The Capacitor
The essential form of a capacitor is made of two conductive plates separated
by an insulating material called the dielectric. Wires are connected to
each plate to make connections to other components. The symbol of the
capacitor is shown here.
The main purpose of a capacitor is to store electrical charge. In storing a charge a capacitor resists changes in voltage. In its uncharged state the capacitor has and equal number of electrons on each plate. When a capacitor is connected to a voltage source such as a battery, electrons flow from one side of the cap and an equal number of electrons are deposited to the opposite plate. Once the capacitor is charged, no more current flows through the circuit. If the battery is disconnected from the capacitor the charge will be retained. The capacitor can be viewed as a sort of battery for the temporary storage of electrical energy.
In a Tesla coil one attribute of a capacitor is that it can be charged slowly and discharged quickly. Batteries typically have significant internal resistance, which limits the available current. Capacitors on the other hand can supply very large currents when discharged because their equivalent internal resistance is very low. Some capacitors are specially made for high impulse currents for such uses as in pulsed lasers.
The amount of charge per unit of voltage that a cap can store is its capacitance. In a formula this is designated C and the unit is the Farad.
C= Q/V
C is the Capacitance in Farads, Q is the charge in Coulombs and V is the voltage. One Farad is the amount of capacitance when one coulomb of charge is stored with one volt across the plates of the capacitor. It used to be very rare to see a capacitor with a value of one Farad or more. Most of the time you will see values in MicroFarads, NanoFarads or PicoFarads These correspond to 1 x 10-6 , 1 x 10-9 and 1 x 10-12 respectively.
Capacitors in series and parallel.
Many Tesla coil makers use what is called an MMC or Multi-Mini
Capacitor. That is a capacitor that is made of multiple miniature commercial
capacitors which are connected together to create one capacitor.
Typically the smaller caps are joined in series strings then the strings
are connected in parallel.
Connecting capacitors in series is the same as increasing the thickness
of the dielectric between the plates.
The combined capacitance is calculated by the following
formula:
1/Ct=1/C1+1/C2+1/C3+…
Series capacitors will have the voltage divided between them based on
their values. If all the capacitance values of the string are the same,
the voltage will be divided equally between them.
When combining capacitors in parallel, the values are simply added. It
is like adding their plate areas.
The voltage will not be divided, all will see the same, full voltage.
An Example
To take my MCC as an example: It is made of seven series strings of 20 capacitors. The seven series strings are wired in parallel. Each individual capacitor has a value of 0.056 microfarad.
To calculate the series values we take the reciprocal of the sum of the
reciprocal values.
1/0.056= 17.857142…
We can multiply this times the number in the string.
20 x 17.857142=357.143
1/357.143 = 0.0028
Now we multiply this times 7 for the seven rows in parallel:
0.0028x7=0.0196
This is real close to the 0.02 that I was shooting for.
Here's what my finished capacitor looks like.
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