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Ryan Parle

Fri Mar 20 2009, 05:41PM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

Hi all

I think this is my first post here... the forum and several usernames seem quite familiar, But i have been reading about high voltatage experiments for years.

I have decided that i'd like to build myself a small tesla coil, I've been reading loads of different sites and i think that i have got quite a good grasp on the theory, and how most of the parts work but i've still got plenty of questions.

Ideally I'd like to make my TC quite small, probably no more than say 2ft overall height, maybe even smaller.

Firstly I am looking to sort out a power supply. I have a few old CRT monitors which i am prepared to sacrifice for their Flyback transformers. I have a MOT that has been in my garage for years awaiting a suitable project, Also i've got plenty of car ignition coils.

I understand that the MOT will need some sort of current limiting and really i would need several MOT's in series to get a high enough voltage. So already the MOT is not looking that favorable.

How suitable is a flyback transformer? Or car ignition coil/s? obviously both of these will need some form of driver circuit. it would be nice if i could run the TC from a low voltage DC source so i can use a battery thus keeping the amount of RFI fed in to my household electrics to a minimum, but i'll use a mains source if need be.

a lot of people use Neon sign transformers, would one of these be significantly better to use over any of the bits i already have?

Another thing i am a little confused about is the differnce between a conventional TC and a SSTC, i understand that the conventional has a spark gap and the SSTC uses IC's. is the difference essentially in the type of power source used?

I think that is all my questions for now.

Thanks.

Shaun

Fri Mar 20 2009, 07:05PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

For a first TC a neon sign transformer (NST) would be easiest; they are already current limited and produce more than enough voltage.

With the power sources you describe, you will be building a spark-gap tesla coil (SGTC). The difference between SGTC and SSTC is an SSTC typically uses a lower-voltage power source, think 12-600V, whereas an SGTC uses a much higher voltage source. SSTCs are much more complex to build and I would not recommend them as a first project.

Finally, if you really want to run the coil off a battery (really not necessary with a little filtering), your best bet would be to use a 12VDC-->120VAC car inverter to run a small NST, but now that I think about it I'm not even sure if the waveform they produce could drive a transformer well.

That's about all I have to say. Keep us updated, as I am sure once you begin building you will have some more questions!

lbenson

Fri Mar 20 2009, 07:43PM

Registered Member #1756 Joined: Tue Oct 14 2008, 07:12AM
Location:
Posts: 32

As far as i understand the main difference between a SGTC and SSTC is the type of switching. SGTC uses a air gap where SSTC uses some form of transistor. MOSfet IGBT etc.

Ryan Parle

Sun Mar 22 2009, 07:50PM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

Thanks Guys.

Shaun. I've spone to a couple of local sign shops about getting hold of NST's they have all given me the "Sorry mate but we dont keep them old things no more" story, but i left my phone number and they said that if they found any they'd give me call

Ideally 'd like to make use of the bits i have already, so i guess its time to move over to the HV board and discuss driver circuitry for flyback or ignition coils in there? or is it ok to discuss them here since it is for use on a TC?

Im very reluctant to perform inital testing on mains power, i have a reasonably large computer network at home and it can be rather time consuming to reboot the whole lot and i darent fire up a TC or any other high voltage experiment with it all turned on. I understand that most of the RFI is filtered out by the commercially avaliable filters, but at the moment i'd rather not risk it.

Download

Sun Mar 22 2009, 08:24PM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

I've never used a filter on my small 500watt coil and I've never had a problem with the 6 computers running in my household. The only problem I have is the TV's picture going funny

Shaun

Sun Mar 22 2009, 09:01PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Concern for your electronics is more than valid, but computer power supplies are highly filtered so you don't have to worry so much.

That aside, Just because you cannot find an NST doesn't mean you have to build a solid state HV supply. You could if you want; it's a project in itself and will be useful for many things besides a TC.

Alternatively you could base your coil off of an MOT. A single MOT could provide more than enough power to run a coil your size. I know you are aware that they use a lower voltage, but you could use some combination of a voltage doubler, tripler, quadrupler, or DC resonant charging.

In my opinion an MOT powered coil would be best, since with a ZVS flyback driver you will have a tough time extracting the several hundred watts needed from it. It has been done, but you may find it difficult.

