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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Bonding the Secondary coil acrylic end caps with Dichloromethane

Got lots of Tesla related stuff done this weekend. Firstly my little bottle of Dichloromethane turned up from from APCPure, a solvents seller on eBay. This meant I could glue (if that's what you call it) the toroid mount to the top cap of the secondary, and the small blanking plug to the bottom cap. I have never used Di-chlo before so was a little apprehensive. I knew if you get it anywhere else other than the bonding surfaces it marks your lovely clear acrylic. Anyway it went OK, I don't think I used quite enough as I ended up with air gaps but the bonds are absolutely solid.

Here's a shot of the toroid mount bonded in place, you can't really see the offending air gaps and the bond is good and completely seals the central hole used for mounting on the winding jig. Glueing the blanking plug to the base cap went much the same. A slight change of design here, I glued the blanking plug to the bottom of the end cap so it protrudes from the base of the secondary. I will adapt the Tesla base so the plug acts as a central locating pin into the primary mounting disc.

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A turn around the secondary coil. Again.

Hi everybody, not done a post for ages. Been busy with a bit of decorating and think I have not been looking forward to re-doing the secondary coil so have kept putting it off. I think it's time to finish the secondary so I can get on with testing and finally fire up this monster. Plus I think I need to move the Tesla to a safer area, at the moment it's in our freshly decorated living room which used to be pretty safe place where it wouldn't get damaged. Not anymore! Sadly we lost our old cat, Coco, a couple of months back but acquired 2 little rescue kittens, Milo and Lulu. They are mad. I haven't yet worked out if Perspex is claw proof and my Tesla looks like the worlds biggest scratching post.

If you want to see Milo and Lulu live take a gander at http://cats.damow.net from 10am till 4pm weekdays or anytime after 11pm any day. Live webcam of our kitchen (cats bedroom).

Anyway on with the Tesla. I have finalised the design for the windings on the secondary coil. The goal was to reduce the number of turns and this was achieved by increasing the diameter of the magnet wire on the secondary and reducing the length of the windings. I used JAVATC to generate the figures I needed.

With 1050 turns on the secondary resonance should be achieved by tapping the primary coil at turn number 11. The predicted resonant frequency is just over 180kHz and can be seen in the data below.

The new secondary coil blank is installed on my trusty winding jig. So glad I decided not to sell the jig on eBay after completing the first doomed secondary.

I made a small change to the winding jig. I replaced the original 148:1 motor/gearbox with a 50:1 unit. This should speed up the winding process, last time it took over 3 hours. The motor/gearbox units are produced by MFA and can be obtained from Maplin in a variety of ratios.

I have already purchased a few bottles of varnish to coat the coil after the windings are done. I am using the same as last time, it's made by a company called AEV (Advanced Electrical Varnishes) and is known as Ultimeg 2000/372 a high build Alkyd air drying anti-tracking varnish. The easiest place to get it is from Brocott on eBay but it's expensive in small amounts (£4.50 for 100ml plus p+p). You can also buy it from Hi-Wire Ltd for about £28 plus p+p but the smallest qty is 5 litres.

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New Toroid mount System

I have started work on the new toroid mounting system which differs in design from the method used on the first secondary coil. This time electrical connection to the toroid from the top of the secondary coil will not pass inside the coil form but will gently spiral around the mount and then finish as a copper disc which will be in direct contact with the underside of the toroid. The final system incorporates good secondary coil design taken from Stefan's Tesla Pages and the fast easy assembly of my first design.

