My first home-made 'Slayer Exciter' Tesla Coil!

Warning - Tesla Coils produce very high frequency RF energy at thousands of volts! This RF interference can cause electronic equipment such as touch screens or pacemakers to malfunction or even break! Please note that I am NOT an electronics expert, and I take NO responsibility for injury or death caused by recreating this circuit!

The Basics

For those who don’t know, a Tesla Coil is essentially a cylindrical air-core transformer consisting of a tall secondary winding, usually wound with hundreds of turns of very thin enamelled wire and a primary winding which surrounds the bottom of the secondary and consists of a few turns of much thicker wire. A high frequency oscillation of current in the primary at the resonant frequency of the secondary induces an extremely high voltage on the secondary resulting in impressive arcs, corona and streamers. Tesla coils can also be used as wireless ‘transmitters’ of power, exciting nearby neon or fluorescent lamps, they can even ‘resonate’ with other nearby coils.

Simply put, Tesla coils are spectacular, dangerous, noisy and very cool. And I have to make one.

Winding the Secondary

So, this Friday I decided to wind (by hand) my very first secondary coil for a Tesla Coil/Slayer Exciter project. The coil has approximately 650 turns (may be slightly less due to my poor maths and the enamelling) of 0.2mm enamelled copper wire I bought from eBay for just 99p on a 100m reel. The winding process took about 5 hours on and off.

Slayer Exciter secondary winding on vitamin tube

I managed to complete the secondary winding without the 0.2mm wire breaking, which is always a possibility when doing this by hand. It’s essential to wind the wire under tension to ensure the tightest fit to whatever you’re winding onto, in my case an old tube of effervescent multivitamins. I took my time and carefully rotated the secondary tube, feeding wire off the spool in the same turning direction, I wanted to minimise having to rework or twist the wire, as the more you do this with copper wire, it becomes stiff and more prone to snapping.

I used a liberal amount of glossy Scotch tape to secure the wire to the tube (making sure to leave a good amount of wire hanging loose for connecting to the circuit) and also to secure the wire when taking breaks - if it had come undone that would have been a nightmare scenario! Once the coil was wound I used yet more Scotch tape to completely surround the winding top to bottom, this protects the windings from moving around and somewhat prevents arcing on the coil. The Scotch tape is temporary, as I really would like to coat the secondary in a good coat of clear coat varnish just like the professional Tesla Coil builders use!

Finished Slayer Exciter secondary next to eBay Tesla coil

The above picture shows my hand-wound secondary next to my eBay mini Tesla coil kit that I soldered back in 2017. This time I will be scaling up, as well as having wound a coil myself, of course.

The Circuit

I knew that for my first test I would use a very simple ‘Slayer Exciter’ circuit, the easiest possible driver for small coils like mine, which tend to have resonant frequencies in the megahertz range. I would be using a 19v laptop switching power supply capable of delivering up to 3.4A. I desperately need a benchtop power supply.

Apparently, laptop/monitor SMPSs (Switch Mode Power Supplies) aren’t the best for powering Tesla Coils, as they can automatically limit the inrush current required by the circuit when we don’t want it to.

The schematic for the Slayer Exciter looks like this:

Slayer Exciter Tesla Coil schematic

I first tried a BD233 NPN transistor which ran very hot after just a few seconds. After running the coil for a further 20 seconds or so, it cut out - I had blown up the BD233, the transistor had gone short-circuit nearly melting my wires! I have quite a selection of NPN transistors and MOSFETs, will have to see which ones perform better and ideally don’t catch fire. I’ve seen some schematics which use a 2N2222A, I’m pretty sure if I used that in mine it would go up in flames instantly. I’m now running it on a BD241B, the arcs seem slightly better and hasn’t blown up yet!

The resistor, R1 in the schematic should be a 22kΩ resistor, although I’m currently using two 10ks.

Notice the LED connected with its cathode to the base of the transistor and the anode to ground? This might seem odd, but is essential for the working of the circuit as well as indicating that the secondary is resonating. After some further reading, I replaced the LED with a fast switching diode, in my case a MUR820, but should ideally be a Schottky type. The purpose of the diode is to clamp the base of the transistor to a negative voltage.

I tried putting a 680μF capacitor across the positive and negative of the supply and was surprised to see a signifcant improvement in the quality and size of the arcs. Does this indicate my power supply can’t sustain a steady voltage when powering the circuit?

I forgot to mention, my primary winding was 3 turns of 20 AWG stranded wire I salvaged from an old PSU, I wonder if I can get better results with a larger or smaller gauge, but 3 turns did seem to be the best number of turns for my primary in this instance, whereas my eBay coil has just one turn.

As you can see, the coil works and the arcs, while not spectacular, are definitely visible and much bigger and nicer than my eBay coil. I love how holding a wire next to the makeshift ‘topload’ appears to show the arc emitting from the wire itself, even though it isn’t connected to anything. Placing a screw on top of the topload resulted in a needle-like, thin arc pointing directly up from the tip of the screw. The trick of lighting a CFL or neon bulb works really well with this coil and is always an amusing trick for people unfamiliar with how Tesla coils work.

I’m wondering where to go from here. Perhaps I’ll look into building a half-wave type circuit next, although can that be done with a small coil like mine? Needless to say, I’ve been bitten by the high voltage bug and want to do a lot more with Tesla coils! For now I’ve got a lot more learning and reading to do.

Further reading & watching