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I built a very basic tesla coil and I'm getting sparks out of the secondary bottom instead of the breakout point in the top. The bottom of the secondary is not connected to anything. The top is connected to the coil toroid and a breakout point. How do I get sparks to get out of the top breakout point?

The coil: coil

Bottom of secondary: bottom of secondary

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2 Answers 2

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The bottom of the secondary should be connected to some sort of ground connection.

If you are operating indoors, then a large sheet of foil underneath the secondary is ideal. I notice you have a wire mesh cage around your electronics in the base. This could possibly do as an alternative to a metal sheet, if all pieces of it are connected together, but increasing its size with a sheet of foil underneath, connected to it, would be even better. The foil, or your mesh cage, must also be connected to the safety/earth ground of your mains supply at the point where it powers the coil.

Put a mains filter at the coil inlet. If you don't have a mains filter, use X2 type capacitors to RF-short both live and neutral conductors to safety ground. This keeps RF interference from conducting back into the electrical network differentially between conductors.

If you are operating outdoors, then a spike into the ground is fine.

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  • \$\begingroup\$ That mesh looks kind of like plastic to me. \$\endgroup\$
    – Jasen Слава Україні
    Commented Jun 8, 2017 at 20:55
  • \$\begingroup\$ Are you sure it is safe to connect the mains ground to the sheet? Won't the coil send spikes through the mains ground and possibly damage all the electrical equipment connected to the mains network? \$\endgroup\$
    – Riccardo Fagiolo
    Commented Jun 9, 2017 at 4:55
  • \$\begingroup\$ It's unsafe not to. It's safer yet to use a mains filter at that point that has capacitors to RF-short all the three mains lead conductors together. \$\endgroup\$
    – Neil_UK
    Commented Jun 9, 2017 at 5:07
  • \$\begingroup\$ @Kopharex are you familiar with the concept of 'grasping the nettle'. It's about taking control of the mains connection. It is there, it is coupled to the electric field from the TC, and sometimes it will get hit by arcs. Connecting it to the secondary bottom and using X2 caps or a filter to RF short all the conductors together at the point of entry to the TC is about defining the relationship of the mains connection to the coil, rather than leaving it to chance. \$\endgroup\$
    – Neil_UK
    Commented Jun 9, 2017 at 6:04
  • \$\begingroup\$ I see. So ground would be connected to the ground plane via some sort of filter. What kind of filter? What are X2 capacitors? Excuse my ignorance, it's the first time I work with mains on a complex project. \$\endgroup\$
    – Riccardo Fagiolo
    Commented Jun 9, 2017 at 6:08
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enter image description here

Your problem is that your secundairy coil is does not have a connection to ground as it should. The primairy setup in your case can look somewhat different but a tesla transformer works always the same.

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  • \$\begingroup\$ Thank you. If i cannot drive a rod in the ground can I use an aluminium foil plane as ground? \$\endgroup\$
    – Riccardo Fagiolo
    Commented Jun 8, 2017 at 18:31
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    \$\begingroup\$ The current in the secondairy circuit is very small. A short rod into the ground is enough. A free floating aluminium foil plane free on an isolated surface is not good. \$\endgroup\$
    – Decapod
    Commented Jun 8, 2017 at 18:39
  • \$\begingroup\$ So there is no hope for using this coil indoors? \$\endgroup\$
    – Riccardo Fagiolo
    Commented Jun 8, 2017 at 18:40
  • \$\begingroup\$ Make sure you work safe. Make a proper earth and run a wire to the basis of the coil. There are more ways to obtain a ground but i do not want to suggest them because i do not know your situation. Remember you are working with very high voltages. So dont forget Murphy \$\endgroup\$
    – Decapod
    Commented Jun 8, 2017 at 18:45
  • \$\begingroup\$ your windings on the secondary seem close together. \$\endgroup\$
    – Jasen Слава Україні
    Commented Jun 8, 2017 at 20:58

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