Grounding point for the Tesla Coil

I am going to need some way to ground my Tesla coil. Obviously dumping these high voltages into your household wiring isn't a good idea. I have heard of several other ways, one suggestion was to ground to your cold water pipes (thanks Mike), others include wire mesh lied flat on the floor etc. I wanted a neat and more permanent solution. I am going to install 3 copper grounding rods into my lawn. These four foot rods are available from B+Q, I have already sank one into the centre of the lawn, the other 2 will be close to the lawn edges as far from the central rod as possible. I have some really meaty 5 awg silicon coated wire which will run from the submerged tops of each rod, under the lawn to the wall of my conservatory.

The 3 wires will run under the drainage gravel around the edge of the conservatory and will run up the wall just to the right of the wall basket.

They will pass through the wall and come together in a single pattress box mounted in the conservatory wall.

The box will be sank into the wall just behind Milo to the left of the double plug socket. There is no existing house wiring to the left of the sockets, the mains wiring travels clockwise around the conservatory. I measured up and picked a drilling point on the outside wall which would bring me out central between the plug socket and the corner in the wall.

Drilled through with my super long 8mm masonry bit, slowing down and reducing pressure approaching breakthrough to prevent blasting a hole in the plasterboard.

I threaded a short piece of the 5 awg silicon wire through the hole just to check hole size was OK.

Here's the components for the earthing point, a 26mm deep single pattress box and a 100mm square 3mm thick copper plate. There will be some kind of terminal central in the copper plate, not decided on it's final design yet.

I masked up the plate and marked up the positions for screw holes to mount the plate to the back box. The 3.5mm holes were drilled and countersunk.

You can simply put the pattress box over the hole for the wire, draw round it and the remove the plaster. I used my Dremel with a spiral cutter, it made short work of the plaster, took about 60 seconds to work around the outline of the pattress. The cut out plaster can then be remove with an old flat bladed screwdriver or chisel.

I attached the copper plate to the pattress box and placed it into the cut-out. I am not going to permanently fit the back box yet. I will do it when I address the wiring to the 3 grounding rods.

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Tesla Coil Extras

Well it was nearly finished.... I am sure I will keep on posting extra additions and also experiences of my first fire-ups and tunings. First addition was mentioned in my last post, the safety bleed resistor for the large 40uF capacitor in the SSG circuit. This was quite a quick fit, the resistor is only small so I decided to fit it on the side of the module that held the 40uF capacitor. I made 2 identical mounting posts out of 10mm diameter brass rod. The posts are 25mm long and the resistor sits in 3mm diameter horizontal holes 12.5mm up each post.

The resistor legs are clamped by M3 grub screws at 90deg to the mounting holes. Electrical connection to the capacitor are ring terminals onto M4 allen key bolts (stainless of course) into the tops of the mounting posts.

I made up the wires to the resistor starting with female 1/4" piggyback blade connectors which meant I didn't have to amend the existing wires to the SSG, they could just plug into the piggybacks.

While the weather was good I decided to sink the grounding rod in the middle of my back lawn. The rod is 1 metre long and I thought it would take several attempts to sink it as the garden seems to have a thick layer of industrial rubble just under the surface. Must have been my lucky day as I hammered the rod well below the surface on the first attempt. I removed a small square of turf before I started. You can just see the refitted turf in the above pic. Next dry weekend I will run the earth lead from the rod under the lawn turf to the patio.

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Tesla Coil Finished!!

Finished off my homebuilt Tesla coil today. This involved finishing the primary coil sliding clamp. It required a way to clamp it to the primary coil and the fitting of an copper lug to make the electrical connection to the MMC bank.

The above pic shows the two parts of the clamp before adding a clamping mechanism. I wanted to keep this mechanism as simple as possible as I may redesign the whole clamp depending on the eventual tap point on the primary coil. I removed a 15mm length from the right hand end of the top piece and silver soldered it to the right hand end of the lower section. I then drilled a horizontal 2.5mm hole lengthways down the centre of the block which was tapped out to take an M3 allen bolt.

This produced a simple clamping method which still permits the two parts to separate to allow for repositioning. Adding a copper lug for the electrical connection just meant adding another threaded hole on the underside of the clamp.

I cut a 1.2m length of GTO wire to complete the electrical connection from the MMC bank to the clamp. 1.2m was needed so the clamp could reach any point around the primary coil. Each end of the GTO wire was coated in solder before fitting into the copper lugs. That's it Tesla coil finished, well almost. Have ordered a 33k ohm 2W resistor that I will fit across the terminals of the 40uF capacitor in the SSG circuit and bleed the stored charge away after each run of the Tesla coil, a simple safety feature.

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Fitting the phase shift Transformer

Another rather productive day. On holiday at the moment so spent a few more hours on the Tesla build. Today I managed to get the step down tansformer mounted in the base of the phase shift module.

As you can see it mounts directly onto the base disc. If you remember the step down transformer was originally mounted on the second level of the Tesla base. The pic below shows its original position.

