Making the Second Tesla Secondary Coil

Well here we go with the build of the replacement secondary coil. Rather than jump straight in and build an identical secondary with thicker gauge wire and hence less turns I decided to do a little more homework on secondary coil designs. One thing had always concerned me with the design of the first secondary I built. This was the termination of the ends of the coil inside the secondary form. The top and bottom copper tails pass through small holes drilled in the wall of the secondary form and make electrical connection through the top and bottom end caps.

The pic above shows the top electrical connection on the old secondary coil. You can see the copper tail clamped into the lug underneath the perspex cap. A very neat set-up but nearly every knowledgeable Tesla site I have seen strongly advises against this. The only one who seems to prefer this method is Alan at teslastuff.com who has had great success with this design. My final decision on the design of the new coil was strongly influenced by a page at www.capturedlightning.org. Stefan really seems to know his stuff when it comes to design and build of secondary coils so I decided to proceed using many of his recommendations.
The overall length and diameter of the new coil form was the same and again it would be made from 150mm diameter 3mm thick clear acrylic tubing with top and bottom caps in 15mm thick clear acrylic. The new secondary will mount to the existing 8 mount holes in the tesla base unit so the secondary base cap is an exact copy of the original. If you want to see the original post showing construction of this you can find it here.

The pic above shows the new secondary coil form with the top and bottom caps in place.

Here is a close-up of the bottom cap of the secondary showing the 8 nylon bolts that pass through mounting holes in the primary coil support disc. The central hole is 10mm in dameter and will only be used to thread the whole form onto my winding jig. After winding the hole will be permanently sealed by a turned acrylic peg turned (very successfully) on my lathe.

The peg is a really close fit and the flat area will provide a good surface to be bonded with Tensol 12.

Above you can see a trail fit of the peg. Obviously it will be fitted on top of the base cap. The coil form is now ready to wound on the winding jig. However, not tonight, time for a quick spin in the new RCZ. Spot the new wheels.

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Secondary Coil (Take 2)

A couple of weeks back I spent a few of hours playing with my oscilloscope. I had seen a few articles on the web showing how to measure the resonant frequency of the primary and secondary coils with an oscilloscope so I thought I would give it a try. The easiest one to measure is the secondary coil, to do it accurately you need to remove the primary coil and simulate streamers from the toroid as these change the resonant frequency. I was not looking for this degree of accuracy at this stage, really just having a go to get a rough idea.
I disconnected the earth lead from the bottom of the secondary coil and connected the coil to the signal generator built in to my oscilloscope.

I plugged a probe into channel one and Damien held the it approximately 3 feet away from the secondary coil.

I set the signal generator to a sine wave of 5v amplitude and starting at 100kHz as I guessed resonance should be between in the range 100kHz to 200 kHz.

The volts per division on channel 1 was set to 10mv and time/div set to 1us. I began to slowly increase the frequency of the signal being supplied to the secondary coil. You can see from the pic above that the trace was flat as 128kHz was passed. This remained the case most of the time as the frequency was increased, there were some signals picked up, presumably harmonics of the true resonance. To my amazement at 175.4kHz the flat trace suddenly transformed into a perfect sine wave, signifying we had hit the resonant frequency of the secondary coil.

As I said this was only done for experimental reasons and in no way to get an accurate resonant frequency, but it did get me thinking more about the relationship between the secondary and primary coils. I had read a lot of references to JAVATC while looking for information about Tesla coil tuning. It's a website based program used to help in the design of Tesla coils. It works by entering lots of component specs and various physical dimensions of the Tesla coil you have built or plan to build. When the program is run it calculates the resonant frequencies of the primary and secondary coil and displays the %tune between them. It can also be used to automatically tune the primary coil to the secondary, in this mode the program will output the correct tap point on the primary (in number of turns) to get resonance with the secondary. It took me a while to work out and gather all the different data required to test my own Tesla coil build but after a lot of measuring I entered the data into JAVATC.

