Pyramider: The evolution from Spark Gap to IGBT

A number of years ago I was approached about a Tesla coil in a deceased estate of a friend of a friend you might say. Naturally this was an opportunity too good to pass up.

Pyramider circa 2006.

Upon first inspection I found out the coil had three 12kV neon sign transformers and a rotary spark gap.


I duly agreed to take responsibility for the creation and it then sat at work for a few years occasionally being tinkered with.

Around 2011 He who must not be named on the internet moved back to little ole NZ bringing back with him enthusiasm for building tesla coils and a REALLY big piece of power silicon: Thus began my renewed interest in power electronics...

2.5kv at 1800A

The idea was to build what's called an Off-Line Tesla Coil (OLTC), in that there is no supply transformer: The incoming mains supply is rectified and fed directly into the tank capacitor.

OLTCs have some interesting design criteria: With the primary voltage being very low (compared to spark gap coil) The tank capacitor has to be huge in order to supply sufficient energy to the system.
The huge tank capacitor in turn means that the primary inductance has to be tiny for the system to have a sensible resonant frequency.

The primary currents involved are pretty massive, as a result the "wiring" has to be pretty heavy duty: In this case the conductors are made from 1.6mm copper sheet, 190mm wide. That is 300 square millimetres of copper!

Initial back of a envelope calculations were based on an input power of around 5 to 10kw to make things reasonably interesting. With a reasonably safe maximum switching frequency of around 7-800Hz this meant the tank capacitor needed to store around 10 to 12 Joules. Charging the capacitor to around 1KV, (more on this later) around 20µF was necessary.

Primary Construction:
As mentioned earlier the primary conductors are made from copper strip 190mm wide, there are a couple of reasons for this besides the ludicrous discharge currents...
The tiny primary inductance: Total inductance, which includes the parasitic inductances of the conductors, transistor, ESL of the capacitors etc had to be about 0.1µH or less. This is so that the resonant frequency of the primary matched the secondary.
The wider a conductor is the lower its inductance, therefore transmission lines made of nice wide strips of copper have lower parasitic inductances.
Flat conductors are also a lot easier to mount closer together: This reduces the parasitic inductance of the system further still. (more on this later)

The "Snorkel"Primary:
Owing to the low inductance needed, the primary had to be very small in diameter (the diameter actually ended up less than the height) this resulted in much headscratching... The required diameter was less than the diameter of the secondary... Eventually the solution of internally mounting the primary was devised by use of a little trickery:

In order to minimise parasitic inductance, I had to devise a suitable feed line to connect the primary turn to the capacitor and transistor assembly. A bit of a hunch and a few hours of sketching later a plan was devised to fabricate a shape from copper strip and then roll it into a combined primary turn and feed line... Below is a sketch of the result:

Sketch of the fabricated primary.

The above shape was made by cutting strips of copper sheet that were then TIG welded together, this approach was taken as there was a shortage of material (it was rather expensive)

The welds were linished flat and all the sharp edges were polished off. I then put the whole part through a sheet metal roller to give the final shape.

The many, many bolts used to connect
the snorkel to the capacitor assembly.

Snorkel featuring polyethylene
sheet to keep the feed line separated. 

Tank Capacitor:

Capacitors in Tesla coils have a pretty hard life, they're charged to high voltages and discharged very quickly... Only to be told to do it many times per second. So naturally they have to be very low loss and rated to huge pulse currents.

Capacitor plate to the left.

Currently about the best off the shelf solution is to connect multiple smaller capacitors in series-parallel to give the appropriate capacitance, voltage and current rating. In this case depending on tuning there are about 40 capacitors in parallel. The capacitors are mounted to a parallel plate feed line (think double sided PCB) made from two sheets of 1.6mm copper spaced 5mm apart. This is an idea I borrowed from Steve Connor and Greg Leyh. The capacitors connect to the plates using threaded brass studs and copper tubing spacers.

Brass studs and copper spacers fitted.

One day I would like to build a coil using water cooled capacitors designed for induction heating from a supplier such as Celem: Megawatts anyone?

Capacitors fitted to the assembly.

Top and bottom mounting bushes.

Mechanical Assembly:
Making the components necessary to hold everything together is a little interesting:
For one tesla coils generate high voltages - Many of the components have to be very good insulators.
Ferrous components can get hot due to induction heating (Tesla coils tend to generate rather large EM fields around the primary)
The result of all this is that I tried to use plastic components as much as possible...

Bottom mounting bush and threaded
nylon rod.

The four mounting rods.

The secondary former is supported by a combination of 10mm threaded nylon rod, 3d printed bushings and PVC pipe columns.

Columns and top bushing fitted.

Secondary former sitting on the bushings.

