My Site, My Stuff

Tesla Coil 1 – SGTC

Nikola Tesla

Firstly, this site was not intended to introduce you to the scientist and inventor by the name of Nikola Tesla (pictured left), so I have not bothered to add an introduction to him, if you want to find out more about this great man, try a web search on his name, you really will be amazed at how our lives are affected by his inventions.

Secondly, Tesla Coils by their very nature are potentially deadly pieces of apparatus, both to living beings and other electrical appliances !. They are easily capable of generating high frequency, high voltage discharges in the range of 1 to 20+ feet !!. If struck by one of these discharges, you can suffer serious RF burns to the inside of the body which can take surgery and a very long time to heal, you may not feel it because of the high frequency, but the damage will be done.

They are definitely not toys and should only be constructed by people with at least some knowledge of electricity, high voltages and spark discharges. If you choose to copy any of my design solutions, you do so entirely at your own risk and must accept the consequences of doing so.

The design

The Spark-Gap Tesla Coil.

This was my first coil, and required a fair bit of research before building it. It follows the same principles as Tesla’s original coils and turned out to be very impressive when running.

The spark gap acts as a very high power, high voltage switch which triggers itself as the voltage across it rises to it’s breakdown point, it does this around 50 times a second. Each time it fires, it discharges the stored energy in the tank capacitor into the primary coil. It is this rapid pulsing which sets up the high voltage gain in the secondary coil. This is a very simplified description as the full operation is quite complex. To stand a fair chance of it working, I used new components wherever possible and did as much testing as possible before connecting everything together.

Following lengthy research on the web and with the kind help of the members of Tesla Coil Builders of the UK, I managed to conjure up a reasonably easy to build design using readily available parts.

+++Primary Coil+++

12 turns 8mm copper tube
Outer Diameter – 606mm
Inductance – 0 to 64uH

+++Secondary Coil+++

Core material – 110mm grey pvc pipe
Wire – 0.5mm enammelled copper
Winding height – 550mm
Overall height – 650mm
Turns – 1000
Inductance – 19.865mH
Capacitance – 8.394pF
Resonant frequency – 239kHz
Insulation – 3 heavy coats of polyurethane varnish

+++Power Supply+++

10kV 50mA Neon sign transformer
Standard line filter at transformer supply, earthed to RF ground
Power factor correction – 25uF 440v
275v MOV accross supply
Custom R/C filter in HT lines to main spark gap
Safety spark gap across transformer output, centre electrode to RF ground

+++MMC Tank Capacitor+++

Value – 16.7nF at 21kv
Made up of 5 strings of 14 * 0.047uF 1500v
polypropylene capacitors

+++Control Box+++

Standard line filter at input, earthed to supply ground
275v MOV across supply outputs
Variac power control
Input ‘Live’ and ‘Ready to fire’ warning lamps
Output voltage and current meters
Key operated safety switch
Two simultaneous press-to-fire buttons and a long extension lead !!

The primary coil

It’s usually best to start at the bottom.

The Base

The base assembly prior to varnishing etc. The coil supports are made from HDPE (chopping boards) and were cut on a table saw which worked quite well. I fixed them to the MDF base using a hot-melt glue gun. The grey socket in the middle is the end of my coil former pipe with the rubber ring seal removed to make assembly easier.

The coil

The finished primary coil. The base had three coats of varnish. The copper pipe is held in place by miniature nylon cable ties, there are no metal fixtures at all in this area.

The secondary coil

Secondary being wound

The freshly wound secondary coil. The coil former is 110mm pvc soil pipe, dried, sanded, varnished, sanded, wound and varnished three more times. As usual, the winding jig is made up of junk box stuff, the motor was a little too slow, at only 9rpm, it took far too long although it did give an excellent result. The dining room table makes a great workbench !

Coils Assembled

The secondary and primary coils assembled.

The tank capacitor


Front view, 70 x 0.47uf 1500v caps in five strings of 14 to give a value of 16.7nf at 21,000v. Looks like a chopping board, but really it’s a high voltage circuit board !