CRAP!!!
According to the software I needed to tap my primary coil at turn 16 to get resonance with the secondary coil. That would be a slight problem. I only have 12.5 turns on my primary!!! The reason for this was a lack of understanding when turning my secondary coil. I had opted for a larger secondary coil during the build after choosing a few other upgrades. I had failed to upgrade the magnet wire gauge used to form the windings. The 0.4mm diameter wire I used on the larger secondary had resulted in far more turns than required. There is only one thing for it, build another secondary with less turns.

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Primary coil strike rail

I have also decided to fit a strike rail to the primary coil assembly. This, in conjunction with the anti-strike disc, should help prevent stray streamers hitting the primary coil. The strike rail is made from a length of 4mm diameter micro-bore copper tubing. It will be mounted around the perimeter of the anti-strike disc and will form an 86cm diameter ring, hence I bought 3m length of tubing from a seller on eBay (86 X 3.141=270cm). The strike rail will be fitted on the underside of the anti-strike disc and will be supported by 16 mounts evenly spaced. Each mount is a cut from a 20mm diameter clear perspex rod, each one 20mm long. The tops and bottoms of each were polished to a glass like finish. These mounts were made and supplied by the excellent Trent Plastics, a company I highly recommend for all your plastic needs. The finish quality was amazing and the price for 16 of these little beauties was very fair.

Here's one of the mounts, I have already drilled this one ready to accept M4 threads. It will also require a horizontal 5mm hole drilling that will accept the 4mm copper tubing.

The mounts are masked up again for the next drill hole and tapping.

Here's the mounting blocks almost in situ. They will actually be bolted to the underside of the anti-strike disc by M4 12mm bolts, probably nylon but possibly stainless as insulation is not an issue here. There are 16 mounts, 8 of which line up with the 8 primary coil support combs, the other 8 are evenly spaced between these. 8 would have been enough to support the strike rail but 16 looks better.

The 3m length of 4mm micro-bore tubing arrived tightly coiled in an A4 size jiffy bag. It took a little work to slowly open the coils up to the approximate correct diameter.


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Winding Jig Upgrades

I finished the secondary coil winding jig a few weeks back and was planning to wind the coil by spinning the perspex tube by hand. After deciding to upgrade to a 6" coil and reading lots more winding stories on the web I have decided to upgrade my jig by adding a motor.

The picture above shows the finished winding jig in its manual mode to speak. The easiest way to rotate the secondary coil would be to drive it directly with a rubber drive belt otherwise I would have to drive the central shaft and fix the perspex former to it. Direct drive to the coil tube via rubber belt was chosen and the parts quest was on.
    I would need a motor with a suitable gearbox, a pulley to fit the output shaft and a rubber drive belt. First place I tried was Maplin, I had bought a similar motor from there in the passed to power the belt on a Van der Graaf generator I made. They had the motor I needed but not the belt or a suitable pulley so I carried on looking. I found everything I needed at Technobots they had an excellent choice of motors, pulleys and drive belts, they took Paypal and delivered the stuff within a few days. I was aiming for  a coil rotational speed of around 15rpm so a bit of maths was needed to work out gearbox ratio, pulley size and drive belt diameter. Final positioning of the motor on the jig is really by trial and error, you just need to get the right tension on the drive belt so it doesn't slip as you are winding. I had to mount the motor on a small piece of wood to get the correct spacing.

The motor I used was a 540 size motor that came with a 148:1 reduction gearbox. The mounting plate comes with it, the output shaft is 6mm and it can be run on 4.5v to 15v.

I needed to make a small cut out on the back panel of the jig to allow clearance for the drive belt. The upgrade was finished and it went quite well to say that I hadn't designed the jig to take a motor to start with.

I also knocked together a spool support for the 26awg wire. This would need to allow the feed spool to rotate freely so I did my typical over engineering on it. Starting with an old piece of wood and a length of 10mm aluminium rod.

