuit until the spark gap has quenched, and that can't happen until the present cycle stops.

With the synchronous system, you arrange for the revolving electrodes to come into alignment with the fixed ones when the AC cycle is