Back view, 70 x 10M 0.5W hv resistors to discharge the possibly lethal power stored after switching off. The row-connecting ‘bus bars’ are 2.5mm solid copper cable.

20/03/2004 – Amendment – Following some more research, I have now doubled the size of the bus bars by soldering another 2.5mm wire along the existing ones. Although only for a very brief period, the currents flowing during discharge can easily reach 100A and any resistance equals lost power – you need to dump every ounce of power into the primary instantly.

The HV input filter

Input Filter

Input filter Mk1, on the left is my adjustable safety gap with the centre electrode grounded. Top and bottom are two 470 ohm 30W wire-wound resistors in series on each 5kV leg, the small blue blobs are 1000pF 3kV caps in a mini MMC setup giving a total of 750pF at 12kV to ground on each supply leg.

I have a feeling that i’ll be letting the smoke out of this board first !! (in a scientifically controlled manner of course)

The power supply and spark-gap board…

Power board

Pictured above is my completed power supply board. The input filter and safety-gap board sits on top of the neon sign transformer. The small grey box (lower left corner) is the supply junction box and also houses the second mains line filter and MOV. The blue tube is the 25uF power factor correction capacitor. Front center is the main RF earthing bar and on the right is the main spark gap (detailed below).

Spark Gap

The main spark gap. The tube is 110mm pvc about 200mm long. The pipes are 15mm x 75mm copper with 0.5mm gaps between each. The bolts sticking out of the top are the variable tapping points for altering the power output of the coil. At present, it runs best at the highest setting but one. At the rear of the tube is the quenching / cooling fan.

I believe this type of gap is named after it’s creator, Richard Quick and are usually just called ‘RQ’ style units.

The control box

This is my finished control box. It is built into a standard toolbox which was the cheapest way of making a safe and easily portable unit.

Control box
Neat and tidy

I had a neat wooden insert made for me which slots into the toolbox. The front panel contains the variac for power control, output voltage and current meters, input and output warning neons, a key-switch and the two fire buttons. The connecting cables fit neatly into the end compartment.

The first top-load design

Top-load fitted

The first top-load sitting atop of my coil, it will eventually be smoothed out with aluminium foil tape. I have now added the strike rail above the primary coil to give a bit more protection against stray arcs. The secondary coil can be raised about 35mm above its lowest position for tuning purposes.

The first test – “First light”

It was cold, damp and trying to rain, perfect weather for mucking about with high voltages outdoors ! So it was on with the warm clothing and out into the garden. Due to the unpredictable nature of an untested Tesla Coil, you can’t really test it indoors because of the high risk of fire etc.

The components were connected with high quality silver plated oxygen free copper loudspeaker cable ! It uses twisted multi strand cores that are then twisted into a thicker core and has a very heavy covering so it seemed a good idea at the time, and I had it in the junk-box.

The wiring was thoroughly checked and an earthed discharge wire was pointed at the top-load about 75mm away. The earthed points of the coil were connected to two 1.2m solid copper ground rods driven into the garden about 6 feet apart and connected with 8sq mm copper cable. You should never use the mains supply earth as it was not designed for the type of discharge that these coils put out and can be extremely hazardous.

I retreated to the safety of my kitchen, checked that the variac(power control) was at zero and turned the power on. I very carefully increased the power and things started happening much sooner than was expected ! I had only reached about 30 volts input and there were very loud sparks jumping across from the topload to the earth probe !!

This indicates two things, one – the coil works, and two – the primary charging circuit is resonant. This is the reason for the seemingly high efficiency but it can be very damaging because it can cause massive over-voltage damage to the tank capacitor and the supply transformer.

To counter the voltage increase caused by this resonance, I am now building in an overvoltage protection device on the capacitor board. The idea is that this large increase in circuit voltage is ok as long as it does not get higher than the design voltage of the components. This is taken care of (hopefully) by safety spark gaps in the power supply and across the capacitor. If the voltage rises too far, one or both of the gaps will fire and discharge the circuit, protecting the components.

