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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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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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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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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2A Variac for Phase Shifting

Last week I took delivery of the first item needed to create the phase shift module. I was going to rely on mechanical adjustment for phase shifting but after a lot of reading up I have decided that the ability to adjust phase remotely while the tesla is operational is a must. If you are pretty new to Tesla coil building (like me) the subject of phase shifting can sound quite complicated. In simple terms, the synchronous motor rotation is locked to the sine wave of the AC power supply meaning that the spark gaps always present themselves at the same point on the sine wave. Without any phase shift the same AC sine wave is applied across the capacitors meaning that the spark gap may not present as full charge is reached in the capacitors. Phase shifting allows you to advance/retard the sine wave to the synchronous motor so you can present the gaps when full charge occurs in the capacitors. Without phase shifting you can only alter this timing by physically rotating the motor in its mount or rotating the spark gap disc relative to the motor shaft. Both of these being impossible (or very dangerous) to do while the TC is in operation and can be very difficult to quantify adjustments.
To make the phase shifter we use a variac wired as a variable inductor, a motor run capacitor and bleed resistor.

First to arrive was a 2A panel mount variac. I bought it from a seller on eBay called Wattbits. This is the second variac I have bought from these guys, the first was a 10A version that is the main power source for the Tesla coil.

Here's the 10A version in the power supply module.

The 2A version is physically about half the size. I want the phase adjust module to look similar in design to the power supply module so I have ordered a length of 185mm diameter clear acrylic tubing and a couple of 185mm acrylic discs for the top and base.

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Connecting the Variac to the mains

Today I finished the wiring to the SRSG (synchronous rotating spark gap), added IEC female connectors to the two outputs from the variac and slapped o a 13amp plu onto the input to the variac. Although I'm not yet ready to test the Tesla coil charging circuitry I can now test the variac up to full voltage.

Here the variac is plugged into a 230v mains outlet. The SRSG 240v feed from the variac module is connected to the correct input on the Tesla coil base. The main Tesla coil feed (0v -110v) is left unconnected but tapped by the volt meter.

The variac dial was set to zero and the on/off switch thrown. The synchronous motor spun up immediately and the meter continued to read zero. Rotating the variac knob resulted in an increase of RMS AC voltage on the meter.

The reading were as expected and hoped. The printed scale around the variac knob isn't that accurate, but I didn't really expect it to be, it's really just a guide. Think I will mark on a couple of accurate markers.


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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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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.

Clamp on strike rail earth connector

Today I fabricated a mock up clamp to earth the strike rail. I started with one of the copper electrical lugs that I bought from Teslastuff.com.

You will not find these listed anywhere on the website or on the Teslastuff eBay shop you will have to email the owner Alan and ask for a price. They are far better quality than anything I have found available in the UK. If you know different drop me an email. I used a length of aluminium 1/2" square rod to make the pieces that form the clamp.

These pieces started off as one block which I drilled with a 12mm bit to form the internal curve and a 6mm bit to form the bolt hole. I used a file to produce the curve on the other end. The block was then cut down the middle using a slotting saw on the milling machine. The two pieces were then clamped back together in a vice on the milling machine and then a 4mm hole drilled exactly on the interface of the two pieces. Using a perfectly flat surface and a piece of fine glass paper I worked the adjoining surfaces this makes the 4mm hole slightly smaller so it clamps tight on the 4mm strike rail. 

Here are the assembled pieces. Straight away I spotted an improvement that could be made when fabricating the actual unit, if I put threads on the bolt hole on the top aluminium clamp plate I can dispense with the nut and use a shorter bolt. Also the actual unit will have copper clamping plates to reduce resistance (aluminium 2.7, copper 1.7ohm.m).

I will mount the finished clamp the other way up than shown above. There will be no need to move this clamp once it is in position on the strike rail. But, obviously, access to the grub screw will be needed to clamp the grounding wire.

With the improvements mentioned I think this clamp will do very nicely. Off to eBay to get some copper square rod.

Primary coil assembly

With all the combs finished and the primary coil approximately shaped to the correct spiral I decided to thread the primary copper tubing into the combs. It was slow going as you have to move the combs along the tubing one at a time working around the spiral.

It took about an hour to get all the combs into the right position. Next I drilled out the mounting holes in the anti-strike disc to 4mm and placed it over the top of the comb assembly. I noticed that the holes in the combs were not going to line up with the holes in the anti-strike disc. The combs were to far out from the centre by about 5mm. After my initial panic I realised all I needed to do is tighten up the spiral to bring the combs closer to the centre. This done, I fitted the 24 10mm M4 nylon bolts through the anti-strike disc into the support combs.

Here you can see the 3 nylon bolts that fasten each comb to the anti-strike disc.

The disc underneath the primary coil is still covered in masking tape as I have yet to make some electrical connections so may need to do some more marking up and drilling.

At the moment the primary coil assembly just sits on top of the perspex disc underneath. I will add some system that will allow the primary to be located into position, perhaps locating pegs or guide blocks.