STEP 5: BUILD YOUR CAPACITOR BANK AND SPARK GAP
The capacitors and spark gap work in tandem in the tesla coil. The capacitors store and
release charge while the spark gap acts as a switch.
A capacitor consists of two conductors separated by a non-conductive region(the
dielectric). It stores and releases an electrical charge.
There are two capacitors in our circuit, one the bank in our primary circuit and the
other is the top load from our secondary circuit. For now we will focus on building
the capacitor bank.
To start, we need to know how much capacitance is needed to hold all the charge
coming in from the power supply.
We can figure this out from a simple calculation:
Plugging in a value of 15Kv and 15mA (the outputs from out flyback) will give us a needed capacitance of 3.1 nano Farads.
Picture taken from
How to build the capacitors
There are two ways to build a capacitor bank.
The first is known as the Layden Jar model. It uses 6 Snapple
jars filled with water, salt and oil. The jars themselves will be
tightly wrapped in aluminum foil and attached in parallel with
each other but in series with the circuit. These capacitors will
store static electricity from the transformer and when enough
power has been stored, it releases it through two electrodes
(spark gap). The oil in the jars has an extremely high resistance
and will keep the charge from arching out the top. Although this
is an effective model, it takes a lot of preparation, is quite bulky
and can be dangerous.
Picture taken from http://amasci.com/tesla/cap2.html
The second is the choice I went with, which is just buying capacitors from DIGIKEY and soldering them together. If you decide to go with this, you need to figure out how many capacitors are needed and whether to attach them in series or in parallel. Remember, capacitances add in parallel and combine as reciprocals in series.
When choosing our capacitors, we must take into account that not only do they need to have a value of 3.1nF, but also be able to hold 15kVolts. After a bit of searching on Digikey, I found capacitors with a DC voltage rating of 1kV and 0.1µF. Since voltage for caps add in series, we can try to add these to see what kind of values they give us.
As we can see, we came up with about double the capacitance of what is needed but more is better than less. I did multiplw testing with 15-20 Capacitors so you get some leeway when it comes to the number of caps you want to use. However, it is important that they can hold all the voltage that the flyback outputs.
Picture taken from http://www.digikey.ca/product-detail/en/0/495-1356-ND
Now for the spark gap!
The spark gap is literally two nails some length apart. It controls the frequency and level of power that goes through the primary coil. Increasing the length of the gap essentially increases that insulation which means that a greater voltage needs to be applied. This length can and should be adjustable for experimentation. The drawbacks of this component is that it creates a lot of light (which can be harmful to the eyes) and is incredibly loud. However, it is the cheapest way and most simple way to create and control the spark gap.
To make this, I took two bolts and made them adjustable on a small board of wood as seen on the right. You should be testing with the spark gap at around a 1/4" gap. Increasing the spark gap allows the capacitors to charge for longer, meaning that more energy can be stored before it breaks down. This is one thing to think about when testing.
So how do these work together in our circuits?
The capacitors discharge into L1 (our Primary coil) through the gap.
Proximity of the two coils creates magnetic coupling which allows the current to transfer to L2 (the secondary coil).
This current travels through L2 until the air surrounding breaks down. Sparks!!
As the high electric field builds, the spark gap becomes a good conductor and breaks down. ie closes!
It initially acts as an open circuit. As the current travels through, the capacitors charge.
