The Tesla Coil
Click images to enlarge
After reading through a book entitled, Experiments with Alternating Current by Nikola Tesla, which was a lecture given before an audience in london in 1894, a book I purchased on ebay. I felt I need to try one of these for myself!
I began searching around the net for more info on contructing a tesla coil and after a few days came up with a basic design and specifications. I'm not saying this was a perfect design as I don't have a huge knowledge of these coils and as they don't act like normal induction tranformers I didn't have a lot of previous knowledge to go by.
A Tesla Coil is an Air-Cored Resonant Transformer. It uses longitudal waves of RF on a primary coil to induces electrical flow in a secondary coil. They don't rely on turns ratio to achieve big step-ups, but the secondary coil is required to be of a large diameter. In it's simplest form, a TC basically a radio transmitter and receiver in one unit. Being very close to the transmitter, the receiver coil is induced with a large voltage spike and with each wave of RF that hits it, it gains strength (like someone on a swing being pushed higher and higher, or Tesla made the analogy of a snowball rolling down a hill, with each revolution the ball gets bigger and bigger). The other point to note, is that a TC is a non-linear output, eg. The input to output ratio changes in an exponential way. This is in contrary to a normal induction coil.
I decided on a 3" secondary, purely because thats what I had available. A piece of PVC drainpipe serves as the secondary form, on which the secondary coil is wound. I used the excellent TeslaMap 5.7 program to work out the specs. I could have done it from basic formulas, but this was quicker!
My primary power source is an 12v automobile ignition coil (Lucas something) running from a 240v dimmer/capacitor circuit arrangement. I know NST (neon sign transformers) would be alot better, but are extremely hard to find round here! The ignition coil puts out about 10-20KV AC at around 5-10mA, guessing as I haven't been able to measure it (for some reason my meter shows only 1KV but it puts out 3" sparks!, I believe this to be because the output is high frequency and normal multimeters cannot measure this).
An RF filter for the mains is advised, as any noise you create from sparks, returns back on the household mains line and anything connected to it can be severly damaged.
To start with I cut a 350mm long peice of PVC drainpipe and coated it with PVU varnish 4 times. (As a side note, I have now found out that black pvc contains a small amount of carbon and can cause the hv to arc through the secondary form, so try to use white or grey pvc if possible). After letting it dry for 2 days it was time to wind the secondary.
To wind the secondary form I used a homemade lathe that I built quickly from an old cordless screwdriver motor and gearbox and some 10mm MDF. The motor was run from a bench power supply and I made an inline footpedal switch so I could control the lathe without letting go of the wire. I used 30SWG enamelled wire that I aquired from a friend who used to work with radar systems. The total winding time only took about 1hr 30mins. I went slowly to make sure that I kept the windings as close together as possible and not overlapping.
The secondary was then varnished a further 3 times. A good idea I found was when I was varnishing the form, I left the lathe running and a desk lamp overhead to help speed the drying process and keep a nice uniform varnish thickness. That was then left for another 2 days to dry.
The primary of my tesla coil is a flat archimedes spiral made from 8mm Copper gas tubing. I needed 10 meters for my perticular primary, leaving enough room for adjustment. The angle of incidence is 0°, this means that the coil lays flat on the wooden support and at a 90° angle to the secondary (a pancake tesla coil). You can get better inductance if you were to angle the spiral into a conical shape but that was not what I had based my calculations on this time. I wound the copper tubing around a 11cm form to get the intial spiral "pipe" pictured above as I thought this would make it easier to work with in forming the spiral. It looked quite impressive in this pipe form! but also quite heavy as this is 10 meters of copper tube!
The spiral was held down with TV coax clips in a 5 point star array as pictured above. The base is made from MDF and contains all the fittings that allow the secondary to be connected and disconnected to it in exaclty the same place everytime. To do this I used a plastic top that fits snuggly into the secondary form and the secondary ground connection is done with a bolt through the MDF and connects to an earthing cable. The primary tap cable is some HV flyback cable connected to a car battery clip and the other end is bolted to the MDF board. A plug leads to the power box. The ground from the primary coil goes through the MDF board and has a plug that goes into the power box.
The Whole primary and secondary coils sit ontop of a plastic storage box, so that it insulated from the ground. The connection to the power box; which sits in-front of it, are made by flying leads.
A ground connection from secondary coil to the earth was made with household earthing cable from a bolt through the MDF and out to a copper pipe that was sunk 1Mtr into the ground.
The Power Box
This is where most of the work is done in a tesla coil. My coil circuit is very basic compared to many others I have read about. It is made from common hardware store components such as 90° L brackets, 5mm bolt and PVU Chopping boards (a good HV insulator!). The boards are seperated by 1" PVC piping and rubber door stops.
The spark gaps are made from 5mm bolts, dome 5mm nuts and L brackets. The primary gap is set at about 6mm and the protection gap is 11mm.
On the input to the circuit from the ignition coil, I have added 2 RF chokes to try and limit the spikes returning to the ignition coil. These are simply 25 turns of 0.6mm wire on 1" PVC pipe. I'm not sure how effective these are, but the coil has survived so far.
As a side note, The earth from the secondary should never be connected to the mains earth as you can fry everything in your household!
The Capacitor bank
My capacitor bank is of the MMC (multi mini cap) type. This is when you series smaller rated caps together to get the required voltage rating. You can then parallel them together to get the required capacitance. I have used RMC .01uF 2-3Kv capacitors. These aren't perticularly good for this application as I believe they are meant for spike protection in computer networks. But never the less they work ok.
One other curious thing about these caps is that they have integral spark gaps (pictured on top of cap) that do not allow the cap to over charge. I have found that these sometimes fire when the coil is running so I assume the bank is seeing over 30Kv in spikes. I have fixed this problem though as I have put some plastic sheet between the spark gaps to prevent them firing.
It consists of 10 series .01uF cap and in a parallel of 3 configuration. This gives me a voltage rating of 25Kv (20-30Kv) and a capacitance of 3.5nF (0.0035uF). The formula for working this out is:
series 1/ct=1/C1 + 1/C2 + 1/C3...
I thought it best to include some bleed resistors in parallel to each capacitor to drain the charge when the system was turned off. I didn't fancy getting a shock from this lot! It is simply 3x 1M ohm (3M ohm total) resistors in series across each cap. So that the resistance is high enough not to allow the charge to pass it when the system is on but is enough to drain them when its off.
My toroid or capacitor is a peice of aluminium ducting from a car shop used to connected the hot box to the air filter. I have wrapped this in aluminium foil to get a semi smooth outside edge. The smoother the toroid, the less charge that leaks into the surrounding air. The secondary connection to the toroid is made with a small peice of aluminium foil and some sticky tape. Its held to the secondary form with some hot melt glue. The bottom centre plate of the toroid comes from an aluminium pie plate that I've cut to fit.
Tuning the coil is easy, as all you need to do is move the primary tap point and adjust your spark gaps to get the best output.
Well this is it, my recreation of one of Nikola Tesla's Coils. The output isn't amazing as of yet. The max arc length was about 4". This is probably due to my lack of a decent HV input transformer (anybody got one for sale?!). When I get a decent transformer I will set this up again and give it another go!
Bitten by the HV bug forever!!
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© Oliver Hunt 2006-2008