I think the first test was very successful and very reassuring being my first coil. At 50 volts input, the spark was jumping about 250mm, not so loud but very encouraging. I am now finishing construction of the control box and base of the coil so that further testing can be done a bit easier.

The second test

The weather was not much better than it was when I first tested my coil, I joined it all together again, retreated to my newly finished control box and pressed the ‘go’ button.

Nothing happened ! After a bit of head scratching, I realised I had wired it up a bit on the wrong side of correct ! A touch of intense fiddling later, all was well and it was running as it was before.

This time, following a few improvements made over the Christmas break, I was a bit bolder and cranked up the power to maximum. It made a bit more noise and nothing smoked so I decided to start opening up the main spark gap, step by step from its present minimum setting. This has the effect of letting the tank capacitor bank charge to a higher voltage and this increases the output voltage from the coil. After opening it to about 50% of maximum, the coil was beginning to really come to life!


I was getting heavy 18-20″ arcs to my ground wire and there was a healthy blue corona glow all around the topload. The noise made by the arcs was beginning to reach the levels where the neighbours were twitching their curtains so I limited it to very short runs.

It really does make a lot of noise!, I was surprised as I had never heard a sound like this before, it really does have to heard to be believed. Anyway, the weather was changing to wet, so it was hurriedly stripped down and returned to the warm again.

Another success! On with the fiddling…

Further testing

After a bit of reasearch and a few more inter-club-member emails, I tried tuning my coil with an oscilloscope and signal generator to see if it made any difference to the output. The theory is to measure the resonant frequency of the secondary coil and topload and then, using this frequency, find the best tapping point on the primary to connect the power tap into.

It turns out that my coil resonates at 238kHz give or take a few hertz, and before building it, I calculated it should be 239kHz which is a very close match from theory to practice.

So off to the garden we went…

The weather was nice, dry and cold with no wind – perfect !. Following the installation of my safety spark gaps, three support legs and a few other minor tweaks, this time I was having it large! I opened up the main spark gap fully, set the input to 125v and pushed the fire buttons.

Not bad

The result was amazing, even at only half input power, opening the spark gap had increased the output dramatically. The streamers were crackling all around the topload and were about 6-8″ long, bringing in my earthed probe, I could draw lovely fat arcs about 20-25″ and it was still only at half power ! I turned up the input to 240v and hesitantly pressed the buttons… Wow !


The streamers grew to about 30″ and were dancing wildly about looking for a target. It was totally amazing, they struck the garden table, the back door and the fence repeatedly. I was a very happy camper ! On the other hand, the noise was now deafening, it echoed around the gardens and quite a few curtains were twitching – oh dear !

Even better

I reduced the power to half way again to keep the noise down a bit and proceeded with some serious technical experiments…

1 – We tried laying a cut flower across the top and brought in the earth rod again, 15 seconds of arcing later and the flower was a smoking wreck, conclusion – I had enough power to roast a flower !
2 – The latest release of AOL browser cd was stood on top and the buttons pressed, conclusion – I had found a use for these discs at last !
3 – A large, empty saucepan was balanced upside down on top, result – it seemed to increase the arc length, especially from the handle !, conclusion – I will probably be trying a larger topload soon as it seems to help.
4 – A 60w clear bulb was put on top, result – it lit up with a plasma globe type of glow, conclusion – will definately be doing this again, also try a flourescent tube next time.

That enough noise for the neighbours this time, also the ozone stinks !!

On display to the public

Yes, there really are public meetings for odd people with Tesla coils!

The Nottingham Gaussfest. (This links to a site run by one of the leading forces in the UK coiling scene, Derek Woodroffe)

It was a great day out and the coil performed well, here are the pictures of my coil performing at the event…

All set up
A youthful me pushing the buttons

It’s a great way to meet other coil builders, the variety of high voltage stuff was great too. I only attended one event to display but did return as a spectator a couple of times before changing hobbies.