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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