Renesis

Sun Mar 22 2009, 09:18PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Finally, if you really want to run the coil off a battery (really not necessary with a little filtering), your best bet would be to use a 12VDC-->120VAC car inverter to run a small NST, but now that I think about it I'm not even sure if the waveform they produce could drive a transformer well.

Some more expensive car inverters actually produces sinusoidal waveforms. I guess they should be perfectly able to run a small NST

Download

Mon Mar 23 2009, 07:14AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

Shaun wrote ...

Concern for your electronics is more than valid, but computer power supplies are highly filtered so you don't have to worry so much.

That aside, Just because you cannot find an NST doesn't mean you have to build a solid state HV supply. You could if you want; it's a project in itself and will be useful for many things besides a TC.

Alternatively you could base your coil off of an MOT. A single MOT could provide more than enough power to run a coil your size. I know you are aware that they use a lower voltage, but you could use some combination of a voltage doubler, tripler, quadrupler, or DC resonant charging.

In my opinion an MOT powered coil would be best, since with a ZVS flyback driver you will have a tough time extracting the several hundred watts needed from it. It has been done, but you may find it difficult.

MOT's are really dangerous, I wouldn't recommend them for a first coil. I've been coiling for 3 years and I only just started using MOT's a few months ago

Ryan Parle

Mon Mar 23 2009, 04:48PM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

I just had a call from one of the sign shops i mentioned

they have had a 7kv 50ma NST arive that i can buy for a small price so i figured i'd start to do a little more planning

i downloaded WinTesla and entered the parameters and here is what it came up with:

Primary Transformer
Input Voltage: 240VAC
Output Voltage: 7000VAC
Output Current: .05A
Input Frequency: 50Hz
Capacitance for Transformer Z Match: 0.0227uf
PFC Capacitance: 14.5067uf

Primary Design
Inside Dia: 4in
Average Radius: 2.31in
Wire Dia: 0.06in
Total Dia: 5.25in
Wire Spacing 0.1in
Wire Length 4.62ft
Total Turns: 3.9
Secondary Res Frequency: 588.3496khz
Inductance: 0.0032mh
Primary Cap to Res: 0.0228uf
Angle of windings: 0deg (flat spiral)

** primary wire is 14AWG enameled **

Secondary Design
Dia: 1.96in
Length: 10.04in
Wire gauge: 29
Wire Length: 420.25ft
Total Turns: 819
1/4 wavelength (W/O top load C): 238.47ft
Inductance: 5.90mh
Res Frequency: 1013.86khz
Medhurst_K:0.81
Self_C: 4.03pf
Top load C: 8.38pf
1/4 wavelength (with top load): 418.23ft
Res Frequency (with Top load): 588.35 khz

Does anything look wrong here? if not i'm probably going to start making the primary and secondary tomorrow.

I have a few alternatives to the primary wire if what i have suggested is not suitable, my alternatives are:
2.5mm2 solid insulated copper, 5mm o/d copper tube. i can also adjust the spacing if needed although i'd like to keep the overall diameter as small as possible. I am considering adding an additional 3 or 4 turns to the outside of the primary for a bit more room when tuning.

The Secondary wire is some that i have reclaimed from the primary coil of an old 12v regulated PSU but i'm not 100% sure i have enough so i may find that i have to use either 30AWG instead as my local supplier does not keep 29AWG

*EDIT* Typo in NST output curent when entering details in to WinTesla. i've corrected the info above

Arcstarter

Tue Mar 24 2009, 04:07AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, a single MOT is what my coil shall be using soon (keep switching my idea from NST to MOT XD). If you have some good capacitors, and diodes, a single MOT coil is very feasible :P.

A ZVS powered flyback driver sgtc is what i would go wit though. You can easily make a driver to power the flyback with enough to get a foot or more spark (:O). Theoretically my driver would get 44 inch sparks on a sgtc. The capacitors can be cheaply homemade, with nothing but alu foil and transparencies. A rotary spark gap would be nice, but static will work. Primary can be some solid (or stranded) copper wire from the wire made for wiring house plugs up. A static gap can be made from carriage bolts or small copper pipes.

For the driver: Some simple 12 volt zeners, some good 20-30-what ever you can get mosfets, high wattage 470-600 ohm (i found that people have sometimes had better performance with more resistance) resistors, fast diodes (simplest ones would be UF4007), and a good capacitor (i like CDE :) ). Just look at the schematics. You can find the components at Newark and other places (digikey, mouser, etc).

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