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# Mechanically Driven Tesla Coil

A simple off-line variation of the SGTC, much easier to make, more reliable and doesn't require high voltage and heavy transformers...

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In this design, I used some leftover components such as an old NC relay, MMC tank capacitor made from 10nF, 1250V MKS capacitors paralelled, an old primary and secondary coil from my school's basement (secondary rewound with 0,17mm wire). I got them from the science teacher in my former school via a friend that still goes there... You can see the schematic on the second picture.

This schematic is unique in the thing that it is the first variation of the SGTC to work on such low voltage AC. Instead of a static gap, it uses the contacts of a relay to act as a spark gap.

1. Because the relay contacts are NC, the current flows through the relay and activates it.

2. When the relay is on, the contacts open for a short moment. So, the current charges up the capacitor to a certain voltage. (Due to resonant charging through the ballast and the relay coil, the peak charging voltage across the MMC was measured at 640V)

3. Because the contacts are open, and the MMC is charged, there is practically no current going through the relay coil, and it restores it's normal (closed) position of the contacts. So, the contacts are now closed, and the MMC is connected in parallel with the primary coil (L1), creating a resonant (Tank) circuit. The resonant circuit begins oscillating at a fixed resonant frequency (In this case- fres=1/(2*pi*sqrt(C1*L1)) ).

4. The resonant circuit oscillates, thus it creates a strong EM field, which induces a current in the secondary coil. The secondary coil and the sum of the coil's parasitics and the capacitance of the topload, create a resonant circuit that has a resonant frequency almost identical to that of the primary resonant circuit (C1 and L1). All of these factors provide significant voltage magnification through resonant action.

5. Because the contacts are closed, besides the current of the resonating primary circuit, there is also a current flowing through the ballast, relay coil and relay contacts. The current flowing through the relay coil creates a fast growing magnetic field that opens the contacts, thus breaking the parallel connection between C1 and L1 and stopping the oscillations.

6. Go back to step 1.

-THE BALLAST LIMITS THE CURRENT GOING THROUGH THIS WHOLE CIRCUIT, AND ALSO HAS A VOLTAGE MAGNIFICATION EFFECT ON THE CAPACITOR CHARGING VOLTAGE.

-IT SHOULD BE ONLY REMOVED IF THIS CIRCUIT OPERATING VOLTAGE IS BELOW 40V.

-YOU MUST NOT OPERATE THIS CIRCUIT ON MAINS POWER WITHOUT THE BALLAST AT ANY COST.

-I AM NOT RESPONSIBLE OF ANY OF YOUR ACTION(S) USING THIS SCHEMATIC.

-YOU SHOULD ALWAYS HAVE SOMEONE TO GIVE YOU FIRST AID JUST IN CASE SOMETHING BAD HAPPENS.

-YOU MUST NOT EVER ATTEMPT TO EXPERIMENT WITH TESLA COILS IF YOU HAVE ANY ELECTRONIC LIFE SUPPORT DEVICE IMPLANTED!

• 1 × 20A, Normally Closed Contact Relay The series resistance of the coil was 5 Ohm, wound with 1mm enamel wire.
• 10 × 10nF, 1250V Capacitors, MKP or FKP (FKP are preffered) I used MKS caps, but the losses were great...
• 1 × 40W Magnetic Ballast Any magnetic ballast from 25-100W, DO NOT USE AN ELECTRONIC BALLAST, THE VOLTAGE SPIKES WILL KILL IT!
• 1 × Helical Primary 2 turns of 1,5mm enamel wire. Diameter is 100mm
• 1 × Helical Secondary 1530 turns of 0,17mm enamel wire, form diameter is 55mm and coil length is 272mm .
• ### The End

Teodor Zafiroski05/29/2015 at 17:58 0 comments

The coil was finally finished, the results were good enough, so the project was donated to the school from where the components were originally sourced. They now have a nice demo coil which the teacher isn't afraid to operate!

• ### Getting Hot!

Teodor Zafiroski03/01/2015 at 19:54 0 comments

In the previous version, the bang energy was small, so the relay contacts were handling it easily, but with the major update, the relay contacts are getting red-hot after 3-4 minutes of operation. Still, that is completely acceptable for a demonstration coil...

• ### A Quick Note

Teodor Zafiroski08/14/2014 at 12:43 0 comments

If you operate this circuit on DC, it will work only a few relay cycles, resulting in a short, but powerful voltage burst at the topload, and then suddenly stops, I don't know why...

• 1
Step 1

Making The Stand

Bend a square piece of steel on both sides (90 degrees) about 2-3cm down. Mark the placement of the components and drill holes for mounting them...

• 2
Step 2

Making The Relay

A good enough relay for this design is hard to find, so I will make instructions on how to make one:

-You will need:

-Tungsten rod, around 3mm diameter

- Coil form

- Enamel wire 0,25mm diameter, around 400-500g.