I had a sealed bearing with a 10mm ID to hand so I turned a 10mm ID X 16mm OD 3mm spacer which went onto the 10mm vertical aluminium shaft followed by the bearing.

Then another 10mm ID X 16mm OD spacer, this time 15mm deep.

Then the spool itself then another spacer/bush to keep the top of the spool centred. Doesn't look pretty but works very nicely.

Secondary top finished

Have just completed the top end disc for the secondary coil. It's pretty much a carbon copy of the base disc but obviously doesn't require the extra holes for mounting to the base.

As well as the 8 4mm holes around the circumference there is also a central 10mm diameter hole which serves two purposes. Firstly, it allows the top-load to be electrically connected to the coil on the secondary. Secondly, in conjunction with the same hole in the base disc, it allows the whole secondary coil assembly to be mounted on the 10mm shaft of my coil winding jig.

The next job is to wind the coil. This will be the first time winding such a large coil but hopefully should go OK.

Secondary base finished

Just got in from the garage.  Spent a couple of hours drilling and tapping the holes in the secondary coil and the bottom disc.

The eight holes around the perimeter are tapped to accept 20mm M4 bolts which will pass through corresponding 4mm holes in the secondary coil. The other eight holes started as 7mm plain holes and were then tapped to M8. They will be used to bolt the secondary coil down to the base.

The holes were drilled with the disc in situ inside the secondary coil. The drill size required is 3.3mm. After drilling the components were separated and the disc tapped to M4. The 3.3mm holes in the secondary were opened up to 4mm.

Here you can see the base disc bolted inside the secondary coil. Think it looks great and, as usual, completely over engineered.

I have used nylon bolts, not sure if it would be OK to use metal so opted for nylon.

Second Thoughts

After deciding to upgrade to a 6" secondary coil I got on the phone to Trent Plastics to order the relevant pieces of perspex. These consisted of a 810mm length of 150mm diameter clear acrylic with a 3mm thick wall and two 144mm diameter 15mm thick clear acrylic discs which will be inserted inside the ends of large tube and form the top and base of the secondary coil. I normally deal with a guy called Levi who is always on the ball, I was a little concerned that the discs would not be a good fit inside the 150mm tube due to variations in wall thickness that occurs in extruded acrylic tube so I asked Levi to check the fit of these discs inside the large tube. Three days later and a long cardboard covered tube turned up. I checked the fit of the discs and they were a perfect close push fit inside the large tube.

The above pic shows the 150mm diameter tube, it still has the protective plastic film and I have added masking tape so I can mark the position of the 8 4mm holes that will accept the M4 nylon bolts to locate the top and bottom perspex end caps.

Here are the 2 end caps. They are sitting on a 12mm thick 500mm clear acylic disc which will be the base for the primary coil. The disc on the left is for the bottom of the secondary coil and will be secured to the primary coil base by 8 M8 40mm Nylon bolts. You can see I have already marked the disc for drilling.

The holes need to be drilled to 6.8mm and then tapped for the M8 bolts. This means the bolts will be captive in the secondary coil base which is needed as there will be no access to the bolt heads once the secondary coil is assembled.

Assembled so you get the idea.

The base disc will attach to the secondary coil tube by 8 X 20mm M4 nylon bolts which will be located equidistant around the circumference at a height of 7.5mm.

Starting to get worryingly large!!

Parts for Secondary Coil

Just about to start construction of the secondary coil. First bits are here, a 1 metre length of 110mm weld pipe and a 1kg roll of 0.4mm class H winding wire. I picked up the pipe from a koi carp dealer and the wire from Brocott who also have an ebay store if you prefer to buy on ebay.



Decided to go for black piping as the copper winding wire stands out really well against the black.



The nasty looking bottle is anti tracking varnish which I will use to seal the windings, also acquired from Brocott. Winding materials were my fathers day present. Thanks Damien.

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