Wednesday, June 25, 2014

Tesla Coil

Any one who does electronics should know about Tesla and if you are into high voltage stuff then there is no escaping the Tesla Coil!
On Christmas break 2010 I was tidying up my room when I stumbled upon my second Tesla coil (my first being a micro coil on a Smarties tube) made from a giant Smarties tube. Suffice to say they were both pretty much a failure...

Misaka is not impressed...

So I decided to build a real Tesla coil! Well, actually it was on my mind for a while, I even managed to pick up a used Hameg HM203-6 20Mhz scope for 125€ (Yes, that is cheap. The lowest price I had found for a second hand scope...) to really help to tune the tank circuit.
After doing much research to properly understand the theory of opperation and minimum design requirements I chose to use JavaTC to do most of the calculations and ended up with a flat primary design to keep things simple. The coil has:
-A 901 turn secondary wound with 0.05mm wire and a lenght of 50cm and a diameter of 9cm.
-A 12 turn primary wound with 6mm wire and an inner diameter of 15cm and outer diameter of 32cm.
-A spherical topload with a diameter of 28cm.
-A 20nF 15kV MMC bank.
-A ballasted quad microwave oven transformer stack (aprox 5400W).
-A static spark gap with 5 18mm diameter pipes with a 4mm spacing in between and another adjustable gap.

The tesla coil is still not finished due to lack of time and funds but here is the proces so far!
From the start I was in a rush, xmas break is only for about 2 weeks and I had plenty of other commitments apart from tinkering with copper! There was no way I would be winding the secondary by hand so I used some wood and some skate wheels in much need of being changed to build a winding jig. I added a DC motor from an HP printer and a model train set power supply. I was crude and build in about 30 minutes. Suprinsingly it's still in one piece somewhere.

The PVC pipe used as the form for the coil got a good cleaning followed by lots of sanding to remove all traces of ink, dirt and other unwanted grime followed again by more washing and drying.
I needed a way to keep track of how many turns I had done so I resorted to the calculator method of "1+1="... on a computer.

The calculators I had either were too simple for this method, were not mine or good quality devices that I would rather not modify. So I cracked open a USB keyboard and traced the Intro key and wired up a magnetic reed switch:

The reed switches are the little black components on the PCB on the winding jig and come from a nice big DMX 2000W color strobe. Just add magnets!

The magnet (fragment) is from a hard drive and works very well with the reed switch. After a while and a few interruptions the coil started to take shape. It's important to do the coil properly and "save" your work with a bit of tape so that it doesn't completly unwind!

After much winding the secondary coil is finished. The coil is designed to be modular so the secondary can be simply droped into the primary so it is easier to transport. A stip of metal tape forms the ground contatc under the primary.

To make the secondary look even better and to stop humidity getting into it I painted it with some varnish. It also stops the coil from unwinding. To stop the varnish from sagging while it dried I applied the coats on the jig. I followed the instructions for the varnish but... It didn't dry properly!! ARRGGGH!!!

Here is a mockup of how the Tesla coil should look, notice how the top load is made from two Ikea bowls placed on the secondary.

The first low power tests were nothing impressive, but hey, no tuning at all!

The coil was being powered by a F.A.R.T neon sign transformer (Yes, they are a company named fart) that I got for free, sadly it was not fully functional but hey, it was free!

After much tweaking I found out that the topload was to big for my home made capacitor (the white box poking out the side of the cabinet) and that my capacitor was too small. Notice the color of the primary coil, it's not copper but steel cable witch is not very good for high frequency and high power jobs. Basically my coil was severly underpowered, the transformer, the capacitor, the tunning, the spark gap, the primary... nothing was perfect! I built an MMC with 10 1500V 100nF capacitors and got a slight improvement.

I needed more power. I wanted more power. More power is always better!!

In comes the 5400W MOT stack. It has a tendency to BURST INTO FLAMES when least expected. I need to get it under oil and improve the wiring. It is ballasted with a giant 2200W MOT with a shorted secondary. 16A breakers don't play nice with a 5400W Tesla coil, I need to find a way to soft start this beast and remove the ballast...

Next up is a better sparkgap with a more uniform separation of the electrodes plus an adjustable sparkgap (I can adjust it with a 3m wooden stick with a screwdriver on the end). It also has a hair drier blowind air over it.

A better MMC was also built, now it has a total capacitance of 20nF. I am still really pushing the 15kV rating... The plan was to build 4 strings of 15 capacitors for 26.5nF and 22kV but... money.

The primary coil got a major upgrade from steel cable to copper cable that was donated from a local workshop. The primary runs a bit cooler and that translates to less power loss!
The supports for the primary (and for the MMC) are made from 5mm acrylic sheets. I got these from a truckload of Point Of Sale units, they are the backlight difuser screen. Easy to work with and very strong.

The top load was also improved with a proper mounting method so I can take it apart easily. Made with more acrylic. Maybe the top load is too high now...

And that is my Tesla Coil so far. I hope to tune it better and get some decent wiring in place (flimsy alligator leads don't work well, they like to melt).

I also purchased a small AC motor from a scrap yard to try and convert it to a synchronous motor for use in a rotating spark gap.

This meant grinding two flats on the rotor. Grind too little and it's not synchronous, grind too much and it will overheat...

I managed to get it to rotate synchronously but the motor just got too hot to use. So that is another thing to improve!

So, how did these improvements affect the Tesla Coil? Quite well actually!! I believe there is plenty of room for improvement, I am getting 70cm long streamers and I am sure that could be improved to up to 90cm with some care.

The top load is provably too high and causes discharges from the secondary, these could be disastrous for the Tesla Coil and are also a safety hazard to the user if it strikes the primary coil. Another addition should be a ground ring just above the primary coil to help prevent secondary to primary strikes.
I honestly have no idea what it was striking... It never striked the grounded rod near the coil nor the step ladders on the wall but liked to strike the wall. Radio frequency high voltage is very hard to predict, so take speciall care around Tesla Coils!

In my case I don't just discharge the MMC bank, but also the top load (holds a decent charge) and the secondary coil (OUCH!). The varnish coating strores a tremendous amount of static charge wich I found out the hard way. Trust nothing, short everything!

1 comment:

igor Tironi said...

The charge escaping for a lateral and a top of your tesla coil in the same instant, this causes loss in efficiency. Attach something Sharp To improve the efficiency of it.