# Why is the primary of the Tesla coil placed at the bottom of the secondary?

The primary winding of the Tesla coil is wound around only the bottom portion of the secondary coil and in Wikipedia, they mention that this configuration illustrates the operation of the secondary as a pumped resonator.

So my question is:

Why is this configuration mostly used? How does this pumped resonance work on the secondary side of the Tesla coil?

• I don't know much about tesla coils, but having the primary located far away from the top seems reasonable for isolation purposes, you don't want to risk any arcing back into the primary coil. Nov 24 '21 at 7:27

The whole purpose of the Tesla coil configuration is to generate a very high voltage.

• The primary is grounded.
• The high voltage end of the secondary has a high voltage on it.
• You want them to be far apart, for insulation.

There are transformers where the primary is wound over the entire secondary. These have far better coupling coefficient, so can operate without needing resonance to overcome the very low coupling coefficient of the Tesla configuration. However, because the primary is now near to the secondary, this severely limits the maximum voltage that the secondary can achieve. We don't call such transformers Tesla coils.

Resonance allows the energy transfer from primary to secondary to take place over a period of many cycles, to cope with the very low coupling coefficient.

A good analogy is a child's swing, as an example of a pumped mechanical resonator. The child drives it, by shifting the position of their centre of mass. However they can only achieve a tiny deflection in the swing position with a single shift in their position. Their coefficient of coupling to the swing is very low. If they drive the swing over many cycles, by timing their body shifts correctly, they can then build up an appreciable amplitude on the swing.

• If one were to place the primary coil more central to the secondary, and provide appropriate insulation & isolation, would this make a more efficient Tesla coil? Nov 24 '21 at 15:06
• @Aaron It would make a more efficient transformer, for certain definitions of 'efficient'. However, if the purpose of a Tesla coil is to generate a high voltage, and usually for making pretty sparks to ground, then it would make a fairly bad Tesla coil, as the output voltage would be half that of an equivalent secondary driven in the normal way. That said, there are a few centre-driven balanced output Tesla coils in existence. Nov 24 '21 at 17:54

Not necessarily has to be like that. There are parameters like isolation, inductance, resistance, coupling ratio, manufacturing difficulty and lastly the aesthetics.

The bottom placed primary is usually made of rigid copper tube, so difficult to tightly bend. Making a flat spiral with increasing diameter the inductance is increasing. With the correct gap, the breakdown voltage between turns is adjusted. As it is rigid, it can be suspended in the air supported by isolating bars. So, the reason behind is the practical implementation.

• The primary of mine was wound with high voltage insulated wire, as it was being driven from the output of a ~10 KV neon sign transformer. No, I didn't calculate or even assess isolation, inductance, or coupling ratio. If it didn't arc (except where intended), the isolation was OK <smile>. Nov 24 '21 at 15:04

The most obvious answer is that if the primary were placed on the top then you would need wires coming down vertically into it, while the HV secondary gets to discharge... somewhere lower down

• Why would it discharge "somewhere lower"? Nov 24 '21 at 15:44
• Because if it discharged somewhere higher it would hit the wires to the primary? Nov 24 '21 at 16:31

When I built mine, many, many years ago, I put the primary coil at the bottom because that was the logical place for it since it was being driven by the output of a heavy neon sign transformer. It also made the whole thing more stable (the secondary coil was 4 or 5 feet tall). And with the arrangement, the high voltage discharge (spark) came out of the top of the coil.