- Magnetic core (Any ferromagnetic material, it needs to fit snugly in the coil form)

- A flexible piece of metal for the moving relay contact

- Various mounting parts

- A schematic (This is a photo from a local magasine, it shows how to make a suitable relay for a Ignition coil tester...)

The coil needs to be wound with around 4500 turns of 0,25mm diameter enamel wire. If you decide on using this design, you should feed the power to this circuit full wave rectified, without a filter cap, and because the relay has such a large inductance, it will limit the current, so the ballast could be removed from the design...

• 3
Step 3

Making The Primary

If you don't have a finished primary coil at hand, you can always make one. You Need:

- 4x PP (polypropylene, 1cm diameter, 4cm long) tubes

-1,5mm thick enamel copper wire.

- A power drill

You need to drill 2 holes through each of the tubes, each spaced 4-5mm from the next. then, you need to mount them to form a square on the stand, if one edge of the square is 10cm.

Then, here comes the hard part... You need to get the wire inside the holes, carefully creating a helical figure, when you get past that, you are finished with the primary coil!

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## Discussions

Bobby John wrote 03/15/2019 at 05:56 point

This design is not unique. Mechanically driven Tesla coils were used in violet ray devices in the early 20th century.

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cschrisstarke wrote 03/02/2015 at 13:54 point

Im looking for a source of the capacitors can you per chance share where you got yours from?

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cschrisstarke wrote 03/02/2015 at 18:31 point

So found some suitable caps,  now I'm looking for a suitable relay

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[deleted]

[this comment has been deleted]

cschrisstarke wrote 03/02/2015 at 19:43 point

I was hoping to use these caps http://za.rs-online.com/web/p/polypropylene-film-capacitors/1908602/ as for the relay ill look a bit more whats available locally

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Teodor Zafiroski wrote 03/02/2015 at 21:26 point

For capacitors in a Tesla coil, you should consider these recomended characteristics of a single capacitor:

- Polypropylene or Mica dielectric

- Foil electrodes (Film-foil types are the best because they have high surge currents and a self healing capability)

- High dV/dT rating (minimum 1000V/us)

- High RMS current capability (minimum 8-10A)

- High peak current rating (minimum 200-300A)

Also, I forgot to mention- where do you live (what is your nominal mains voltage), because if your mains voltage is 230V, I'm not sure that these caps are suitable.

If your peak current rating per cap is smaller than my recommendations, when you connect them in parallel these values sum up...

I will make a tutorial on how to make a suitable relay for this application...

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Teodor Zafiroski wrote 03/02/2015 at 19:24 point
What caps have you got? Also, my relay dates back from the 60s and has tungsten contacts, any relay with sufficiently low coil resistance could work if you reduced the rated power of the ballast or had enough cooling of the contacts... Best option is to make your own though, because a relay with a coil resistance so small is incredibly rare...

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J Groff wrote 08/14/2014 at 15:33 point
Ok, Sir, you are now exactly half correct. Its a Tesla coil if you use AC. Using DC automatically makes it an oversized automotive ignition coil. You do have an LC circuit, I stand erected on that point, and I believe your design is analogous to the rotary spark gaps that made 1930's monster movies so cool, albeit far less efficient and reliable as it has noneof the nifty cooling and plasma suppression features. My guess is that you are using a sealed relay and cant see whats happening to the contacts but my guess is that using AC the poles flutter between contacts creating more of a traditional spark gap but on DC they are driven hard against the contacts, fusing them thusly. Anyway, I appreciate your effort, every science fair has to have at least one Tesla coil.

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Teodor Zafiroski wrote 08/14/2014 at 19:18 point
The relay is not sealed, the hardness of the contacts can be adjusted by a screw. When I built this, cooling it wasn't necessary because I was using a 15V, ~50W U core transformer from an old "Ei Niš" TV set..

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K.C. Lee wrote 08/16/2014 at 01:56 point
In the LC tank, there will be AC by definition! AT each cycle of the relay switching, the cap is charged and subsequently connected to the coil.
There would be an oscillation at its resonant frequency as energy is converting between stored energy inside a cap into magnetic field. Magnetic field collapses and energy goes back to cap. The amplitude exponentially decays as there are losses and energy transfered to secondary side.

There is a 2nd tank circuit from the secondary coil and parasitic capacitance (between windings and most importantly between the top load and ground). It is here where the amplitude of a Tesla coil builds up.

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K.C. Lee wrote 08/16/2014 at 01:57 point
http://en.wikipedia.org/wiki/LC_circuit
Watch the changing direction of the current flow in the animated figure on the right hand side.

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J Groff wrote 08/13/2014 at 16:31 point
Sorry, this is not a Tesla coil, its a spark generator. No resonant tank circuit = (!Tesla coil)

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Teodor Zafiroski wrote 08/13/2014 at 19:24 point

This is a Tesla coil, there is a tank circuit (the 100nF MMC and the primary coil), also, the secondary is in no way connected to the primary!

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