Tesla Coil Primary Tank Capacitors.

Requirements of Tesla Coil Primary Tank Capacitors.

The design of the primary tank capacitor of a Tesla Coil arrangement is of crucial importance. This component is usually the component that fails the most in a Tesla Coil arrangement, but it is also usually the component that gives the biggest potential for improvement. It must have the following properties:

• Good dielectric strength. This is perhaps the most crucial factor, and is the reason that most capacitors fail. The capacitor will be subject to very high A.C. voltages, and even higher voltage peaks or 'spikes'. Thin dielectrics of low dielectric strength will most likely fail.
• Low dielectric loss. Surprisingly, not all of the energy that is stored in a capacitor will come back out. Some is lost as heat in the dielectric. At low frequencies, this effect is usually negilgable. At Tesla Coil frequencies, however, the loss can be enough to melt the dielectric! Some dielectrics are worse than others, and to complicate matters the losses tend to vary with frequency. The best all round dielectric for Tesla Capacitor use is polyethylene. This material has a good dielectric strength, and low dielectric loss. It is better than glass.
• Low ESR (Equivalent Series Resistance). Capacitors use metal plates, and all conductors (including metals) possess some electrical resistance. This is usually not a problem. For Tesla work, however, it is desirable for the resistance to be very low. This means using a good conductor (copper would be best, but aluminium is a good, cheap, readily available alternative). Also, thick plates are better than thin.
• Low inductance. Any conductor, even the metal plates of a capacitor, will have inductance. This is because whenever a current flows through a conductor, a magnetic field is set up. Energy is stored in the magnetic field, and that constitutes inductance. Of course the inductance of the metal plates in a capacitor is usually negligable, but if the capacitor is of a 'rolled' construction, as is sometimes used for Tesla Capacitors, then the inductance can be significant. At low frequencies even the inductance of a 'rolled' capacitor is insignificant, but at the high frequencies used in a Tesla Tank circuit, the inductance can present a high inductive reactance. High inductive reactance is undesirable, as it limits the 'ringing' current that will flow in the Primary Tank circuit. Some ways to keep the inductance of a capacitor low are suggested below:
  • Use a flat plate construction rather than a 'rolled' capacitor.
  • Keep the plates of a flat plate capacitor as close to a square shape as is practical (in fact a circular shape is the best, but is not often practical). This is because long rectangular plates have higher inductance.
  • If possible, take the connections from the centre of the plates (imagine current flowing from every point on the plate towards the connection - the magnetic fields created tend to cancel, thus minimising inductance).

Types of Tesla Coil Primary Tank Capacitor.

The following types of capacitor are common in amateur Tesla Coil Usage:

• The flat plate capacitor
• The Rolled Capacitor
• The Salt Water Leyden Jar

These types are now dealt with in turn:

The flat plate capacitor

The Rolled Capacitor

The diagram below shows how a rolled capacitor is constructed. The black layers are made of conducting metal foil that form the plates of the capacitor. They do not touch each other. The red layers represent the insulating film dielectric. The metal most commonly used for the plates is aluminium, and for the dielectric is polyethylene. The diagram shows only 1.5 turns in the roll, but a practical capacitor is likely to have many more.

The whole capacitor roll is usually immersed in oil to reduce corona losses.

The Salt Water Leyden Jar

One dielectric layer versus many layers.

It is better to make a dielectric for a high voltage capacitor from several thin sheets of plastic film instead of one single thick sheet. The reason is that if a single thick dielectric layer is used and it has a manufacturing defect, it is likely that the defect extends all the way through the plastic. If such a capacitor is charged to a high voltage, it will tend to fail at the single defect. With multi-layer dielectrics, however, even though every layer might conceivably have a defect, it would be unlikely that all the defects would be lined up to allow an arc to form.

Capacitor Dielectric Properties.

Polymer	Dielectric Constant 50Hz	Dielectric Constant 1Mhz	Dielectric Strength (Kv/cm)	Dissipation Factor (x10^-3) 50Hz	Dissipation Factor (x10^-3) 1Mhz
LDPE	2.29	2.28	370	.15	.08
HDPE	2.35	2.34	--	.24	.20
PP	2.27	2.25	240	.40	.50
PVC-plasticized	4-8	4-5	270	80	120
PS-polystyrene	2.5	2.5	200-300	.1-.4	.05-.4
ABS	2.4-5	2.4-3.8	~400	3-8	2-15
PMMA-Plexiglass	3.3-3.9	2.2-3.2	140	40-60	4-40
POM	3.7	3.7	400	5	5
PTFE-Teflon	2.1	2.1	480	.2	.2
PCTFE	2.3-2.8	2.3-2.5	550	1	20
PA-6-Nylon 6	3.8	3.4	400	10	30
PA-66-Nylon 66	8	4	600	140	80
PC-Lexan	3.0	2.9	380	.7	10
PET-Mylar	4.0	4.0	420	2	20
PI	3.5	3.4	560	2	5
PUR-linear	5.8	4.0	>300	120	70
PUR-thermoset	3.6	3.4	240	50	50
PUR-thermoplas	6.6	5.6	300	30	60
CAB	3.7	3.5	400	6	21
Silicone	3.6		200	5-13	7
Pure Mineral Oil	2.2		79