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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Primary thoughts

As the secondary coil is pretty much finished I have already been thinking about construction of the primary coil. Both coils will sit on a perspex 50cm 12mm thick disc which is supported above the second layer of the main module on 8 30mm diameter 20cm long perspex pillars.

The second level contains the MMC module, the spark gap module and a step down transformer module for the spark gap motor. All modules are mounted on ceramic standoffs.

I have yet to finish off wiring and the support disc is not yet fitted. As you can see I have coated it in masking tape ready for marking up the holes for drilling.

The pillars are drilled out to 5mm and then threaded to take M6 bolts. If you want some advice for drilling and tapping perspex takes a look at my Drilling and Tapping Perspex page.

The primary and secondary coil will sit on the top perspex disc (covered in masking tape above). At this point I have decided to upgrade the design from the original plans (yet again). I thought it would be a good idea to mount the primary coil underneath a perspex disc which should help prevent strikes to the primary coil. Also I heard if you do this any strikes that hit the perspex create an interesting effect as they spread out over the perspex. I will install a more traditional strike rail around the edge of this protection disc. The disc diameter was decided at 90cm and it would need a 170mm diameter central cut-out to allow the 150mm diameter secondary coil to pass through.

And here it is. Ordered from Trent Plastics, it turned up at work this morning. Took me about 10 minutes to remove the 3 layers of cardboard packaging.

The thickness is 5mm and I paid extra to have the inside and outside edges polished. It's a big piece of plastic and does flex a little, but there will be 8 radial combs from the centre outwards to support the primary coil and these will add extra stiffness.

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!!

First layer plastic surgery

Spent a couple of hours in the garage drilling the various mounting holes in the base. 4 X 6mm holes for the neon transformer and 16 X 4mm holes for the Terry filter, the emf filter and the mains connector module.

Another 12 X 4mm holes were needed to mount the 10x10mm perspex square rod that runs around the perimeter of the base.

These 10x10mm perspex rods provide a means to screw the side panels onto the base.

Looks a bit like ORAC

I have decided to enclose the whole assembly with clear 10mm perspex. Maybe this may help prevent strikes but mainly I just think it will look good. I will have to sort out some ventilation, perhaps a fan mounted in one of the perspex walls, not sure yet. For those of you who remember Blakes 7, I can't help thinking about ORAC as I envisage the finished enclosure.

I am using 10mm square rod as seams on the inside of the side walls which allows me to bolt the box together rather than glueing.

Our glass dinning table made a great flat surface to align the side panels.

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Multi Mini Capacitor MMC

Today I started on the MMC (multi mini capacitor). The capacitors had arrived from Teslastuff.com a few weeks earlier. The MMC is built up of 40 capacitors, 2 banks of 20 caps in series connected in parallel. Each capacitor is rated at 0.15uf at 2000v as per the original Teslastuff plans.






A bag of 20 capacitors for the first layer of the MMC. This layer would be mounted on a 8mm thick 320mm by 220mm clear perspex board.







The board was covered in masking tape and marked out for drilling.I drilled 1.75mm holes for the cap legs (40 in total), some 4mm holes for mounting onto 1" ceramic stand-offs and 2 6mm holes for the copper lugs. After marking out I attached an identical piece of perspex to the back so I could drill both layers at the same time.







After drilling I mounted the 20 caps to the first level. I twisted the adjacent caps legs together and then laid the bleed capacitors across each cap before soldering.







Also soldered in some copper wire to create the series links between the rows and a couple of terminal loops on the ends.







Above is the first layer completed. The 5 support uprights are 12mm diameter perspex drilled and tapped to accept 4mm bolts. Now for the second layer.






The finished article.






Very pleased with the end result.






Will also be easy to add more layers in the future.






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Proper Workshop

Not working on the tesla tonight, waiting for quite a lot of parts to turn up, hopefully some will come tomorrow so I can finish the Terry filter. Took some pics of my workshop first is my most useful tool, my little milling machine bought from Machinemart. It's a Clarke CMD300, I modified it by fitting a belt drive system and also x and y digital vernier scales which negates any slop in the bed.

Here's a close-up of the digital readouts. Makes a good milling machine great.

This is my little Hobbymat lathe, would like something a little larger but don't think I can fit anything else in.

As you can see, not a lot of room left.

If your interested the car is a Westfield I built a few years back. Here's another pic with it out in the sun outside the SVA centre.

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From the bottom up

The original plans from Teslastuff.com show how to construct the base from circular pine sheet and uprights made from pvc pipe. I decided to carry on with my perspex theme so all the components will be visible but still enclosed. I purchased a hefty piece of 15mm thick 50cm by 50cm clear perspex from Trent Plastics which should give me enough room to mount the Neon Transformer, Terry filter and line filter plus a terminal block for line in from the variac. Other components would be mounted on a second level. To allow the whole thing to be easily moved I fitted good quality castors. I could have drilled and threaded some holes to fit them but decided to try some threaded brass inserts that are hammered in to 8mm holes and have an M6 internal thread. Here's a pic of the perspex base, its still covered in the protective plastic and a full coat of masking tape.

Here is one of the 16 brass inserts used to fit the castors. It's a bit easier using these than having to drill and tap 16 holes. They produce a very strong fitting as when the bolt is inserted they expand into the drilled hole.



Here's one of the castors fitted, should make moving the Tesla coil a little easier.

There is plenty of space for the Neon Transformer, Terry filter and line filter. Both filters will be mounted on perspex bases partly for look and partly to allow easy removal of any single component.

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