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200 watt spark gap Tesla Coil project

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Teslahed

Mon Mar 13 2017, 07:58PM

Registered Member #61513 Joined: Mon Mar 13 2017, 10:43AM
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Posts: 4

Last weekend I had the chance to attend the Nottingham 'Gaussfest' Teslathon event. There was an impressive selection of high voltage equipment on display and being demonstrated. I took a small Tesla coil I'd built the previous week with me. Since returning from the event I've been putting a lot of effort into neatening my little Tesla coil up. I've been 3D printing as many components as possible for it as well, just so that I didn't waste my money on that 3D printer I bought. I thought I'd do a write up of the project in case anyone is interested.

http://www.extremeelectronics.co.uk/nottingham-gaussfest/

Here's the video of the event in Nottingham;

http://youtu.be/fTw6eyZH19o

And here is the write up of my Tesla coil project. I wrote this as a general outline of my project for people that might have some electrical knowledge but not necessarily any Tesla coil experience, so some of the details are probably unnecessary on this forum;

Warning

Tesla coils are potentially dangerous. Even a small Tesla coil like the one I give details of below could possibly kill someone if it struck them, but the high voltage capacitor bank could definitely kill someone if they managed to discharge that across themselves. I've given myself worse injuries with multi-rotors than I've ever managed with a Tesla coil but both deserve caution and respect and both could send you to hospital or worse if you made a bad enough mistake.

Tesla Coil

A Tesla coil is an air cored resonance transformer capable of producing high frequencies and high voltages. They produce visible electric discharges from the high voltage terminal which can be quite impressive. Tesla coils were invented by Nikola Tesla in the late 19th century and used as radio transmitters and for high voltage research until better equipment came along. More recently they were used for special effects in movies and on stage until CGI mostly replaced them there too. Small Tesla coils are still used in industry for electrical insulation testing.

The main reason to build a Tesla coil these days is as a fun electrical project and to use one for giving short demonstrations for educational purposes and similar. They are more interesting than useful.

Theory

A basic Tesla coil consists of two separate inductor (L) and capacitor (C) circuits tuned to run at the same frequency. The inductors make up the primary and secondary coils of an air cored transformer so there is a voltage rise from the ratio of turns used. Because both LC circuits are tuned to the same high (RF) frequency there is also a resonance rise effect which further increases the voltage. If the tesla coil had a metal core like a regular transformer then it wouldn't oscillate at such a high frequency and it wouldn't produce such high voltages and frequencies in the way that it does.

In the circuit diagram above both LC circuits are to the right of the spark gap. In the first LC circuit the high voltage capacitor charges up from the high voltage power supply until the voltage across that and the spark gap is high enough for the gap to break down and spark. When the gap sparks it switches 'on' and the capacitor and inductor (Tesla coil primary) are connected in parallel. This LC circuit then oscillates at the frequency it's tuned at until all the energy has died away and the spark gap opens again as the voltage drops and the spark dies. Spark gaps work very well as simple high power high frequency switches like this. The capacitor then charges up again for the cycle to repeat many times a second.

The second LC circuit is less obvious. The Inductor is clearly the Tesla coil's secondary coil. The capacitor is the spherical or toroidal metal object placed on top of the coil. This acts as one plate of a capacitor. The other plate of the capacitor is the ground connected to the other end of the secondary coil. The insulator between these two plates is the air itself. The amount of capacitance generated in this fashion is very low but it still counts as part of an LC circuit.

This means that the tesla coil's output will run at the frequency defined by the tuning of the LC circuits but that frequency will be modulated at the frequency at which the spark gap fires. I have calculated that my Tesla coil runs at about 400khz.

This is the basic circuit that Nikola Tesla would have used back in the 19th century. More modern designs of Tesla coil exist that use vacuum tubes or high powered transistors or mosfets to achieve the same high frequency high power switching that the spark gap provides in the original version. Spark gap Tesla coils tend to be less efficient than well designed solid state or vacuum tube coils because of the losses in the spark gap, but they are much easier to construct and harder to blow up.

My Tesla coil

Recently I decided to build myself a small Tesla coil. My aim was to build something of a moderate size that could be used for demonstration purposes without too much danger and which would be portable and easy to set up.

Components

The high voltage power supply.

Getting hold of a high voltage power supply used to be tricky. These days it's pretty easy thanks to ebay and the Chinese; you can buy a Zero Volt Switching device for Â£15 that's designed to run an induction heater that can also be used to drive a TV flyback transformer to provide about 15KV at 12mA (150 to 200 watt output). This is enough to power a small Tesla coil.

I won't go into detail on the ZVS device; just check this website out;

http://adammunich.com/zvs-driver/

and search ebay for 'ZVS Tesla coil flyback driver'

They are fairly simple to use and can be run from 12 to about 36 volts DC input. I'm running mine from a 6 cell 10,000mAh LiPo I had lying around so it's getting 24 volts and seems fine. It doesn't heat up. Running the circuit from a battery also has the bonus of helping separate the Tesla coil from the mains supply cutting down on RF interference problems.

I mounted mine on a cheap plastic breadboard along with a switch and 3D printed a cover for the electronics, then attached everything using M3 nuts and bolts.

Spark Gap

I used short sections of 24mm diameter copper pipe to construct my spark gap. Using a larger number of smaller gaps in series instead of one big gap helps spread out the heat and makes the spark gap function better. I 3d printed a support piece that holds the copper pipes the correct distance apart (0.6mm) with holes for cable ties. This means I can tune the spark gap to different voltages by varying the number of gaps I connect up. I use crocodile clips on the wires that terminate at the spark gap as these grip the copper pipes very well and make adjusting the gap easy.