I removed the whole module including the perspex base. After separating the transformer from the base I used the old base to mark drill points onto the base disc of the phase shift module. After drilling these holes I fixed the transformer to the base disc using the original ceramic pillars.

This left a couple of jobs to finish up. Two M12 flex glands were fitted by drilling two 10.7mm clearance holes and tapping out to 12mm. I also drilled six 12mm holes in the top disc, these are purely for ventilation. There are 7 similar holes in the base around the transformer and as the base stands on small cork pads air should be able to circulate up through these holes and out the vent holes in the top.

I took a picture of the main power module and the phase shift module together just to show you how I have tried to design them with the same look.

Still have wiring to finish up and few other touches to the phase shift module but I think I have achieved a matching module.

Here's the module that replaces the repositioned step down module. It's a rather large 40microfarad capacitor and is part of the phase shift circuit.

The capacitor has a central M8 mounting bolt which screws into a vertical perspex 8mm thick square. This square is bolted to the perspex base and is reinforced by a secondary perspex square flush up against it and also bolted to the base.

Again, just a bit of wiring needed to finish off.

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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 Coil Clamp Pin

I spent sometime yesterday trying to figure out a good way to make the required electrical connection to the innermost end of the primary coil.

As you can see in the picture the connection needs to clamp onto the end of the 1/4" copper tubing and then pass through the 12mm perspex disc underneath the primary coil. In this pic the disc is still covered with protective plastic coating.

This is a side on pic taken level with the primary coil. You can see the anti-strike perspex disc above the primary coil and the 12mm thick disc below. After long consideration I decided that I needed a fitting that would clamp on to the end of the primary and have a protrusion that would pass through a clearance hole in the 12mm disc below. This "protrusion" would need to be threaded to allow a copper lug to be fitted on the underside. I had bought a drawing app for my ipad called iDraw and thought I would throw a quick design together. I've used this app before for designing the toroid mounting pin, it's not the most technical drawing app but it matches my abilities.

I started with a small length of 16mm diameter round copper bar. I turned down 25mm of it to a diameter of 8mm and then the next 25mm to 14mm diameter.

I cut the turned section off with a hacksaw then cleaned up the cut end in the lathe finishing the 14mm diameter section to a length of 22.375mm.

Here's the finished blank. Next I drilled the hole for the 1/4" copper tube (6.75mm drill bit used) and the holes for the clamping M3 grub screws. Decided to deviate from the plan and have 2 clamping grub screws at 180 degree to each other.

Using an M8 X 1.25 die I cut threads on the end of the 8mm diameter section (about 15mm of threads) and then cut the M3 threads for the grub screws.

Here's the finished article fitted with an M8 stainless nut and copper electrical lug.

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Checking the Variac with the Oscilloscope

Started on a little more of the wiring today. My task was to add a feed wire from the variac unit which would supply 240v to the SRSG feed transformer. This wire would just tap into the input feed after the on/off switch within the variac housing. I thought it would be a good idea to test the variac electrical connections before adding anymore wiring. The safest way to do this was to use the function generator on my oscilloscope to supply a low amplitude (about 5v) 50Hz signal across the input terminals of the variac and test the output with one of the channels on the oscilloscope.

Here's the setup I used. The function generator is plugged into the inputs of the variac and also channel 2 of the oscilloscope. The oscilloscope probe is connected across the output of the variac and feed into channel 1 on the oscilloscope. With both channels set to the same volts/div and the scope set to dual trace (shows both channel traces on same screen) it's very easy to see to compare input and output of the variac. Far safer than slapping a plug on the variac, plugging into the mains and hoping all is OK.

You can see from the video when the variac control knob is set to zero there is no output. As the knob is rotated the flat trace rises till it reaches the same amplitude as the output of the function generator. This is the required result and shows that the variac is functioning correctly. When in use with the tesla coil I will only need to use about half the range of the variac as the NST is rated at 110v. Tomorrow I will add the wiring for the SRSG. 

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Spinning up the Spark Gap Motor

Finished the last bit of wiring on the synchronous motor and the step down transformer supplying it. Annoyingly the little ceramic 3 pole terminal block cracked when re-mounting it, on closer inspection it really is a horribly made piece of crap from B+Q. I have ordered a similar looking unit from eBay tonight, hopefully it's better quality and will fit the mount holes already drilled, if not I may create my own from a block of perspex. Anyway time to plug in the the spark gap module. Fingers crossed.

Well that went pretty well. The spark gap disc seems to be well balanced and the motor is very quiet, not that that will make any difference once the gaps are firing. The motor took more than 40 seconds to come to a complete rest after unplugging. There is a fair bit of weight involved in the spark gap disc but I still think this is a good indication to the quality of the bearings in this little motor.

Hers's a close-up of the cracked terminal block. I had to remove it when I was wiring in the cable in the pic as I had trouble tightening the cable clamp screws. When I checked the screw heads non of the slots were formed correctly. I used the 2 best formed clamp screws to secure the new wiring and refitted the block, this is when it cracked. Two steps forward, one step back.


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