Above is a quick sketch showing the basics of the new mount. Here we see 3 acrylic disc which are 107mm in diameter and 15mm  thick. The middle disc has an central 25mm hole. All bolts are nylon with the center bolt being M8 and the four others are M6. The diagram only shows the 2 side M6 bolts, there are another 2 front and rear. The bottom disc will be Tensoled to the top cap of the secondary coil and the four holes will be threaded to take the M6 bolts. The middle and top disc corresponding holes will be 6mm clearance holes with the top disc holes being countersunk. The central hole in the top disc will be threaded M8. The copper electrode disc will sit on top of the uppermost disc (probably bonded to it) and the the toroid mounts over the M8 bolt sandwiching the copper plate. The copper plate is soldered to the copper tail of the secondary with the tail gently spiralling (with some excess) around the toroid mount. This system will allow extra acrylic discs to be fitted to adjust the spacing between the toroid and the secondary coil.

I ordered four discs from Trent Plastics. The 3 required for the construction of the mount plus an extra to be inserted as a spacer. All the discs were the same apart from the one with the 25mm central cut-out. Above you can see the top 3 discs including the extra spacer. The lefthand disc has just been drilled to form the four 6mm clearance holes and the central hole which will be threaded to M8.

I the stacked the drilled disc on top of the other 2 and taped them tightly together. This allowed me to use the existing 6mm holes in the top disc as a template to drill 6mm clearance holes through the other 2 discs. After this was done I the added the fourth disc to the botton of the stack of disc, again taping it tightly to the others. I then passed the 6mm drill down each hole with just enough pressure to mark the center of the drill bit on the bottom disc. The stack was then separated and I could use the drill dimples on the surface of the bottom disc to centre the 5mm drill bit before drilling.

Here I am taping the four 5mm holes in the bottom acrylic disc to accept the M6 nylon bolts. Remember, this is the disc that will be bonded to the top cap of the secondary.

After removing all the protective masking tape I did a quick trial assembly just to make sure everything lined up OK.

And here's the mount in situ, not bonded yet, that will have to wait till the secondary is wound. That's the next job.

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Testing the Synchronous Spark Gap Circuit

Tonight I have just done a little bit of testing. I wanted to make sure all was OK with the spark gap circuit so first I made sure everything was physically OK. I checked that the disc holding the four tungsten spark gap electrodes was fastened securely to the mandrel. Also that the tungsten electrodes were secure and the spark gaps were set at the correct distance (0.5mm + or - 0.2mm).

All was fine so I proceeded. I plugged the main power supply module into the mains and connected the phase shift module to it. Then connected the phase shift module to the SSG circuit iec socket on the side of the main tesla base module. The main supply to the neon transformer was left disconnected as, at this stage, I only want to test the SSG circuit.
Turned the phase shift module to its minimum setting, a final check then I threw ON/OFF switch.
The SSG motor hummed into life and spun up to full speed very quickly. I sat and watched for a while. Nothing seemed to be smoking or melting and the motor was running smooth. There was virtually no vibration, the electrode disc seemed to be balanced very well. I decided to give the phase module a try and  rotated the variac knob a few degrees. Obviously there was no viewable change to the rotation of the SSG but I could hear the motor lose and regain its lock on the frequency each time I changed the position of the phase shift variac (well I think that's what I was hearing). On further increasing the phase module the SSG motor would completely lose lock and stop but regain and restart when the setting was reduced. By this time perspex case around the phase shift module was starting to cloud up with what looked like condensation. A good time to end this first test run.
After unplugging and further inspection I concluded that the fogging was just solvent being driven out of the varnish on the coil windings in the phase shift module.
I wanted a visible confirmation that I was actually achieving some degree of phase shift. First, I thought if I illuminated the SSG with a fluorescent strip light I might be able to "see" some phase shift as the phase shift module is adjusted due to the (50HZ) flashing of the strip light. Unfortunately no access to a strip light prevented that little test. After explaining what I was trying to achieve Damien came up with an idea. He could create a 50HZ strobe with a simple little program on an Arduino microcontroller.

