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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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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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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Electrical connection to the Primary Coil

A few weeks back I made a very nice copper clamp pin which would be used to connect the inner end of the primary coil to the HT lead from the synchronous spark gap (SSG).

The hole at the top will accomodate the inner end of the primary coil and the threaded section will pass through a hole in the primary coil support disc and then the electrical lug will accept the HT lead to the SSG.
First job today was to drill the hole for this clamp pin.

The picture above clearly shows the primary coil support disc, it's the white disc sitting under the radial primary combs. It's white as it still has its protective coating and a layer of masking tape. To drill the hole needed I would need to remove this disc so after marking the hole position I started disassembly. I removed the toroid, the secondary coil and the primary coil assembly which only took a few minutes. The primary coil support disc was removed by undoing the 8 allen key bolts (stainless of course) that screw through the disc into the perspex support columns. I drilled  the 9mm hole required for the clamp pin and then removed all the protective coating from the primary coil support disc.

The primary support disc was refitted and the clamp pin slipped over the inner end of the primary coil and the grub screws tightened. The primary coil assembly was then lowered down onto the support disc making sure the clamp pin threaded end lined up with the clearance hole. It did!!

Above is a close look at the clamp pin in situ. Very happy with design, it allows an easily removable electrical connection to the primary coil.

A shot from underneath shows the copper lug bolted to the threaded end of the clamp pin.

I've done a few more pics of the assembled tesla base as this is the first time all protective coatings and masking tape have been removed.

And finally a full length shot with the secondary and toroid back in place.

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