Large Tesla Coil

Tesla_coil

Figure 1: Our Tesla coil (Spark Gap)

Our home-made Tesla coil can generate 1.2 million Volts of high frequency alternating current. It can produce long noisy sparks, and is one of students’ favorite demonstrations. A 12″ tall tabletop version of a Tesla coil is also available.

Equipment:

  • Tesla coil
  • Long fluorescent tube light bulb
  • Demonstrator (only experienced instructors should do this alone!)

Demo:

  1. Place the Tesla coil far from any metal objects, laptops, and people (at least a few meters away). Lay out the cable with the red button on the end a good distance from the coil.
  2. Turn the key to the “ON” position, then turn on the fan.
  3. Dim the lights.
  4. Stand a few meters away from the coil and push the red button on the cable to make lighting erupt from the metal sphere.
  5. Place the fluorescent light on a table near the coil to show that the Tesla coil can power it through wireless energy transmission

Explanation:

All Tesla coils are comprised of 4 essential components: primary coil, secondary coil, topload, and control circuitry. When combined properly, these pieces allow Tesla coils to build up extreme voltages in the topload which allow for large arcs of electricity through the air.

Figure 2: Tesla coil diagram, Live Science

For the general Tesla coil, this spectacular display relies on energy storage in the control circuitry and energy delivery from primary coil to secondary coil. First, energy is stored up in a large capacitor bank (grouping of capacitors in series or parallel) below the main coils. This capacitor energy storage allows for short bursts of high power to be delivered to the primary coil. Once this high frequency current enters the primary coil, the next power transfer stage relies on the coupling of primary and secondary coils. The primary coil, a large copper tube wrapped around the base of the Tesla coil, has a much smaller number of turns compared to the secondary coil, the thin red wire wrapping the entire Tesla coil up to the topload. This combination of coils effectively creates a step up transformer, converting low voltage, high current electricity in the primary coil into high voltage, low current electricity in the secondary coil. The secondary coil is connected to the topload, a large metallic ball in our case, while the other end is grounded. These connection can be seen in Figure 3:

Figure 3: Tesla coil diagram, Wikipedia

At this stage, our Tesla coil has a large differential voltage (1.2 million volts) between the topload and ground. The only reason it is able to build up such large voltages is due to the effective capacitance (C2 in Figure 3 above) between the topload and ground created by the air gap in between these two. However, once the voltage climbs high enough this air gap will appear to be a short to our high voltage electricity. Finally, our Tesla coil sends long branches of electricity shooting though the air.

In reality, the coupling between primary and secondary coils isn’t as simple as a common step-up transformer. Transformers typically deal with DC current, and therefore a constant magnetic field at their core. In contrast, Tesla coils deal with AC current making it’s coupling of coils a resonant harmonic oscillator. Each coil has its respective resonant frequency:

f_{1}={1 \over 2\pi }{\sqrt {1 \over L_{1}C_{1}}}\qquad \qquad f_{2}={1 \over 2\pi }{\sqrt {1 \over L_{2}C_{2}}}\,

Setting these frequencies equal yields the relation L1C1=L2C2

When the resonant frequencies of these two coils are equal the oscillation achieves maximum power transfer. To match these two frequencies it is easiest to alter the number of windings in both the primary and secondary coils. The capacitance of the primary coil circuit is simple enough to adjust, as this is the capacitor bank used in the control circuitry. The secondary circuit capacitance is much more difficult to fine tune however, because it is related to the shape of the topload and the distance which the sparks will arc through the air. Typically these resonant frequencies are in the radio frequency range (100kHz to 1Mhz).

From the discussion so far, one might assume that the primary and secondary coils are the most prominent pieces of the Tesla coil assembly. Yet the control circuitry plays an undeniably large role in the operation of this spark-shooting contraption. Due to it’s complex electrical component composition, the control circuitry design falls under the field of electrical engineering. On the other hand, the coupling of primary and secondary coils relies more heavily on physics in the form of induced magnetic fields caused by moving charged particles.

The design of these circuits feeding current to the primary coil heavily depends on which type of Tesla coil one sets out to construct. The most common forms of Tesla coil include spark gap and solid state.

Our Tesla coil is of the spark gap variety. Spark gap control circuitry is much simpler than its solid state counterparts because it only consists of a charging capacitor and spark gap, as the name implies. The capacitor builds up current until it reaches a high enough voltage for the spark gap to short (completing of a circuit with little to zero resistance) , sending high frequency AC current to the primary coil. A basic diagram of such a circuit can be seen in Figure 4 below:

 

Figure 4: Simplified spark gap TC, Wikipedia

Figure 5: Spark gap TC in operation, Wikipedia

On the more complicated side of design lies the solid state Tesla coil (SSTC). A SSTC differs from the spark gap equivalent because it utilizes transistors (BJT, MOSFET, and Thyristors) and op amp (operational amplifier) chips to deliver power to the primary coil. The increased complexity of this design provides many benefits. First, using transistors to deliver current allows for an adjustable range of output frequencies. This allows for easy optimization of power transfer between primary and secondary coils. Second, this setup supplies the current much more quietly because it lacks a gap through which sparks noisily jump across. Lastly, an SSTC allows for a wider range of changes to the sparks produced. Changing the duty cycle on the output of your control circuitry creates the potential to change the shape of arcs (brush, streamer, or corona discharges). In addition, supplying an output which combines two frequencies can create audible sound, also known as a singing Tesla coil. An example of the SSTC design:

Figure 6: Solid State TC diagram, Steve Ward’s Tesla Coils

Figure 7: SSTC producing brush discharges, Steve Ward’s Tesla Coils

These high levels of sophistication allow for increased efficiency and power delivery over their spark gap alternatives. Beyond these differences, spark gap and SSTCs are very similar, and will produce similar results.

Demonstration Photos:

Figure 8

Figure 8: Our Tesla coil in Thimann 3

Figure 9: Tesla coil and fluorescent light

Figure 10: Our Tesla coil in action

Figure 11: Tesla coil powering the fluorescent bulb without contact

Notes:

  • Caution: This apparatus is dangerous for people with pacemakers, and could damage computers or sensitive equipment. Keep coil several meters away from the nearest observer!
  • This demo can only be shown in Thimann 1 or 3.

Related Demos:

Written by Noah Peake