Damien explains:

A 50Hz wave has a period of 0.02 seconds.  This is composed of half-on and half-off output.  Repeat this procedure and you get a fairly accurate 50Hz square wave.  Hook this output to an LED and you have a simulated strip light running at 50Hz.  While it isn't in phase with the mains (Unless you're really lucky when you turn on the Arduino!) and it's not massively accurate because of the inherent overheads introduced by the delays in the microcontroller and timers, it's a good way to get a strobing effect and get an idea whether the phase-shifter is working or not.  Back to dad...

Here's a pic of the little unit Damien put together. We switched off the lights and fired up the SSG circuit.

Without adjusting the phase module the Arduino strobe was shone on the spinning electrode disc. The strobe effect worked well, the electrodes appeared to be rotating anticlockwise very slowly. This was due to the slight difference in frequency between mains and the Arduino strobe. If the frequencies were exactly the same the electrodes would appear static. I adjusted the phase shift module and this resulted in a change to the slow rotation of the electrodes. This change only occurred during the adjustment of the phase shift module. I think this pretty basic test is a good indication that there is a change in phase. Below is a video of the process, it's just about possible to see the moving electrodes as the phase module is adjusted, it was more obvious to the naked eye. I think the video camera frequency also combines with the other frequencies to complicate matters.

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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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Tesla coil ground connections

Today I decided to finish some of the electrical connections. Firstly I needed to complete some ground connections, 3 to be exact, and when I say ground I mean literally ground not mains earth ground. These are :-

1. Strike rail to ground.
2. Secondary coil to ground.
3. Neon transformer to ground.

I decided all 3 of the above will start from the ground post on the neon transformer as this has a large enough post to accomodate the multiple ring terminals.

You can see this terminal in the pic above, it's the lower terminal with the blue sheathed wire attached. In this pic I have removed the perspex side panel.

As the 3 grounds will exit the lower level perspex box I needed to drill 3 holes in the perspex side panel. I masked up the side panel so to protect the perspex and so I could mark my drill points. The welding wire for the grounds is 8mm in diameter but I decided to drill 12mm holes so the wires could pass through with the ring terminals attached. This would make assembly and maintenance much easier.

The left hole would house the secondary coil connection, the right the strike rail connection and the centre would be to the grounding spike. I marked the top on the masking tape mainly to show this side is the outside as the panels do fit better one way than the other. I drilled the three 12mm holes and then removed the masking tape. But unfortunately failed to note which side was marked top. So I took my best guess and offered up the panel and proceeded to refit the mounting bolts. Guess what?

Wrong way! the bolts did not line up correctly and the result is shown above. The perspex corner post cracked. The bolts did feel a little tight, I should have realised the panel was the wrong way round. Took me an extra hour to make the replacement post.

Here's the side panel and new corner post back in situ. Now to make up the wire lengths required for the two ground connections.

The first is the strike rail connection. The wire end at the strike rail fastened inside a copper lug so I impregnated the copper strands with solder to stop the wire end disintegrating due to repeated clamping. The other end terminates in a ring terminal which was also soldered.

Next I did the ground connection to the lower end of the secondary coil. Both ends of this wire were fitted with ring terminals, again soldered.

Above you can see the routing and end connections of these two ground wires.

I am not going to fit the wire to the ground spike yet, there's no point as it will just get in the way. It will pass through the central hole in the side panel and attach to the ground post of the neon transformer thus connecting the neon transformer, strike rail and secondary coil to ground. The ground spike will be situated in the middle of my back lawn. It will consist of a 4 foot copper rod driven fully into the ground. I will run the connecting wire under the turf across the lawn and out on to the patio.

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Primary Slider Clamp

Todays horrible weather gave me a good excuse to do some more work on the Tesla. My next task is to make some kind of easily moveable clamp for the outer connection to the primary coil. It needs to be easily moveable so you can adjust its position (tap point) on the primary coil. The position of the clamp on the primary coil determines the energised length of the primary coil and hence its resonance. The tap point is moved so you can adjust the resonance of the primary to match that of the secondary coil. Well there's the basic tech stuff, here's how I made the clamp.