High voltage capacitor bank

Getting hold of capacitors rated to 15KV is tricky. I managed to find a load of 1.6KV rated capacitors that I put into strings of 13 to get 20KV total. I then stuck 3 strings in parallel to get 15nF of capacitance. Each capacitor has a 2.8kohm resistor across it to ensure that the capacitor bank slowly self discharges. This should have no effect at the frequencies at which the coil operates but should mean that if you touch the Tesla coil circuitry after you've switched the coil off you won't get a very nasty shock from all the capacitors discharging.

I mounted the capacitors to another breadboard using cable ties running through holes drilled in the board.

I drilled holes in the corners of all 3 of the breadboards and used some M12 plastic bolts and spacers which I printed to put everything into a stack.

Primary Coil

The primary coil needs to have enough turns to tune your primary LC circuit to your secondary LC circuit. As you have much more capacitance in your primary LC circuit you need much less inductance to tune to the secondary where you've got much less capacitance but much more inductance. You also need to be able to vary the number of turns on the primary coil to fine tune it to get the best performance out of your Tesla coil.

I 3d printed a coil former in several parts that I use to wind 2mm diameter copper pipe onto. I can use crocodile clips to connect up different numbers of turns and am currently finding 5 works best.

Secondary coil

The secondary coil is made from 28awg copper enamelled wire wound onto 9cm diameter ABS pipe. It measures 9cm diameter by 32.5 cm long and consists of about 800 turns. I wound it over the course of several days using a jig that held the pipe horizontally and allowed me to turn it slowly by hand. Afterwards I varnished it with many layers of polyurethane varnish to add insulation and physical protection to the wires. I've measured the inductance at about 15mH. I made the thing about 18 years ago for an older Tesla coil project and have had it sitting around ever since.

Top Load

I managed to find a 15cm metal sphere on ebay for about Â£10 that was designed as a garden ornament. It seems to work very well as a discharge terminal.

Results;

The finished Tesla coil;

The next step in the project is to find a nice enclosed box with a handle that I can stick the stack of electrical components into. This would help make everything safer and more portable. Other than that it's pretty much done.

Here are a couple of videos of the coil running;

http://youtu.be/kW7m7EwWPDU

http://youtu.be/msgyQQGdSiw

Please note the second video was recorded before I neatened up the electrical components. I'll get another video soon.

Here are some time lapse photos of the coil running. Because the shutter was open for half a second or longer it produces a very different effect than the video.

I estimate that I'm getting just over 20cm arcs from the coil. I might be able to increase this a little with further tuning. For a fairly small and simple Tesla coil operating from a maximum of 200 watts this is quite a good level of performance.

For sake of comparison here is an older Tesla coil I built. This one looks a bit more impressive but runs from 1KW of power instead of 200 watts. It wouldn't be anywhere near as portable or easy to demonstrate with;

http://youtu.be/JWnVtO6wUR0

Teslahed

Wed Mar 15 2017, 08:58PM

Registered Member #61513 Joined: Mon Mar 13 2017, 10:43AM
Location:
Posts: 4

I would like to try to run a Tesla coil directly from a ZVS driver without using a spark gap. I believe it's possible but have so far failed to find any detailed information on the subject. I think it involved replacing the capacitors on the ZVS board with smaller ones to make that oscillate at higher frequencies.
If anyone knows more I'd love to hear about it.

dexter

Wed Mar 15 2017, 11:28PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195

Teslahed wrote ...

I would like to try to run a Tesla coil directly from a ZVS driver without using a spark gap. I believe it's possible but have so far failed to find any detailed information on the subject. I think it involved replacing the capacitors on the ZVS board with smaller ones to make that oscillate at higher frequencies.
If anyone knows more I'd love to hear about it.

yes it is doable drive the TC primary coil directly from the zvs but is harder to tune because you can only adjust the tank capacitor

Teslahed

Sun Mar 19 2017, 07:40AM

Registered Member #61513 Joined: Mon Mar 13 2017, 10:43AM
Location:
Posts: 4

When using a ZVS to drive a tesla coil primary directly, why can't you vary the number of turns on the primary to drive the coil at different frequencies the same as normal? Does the ZVS device only work with exactly 6 turns on the primary? I thought if I got the capacitance value close I could then just tune the primary a bit.

And when you talk about the 'tank capacitor' in terms of the no spark gap ZVS driven tesla coil, do you mean the capacitors associated with the ZVS driver itself? That's been my assumption so far, which is why I removed the surface mount capacitors from my ebay purchased ZVS driver and replaced them with lower capacitance capacitors to up the frequency.

I guess the best way to do it in that case would be to build a capacitor bank with lots of taps around the correct value such that that + 6 turns on the primary = the secondary coil frequency.

I had the Tesla coil round my mates house the other night. He's a ham radio enthusiast and tells me my coil puts out interference at 465khz when I've got my shiny new metal toroid on top, which is pretty close to what the Tesla coil calculator I used predicted.

Teslahed

Tue Apr 18 2017, 09:43PM

Registered Member #61513 Joined: Mon Mar 13 2017, 10:43AM
Location:
Posts: 4

I blew my flyback up by putting too many volts through it (the ZVS was fine) so I ended up rewinding it and building myself a DC rectifier / voltage doubler circuit for the output. I seem to be getting almost twice as much power out of the power supply as previously so I'm happy with the results. Here's a video;

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