Water Cooling System:
For a lot of the initial running, the power consumption was limited to around 5kw. This changed quite dramatically when I finished building a three phase input supply. A brainwave to do with the implementing a pulse forming in the charging circuit made things even more dramatic.

The side effect of much more power is well, much more heat... Until this point we had been using a fan cooled aluminium heatsink, this was ok but things were getting a little hot.

The shiny new waterblock.

The transistor with heat sink.

Enter water cooling: I have a friend who's a rather brilliant machinist. We spent an evening drawing up a water cooling block to suit the transistor. An hour or so on the CNC machine later and presto.

Bolted together.

Getting ready to bolt the transistor to the waterblock: Toons,
safety glasses, and two finished cups of coffee... Serious work.

Nobody breathe.

For the first test I lashed the system together using a plastic bucket and a spare PC water cooling radiator I had lying around.

Hose restraint.

The first results were more than a little exciting.... The cooling water stayed below 27 degrees celsius during many power runs like the video below... Making Toccata and Fugue even more epic.

Zappy.

So What's Next??
I've made a better water reservoir than a bucket, Hansen plumbing fittings are great for building crazy stuff like this.

10 Litre reservoir.

Still to be fitted is a new radiator from Koolance: As a final teaser here's the CNC machined mounting brackets for it... Next post will detail the power system.

Problem: "Radiator bracket for water cooled Tesla coil"
is not a part that exists...
Solution: CNC machine.

Rapman Extruder Conduit Clamp

For a very long time now the Rapman's extruder conduit has been attached to the frame with a couple of velcro cable ties:

I decided finally that the cable ties were a bit too crude and set about designing something a little more elagent. Twenty minutes later I designed a clamp:

 Now it was only then I realised that the clamp needed an offset between the threaded rod and the conduit. So I had to throw away the nicely rounded clamp and start again.

Mark two was a little uglier - The part needs at least one flat face with no overhangs. That is, the face which will be laying down on the bed of the printer.

A couple of hours on the printer later and these were the result:

Still not perfect (I misread the protractor in my haste) but it clamps the conduit and the threaded rod OK. Mark three will be the correct angle :-)

If I say so myself this clamp goes pretty well with the first one made for this machine.

Building a Ballet Barre

My Sister has been getting more into her ballet recently, she asked me about the possibility of building her a portable barre.

Not one to do things by half and being rather inclined to build industrial furniture and lighting, there was only one material to build this creation out of...... STEEL!

This will probably be the final use for my pile of 32mm steel pipe and matching Kee Fittings. The stash has seen in previous lives, use in a bed been a scaffolding in a music video and more recently as a frame to support the big scary light.

I set about cutting the steel pipe into the appropriate lengths with my friction cutoff saw,

and assembled the frame:

Presently the frame is sitting on four corner couplers, I intend to make something fancy up involving locking casters and perhaps some 3D printed clamps.

New Conduit Clamp for the Rapman

For about a year now the Rapman has had a clamp holding the plastic conduit. The clamp was inserted into the original 10073 conduit mount plate. The plate is laser cut acrylic and on a recent trip to a secondary school I managed to smash it getting into the car.

I decided to design a part that took place of both the clamp and the mounting plate in one piece... 30 mins in google sketchup later and I had this:

Into the printer and 50 minutes later I installed the new part :) 

Time Lapse Panning System:

I'm shooting another video in the not too distant future which is going to feature a number of different time lapse scenes. After seeing a rather epic video by Jared Brandon I knew that some kind of motion system was needed:

Mt Ruapehu Timelapse from Jared Brandon Productions on Vimeo.

Staring blankly at the 3D Printer I had a brain wave: Sitting in the ceiling was my reflector telescope with a perfectly good equatorial mount... This mount allows the operator of the telescope to easily track a star or planet across the night sky as the earth rotates.

The brainwave involving the printer was to adapt the mount to take a DSLR camera for time lapses.

An hour with Google Sketchup and my vernier calipers I had a few parts drawn up.

The part on the right is an adapter plate the attaches a Manfrotto quick release adapter to the equatorial mount. The part on the left is a clamp for mounting a cordless drill motor (more on this later)

Then on to printing :-)

The adapter plate:

And the motor clamp:

Measure 200000000 times, draw, redraw and redraw meant that the parts bolted together without a hitch.

Mount with legs attached and a camera attached to the QR adapter:

So are you wondering what the cordless drill motor is for yet? The whole idea behind this system is that the camera can be moved a fraction of a degree in pan and or tilt between each of the hundreds of exposures required to make up a decent time lapse. Now I don't know about other photographers or engineers out there but I'm not doing that manually!

Enter the cordless drill motor: With the chuck it mounts onto the driveshaft of the pan head very easily. The clamp simply serves to take some of the motor's weight and to stop it from turning.

A test of the head with the camera in video mode:

Control electronics are in the works, likely to be based on the Arduino platform... Stay Tuned :-)