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.

If you like this blog you can show your support by one or all of these. 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.

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.

If you like this blog you can show your support by one or all of these.
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.

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.


If you like this blog you can show your support by one or all of these.

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.

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.


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.

Wiring in the Spark Gap

Had a spare hour today to do a bit of wiring on the tesla. I'm glad I put a bit of thought into the design of the main base unit as access will always be needed for tweaks and modifications. Access to the main base can be achieved by removing any of the side panels.

 For this job I removed the right hand side panel which gave me easy access to the safety gap end of the Terry filter. I needed to connect the left and right terminals to the left and right terminals on the synchronous spark gap module situated above on the next level.

I had previously drilled the 2 6mm holes for the passage of these wires from one level to the next so it was just a matter of making up the two wires and then threading them through.

The wire used is high quality GTO (Gas tube only) wire rated at 15,000V. This wire was sourced from Teslastuff.com and ordered from their eBay store. Click here if you want to take a look at the store.

I "sealed" the ends of the wires by coating in solder before fitting. This prevents repeated clamping from damaging the ends of the wires.

I tried to keep the wires reasonably short so they didn't pass to close to other components. This type of wire is almost all insulation but it won't do any harm to try to keep separation to a maximum.

Synchronous Spark Gap Module Part 2

Just a quick update with a few more pictures.

The finished article looks really good.

The two perspex blocks used to mount the motor have the cut sides polished. This is an extra service offered by Trent Plastics when you order any pieces of perspex. It's only a couple of quid extra and really adds a finishing touch.

The brass disc fitted to the back of the rotor disc provides electrical contact between the four tungsten rods. It has the centre removed to isolate it from the motor shaft. It's a 2mm thick brass disc of 100mm diameter with a central cut out of 40mm diameter. It was made by http://www.ringwoodprecision.com/ all I had to do is drill out the 4 1/4" holes for the tungsten rods.

Synchronous Spark Gap Module Part 1

Not done much on the Tesla for a few weeks due to the summer hols. First task is the synchronous spark gap.  The base is a piece of 8mm thick clear acrylic measuring 320mm by 160mm. I wrapped it in masking tape again to protect it when drilling and also so you can mark it up for drilling.

The motor was mounted to a small block of 15mm perspex by 2 8-32 bolts kindly supplied by Alan from Teslastuff. In turn this block was mounted to the base and supported by another block of 15mm perspex directly behind it.

Here you can see the motor attached to the perspex mounting block. The disc on the mandrel is a piece of 5mm black acrylic perspex and holds the 4 1/4" diameter tungsten rods which are held in situ with collars on the front and rear.

The tungsten rods were sourced from Teslastuff.com and the collars came from a power tool dealer on ebay, think they are something to do with router tooling.

The collars have 2 small grub screws at 90 degrees to each other and clamp the rods up very securely.

Parts for Synchronous Spark Gap

Another box arrived swiftly from the US. Eagerly unpacked to find a synchronous salient pole motor from Teslastuff.com. This will drive my rotary spark gap. I am informed this is the best way to go for a reasonably large Tesla coil.






As far as I understand (limited but growing knowledge) this motor rotates directly proportional to the AC frequency. This allows the spark gap to be timed so the gap is presented as the capacitor bank reaches an optimum charge level. I found an excellent article on the different types of spark gaps used explaining how they work and pros and cons. The article is by Richie Burnett and can be found at http://www.richieburnett.co.uk/rotary.html






Will need small step down transformer to power it at 115v.






- Posted using BlogPress from my iPad