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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Finishing the Phase Shift module casing

Hi Tesla fans. Today I spent several hours on the phase shift module case. Last post described marking out and drilling the top and bottom perspex discs. I've not been looking forward to the next task but today I decided to get on with it. The phase shift module will contain the 2 Amp Variac and the 110V step down transformer, this means the perspex tube required is quite tall at 195mm. This presents a problem as it's too tall to fit under the chuck of my mill/drill and that means I would have to freehand drill the holes to take the 3mm bolts that fasten the end discs onto the tube. Doesn't sound to bad? Well remember that the perspex tube walls are 5mm thick that only leaves 1mm either side of the 3mm bolts. Now thats why I haven't been looking forward to this. I picked 15mm long M3 socket head screws to use to bolt the top 8mm thick perspex to the tube. This meant they would only protrude 7mm into the tube walls and the holes required for tapping could be kept reasonably short. Drilling went pretty well. I taped the top disc to the tube end so I could use the pre-drilled holes in the disc. I found the best way to keep as vertical as possible was to stop frequently an reposition 90 degree to the work. That way I could try to keep vertical in multiple planes.

The resulting tap holes were pretty tidy, at least none of them had broke through the tube walls due to being so far off the vertical. The 8 shallow holes were tapped out to take the M3 bolts (stainless of course).
My last excuse for a lull in my Tesla activities was the purchase of a Sony PS Vita. This months excuse was a little more impressive, took a quick snap of it before attempting the holes for the base disc.

A Peugeot RCZ GT 200. Second childhood, third mid-life crisis. Whatever!!

The base was taped to the tube and drilling was just a re-run of the top. The base disc is black 5mm perspex, black as clear would be pointless for the base, slightly thinner than the top as the top needs extra thickness to support the carry handles.

Here's the assembled phase shift module case. Next to it are a couple of lengths of 1/2" diameter aluminium rod. I will use these to make the carry handles. Below is a pic of the main power module containing the 10A Variac. I am trying to create the phase shift module in the same style.

The 2 aluminium rods were cut to 108mm and 17mm long flats were milled on each end. I drilled 6mm holes central on the flats on each end.

The handle will be supported on 22mm lengths of 6mm ID 8mm ED aluminium tubing. To allow the handles to sit flush on the tubes 8mm seats were milled on the underside of the handles centred on the 6mm mounting holes.

Below is an assembled handle. The supporting tubes fit nicely into the 8mm seats. I will be using socket head screws in the final asssembly (stainless of course).

I span up the handles on my lathe and used varying grades of emery clothes to create a nice brushed finish.

I fitted the 2A variac to the top disc incorporating the handles. The plastic bolts were the only long bolts I had to hand but I now know I need 4 60mm socket head bolts for the final assembly (stainless of course), off to ebay to source some.

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Building the Phase Shift Module

Hi there, it's been over a month since I last posted (blaming the purchase of a PS Vita). Haven't done anything with the tesla so you've not missed anything. Last post I did was about the 2 Amp variac purchased for phase shifting. I am still in phase shifting mode so I bought some perspex to house the new module.

The phase shift module will be housed in the perspex tube and will look similar in design to the tesla power source module I constructed. It will contain the 2 Amp variac and the 240v to 120v step down transformer that, at the moment, is mounted on the second level in the tesla base. This change in design is due to some advice I obtained from the very clever people at http://www.pupman.com. The advice was to step down the voltage to 120v before the phase shifting variac, hence the step down transformer will have to be external of the main tesla and it makes sense to house it within the new phase shift module.

Above is a pic of the present situ of the step down transformer. This will me removed and replaced by a module that houses the large capacitor needed in the phase shift circuit. Tonight I started on the housing for the new phase shift module. As usual the plastic was masked up ready for marking up and drill. The top and bottom of the housing will be held in place by 3mm allen bolts so first I marked up the holes needed.

I marked up both discs, then realised I could tape both discs together to drill at the same time. Both discs are 185mm diameter, the top disc is clear 8mm thick, the bottom is black and 5mm thick. I went for 8mm thick on the top as this disc will have the rather heavy 2 Amp variac bolted to it.

Here's both discs taped tightly together ready for drilling the 8 3mm clearing holes for the bolts.

Drilling went without any mishaps. If you want a few tips on drilling perspex take a look at my Drilling and Tapping Perspex page. The top disc needs an additional 10mm hole drilling dead centre to allow the variac shaft to pass through. After drilling this hole it was possible to rest the variac on top of the disc with the shaft passing through the hole to allow the marking of the four 6mm holes needed to mount the variac to the perspex disc.

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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.