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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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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1. +1 my blog and email it to a friend.
2. Follow me.... It's good to know someones interested.
3. Leave a comment.... All are appreciated.

Proper Workshop

Not working on the tesla tonight, waiting for quite a lot of parts to turn up, hopefully some will come tomorrow so I can finish the Terry filter. Took some pics of my workshop first is my most useful tool, my little milling machine bought from Machinemart. It's a Clarke CMD300, I modified it by fitting a belt drive system and also x and y digital vernier scales which negates any slop in the bed.

Here's a close-up of the digital readouts. Makes a good milling machine great.

This is my little Hobbymat lathe, would like something a little larger but don't think I can fit anything else in.

As you can see, not a lot of room left.

If your interested the car is a Westfield I built a few years back. Here's another pic with it out in the sun outside the SVA centre.

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From the bottom up

The original plans from Teslastuff.com show how to construct the base from circular pine sheet and uprights made from pvc pipe. I decided to carry on with my perspex theme so all the components will be visible but still enclosed. I purchased a hefty piece of 15mm thick 50cm by 50cm clear perspex from Trent Plastics which should give me enough room to mount the Neon Transformer, Terry filter and line filter plus a terminal block for line in from the variac. Other components would be mounted on a second level. To allow the whole thing to be easily moved I fitted good quality castors. I could have drilled and threaded some holes to fit them but decided to try some threaded brass inserts that are hammered in to 8mm holes and have an M6 internal thread. Here's a pic of the perspex base, its still covered in the protective plastic and a full coat of masking tape.

Here is one of the 16 brass inserts used to fit the castors. It's a bit easier using these than having to drill and tap 16 holes. They produce a very strong fitting as when the bolt is inserted they expand into the drilled hole.



Here's one of the castors fitted, should make moving the Tesla coil a little easier.

There is plenty of space for the Neon Transformer, Terry filter and line filter. Both filters will be mounted on perspex bases partly for look and partly to allow easy removal of any single component.

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Worlds nicest Variac

The first bit of the build was to create a case for the variac. I thought it looked really good and it would be a shame to stick it in a metal box. Decided to keep it on display by casing it in perspex. Had lots of small taps and stainless allen key bolts hanging around so decided to bolt it together rather than glue. Trent Plastics was found (eBay again), a supply of many types of plastic sheet and tube. They can supply cut to size so saved me hours of measuring and cutting. The main tube is 250mm diameter 6mm wall, the top holds all the weight so I went for 8mm thick clear perspex. The base is 5mm thick in black cos I thought the base would get scratched pretty quick so clear was probably not a good idea. The next pic shows Variac fitted to top plate and top plate bolted to main tube. Bolts are M3 so not a lot of room for error when drilling holes in the main tube.








After drilling and tapping to fit the base I decided to fabricate some handles which would be fitted to the 4 bolts that mount the Variac to the top plate which meant there would be no stress to the perspex when lifting. Also drilled some vent holes in the base and the top plate, small feet on the base provides clearance for air to enter circulate passed the coils and then out through vent holes in the top plate. Drilled 12mm holes for retro style 20amp switch and 3 10.7mm holes for the glands. The glands have a 12mm 1.5 tpi thread so another ebay purchase was required for the tap and 10.7mm drill bit.







Well happy with the result, handles are solid 16mm aluminium rod, I milled the ends so the M8 x 70 bolts sit nice.







Vent holes are 12mm, you have to go quite slow when drilling otherwise the perspex just melts. Worked well clearing the swarf regularly off the bit. May need more vents, just have to see how it goes.







Switch sits in another 12mm hole. Room for extra vent holes either side of the switch if needed.







Here you can see the 3 glands. One will be 240v mains in, one will be 240v out via switch (may use for quench fan) the other is variable voltage out via the variac. Wiring is the next job.


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Neon Transformer Arrives

Next to turn up was the neon transformer and it weighed 56lbs and looked very scary. Found it on ebay from a company called Teslastuff based in Minnesota US. It's rated at 60mA 15kv and has no GFI (ground force interrupter?). Apparently that's what required when building a Tesla coil. The guy who runs this company also supplies loads of other goodies for Tesla coil building including a great set of plans for the build.

Very heavy small wooden coffin from US

Inside one very scary 15kv Neon Transformer

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New Tesla arrivals

I am a few weeks into the build of a fairly large Tesla coil and I thought I would create this blog to record my progress and hopefully get some positive feedback if I get stuck along the way. I have already got a couple of the main components. The first to arrive was a 10 amp variac which I purchased on ebay from a company called Wattbits, ebay shop can be found at http://stores.ebay.co.uk/Wattbits?_trksid=p4340.l2563

I bought the panel mount version with no casing partly due to being a lot cheaper than the cased version and I decided to make a perspex casing.

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