I started off with a chunk of copper that I picked up off ebay for about £8. It was about 3" long, 1" wide and 3/8" thick. The clamp would have to be of an unusual design because of the perspex disc that covers the whole of the primary coil. I will have to clamp onto the primary coil from below and, depending on the eventual tap point, may have restricted access due to the perspex disc underneath the primary.

After taking a few measurements I made a quick drawing using iDraw on my MacBook just to work out if this design was feasible. I decided this design should work even if the tap point is in the more inaccessible places on the primary coil. The length and shape of the righthand end of clamp may change as the design develops.

The copper block was marked up and sawn into the rough shapes required. The parts were cut slightly larger than required as they would be milled down to the required dimensions.

The off-cut removed was marked up to form the second part and roughly sawn out again oversize to allow for milling.

I fitted a 10mm milling bit and did the first lengthways run to produce the correct thickness required (6mm).

Here's a vid of a bit of the milling process. Milling copper is pretty easy as it's quite soft, I had to use the milling vice because the small height of the pieces I was working on. It's good practice to clamp work as close to the milling bed as possible so any slop in the bed is reduced to a minium. I couldn't do this with these pieces so I kept cut depths small.

After milling the two pieces were clamped together and drilled at the intersect to form the clamp jaws. The primary coil is 1/4" tubing so I used a 1/4" drill bit to form the jaws. To get the jaws to clamp tight on the tube I will remove some of the material at the vertical intersect just below the drilled hole.

A test fit of the piece works fine. It's a tight fit between the coil and the perspex disc underneath so I will mill another 0.5mm off the bottom of the lower assembly.

This picture really shows the need for this particular design of clamp especially if the tap point lies inside the external diameter of the primary coils support disc. Next session I will reduce the clamp thickness, join the two pieces together with some kind of slide mechanism and add the electrical connection.

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230V to 110V for Synchronous Motor

Did a little more wiring today. It consisted of connecting one of the female IEC socket in the front panel to the step down transformer on the second level of the tesla coil. I used a length of standard 3A 2 core round black flex. The synchronous motor is rated at 7.5W at 110V so by the equation

                  Power (Watts) = Current (Amps) X Voltage (Volts)

            so   Current (Amps) = Power (Watts)
                                                ------------------
                                                Voltage (Volts)

                                             = 7.5
                                                ----
                                                110

                                             = 0.068 Amps

So, as you can see it draws very little current and even the lightweight 3A cable is a bit of an overkill.

The top IEC socket will supply 240V to the step down transformer which in turn will supply 110V to the synchronous motor. The transformer was sourced from Newlec and came in panel form (uncased) but mounted on a pressed steel bracket. I removed the bracket so I could mount the transformer on a perspex base. This meant that I had to remove the connection strip. This task was undertaken several months back and I thought it would be wise to check that the connections were correct and the transformer is doing what's expected, also a good reason to play with my Xmas present.

The transformer has inputs for 230V and 240V with an output of 110V. I decided to supply a 50Hz AC voltage across the 230V inputs and measure the output of the terminals marked as 110V. I used the function generator built into my oscilloscope to supply this signal to the transformer at a very safe amplitude of about 3 volts. The same signal was feed into channel 2 on the oscilloscope.

The oscilloscope probe was plugged into channel 1 and then connected across the 110V output terminals. With the same volts per division set on both channels and the scope set to dual I could view the input and output waves at the same time. I did take a pic to show you the result but my camera only got an image of a few dots on the oscilloscope screen due to the high speed of the trace. So I took a quick video with my Sony Bloggie.

As you can see, the terminal connection are correct and the transformer is stepping down the voltage by  a factor of about 2 so 230V would be output at about 110V.

I can now finish the wiring to the transformer with confidence. Very pleasing to see actual inputs and outputs on the oscilloscope screen also interesting to see how the 2 signals stay in phase.