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Entering the field with a small flyback SGTC

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Shrad

Sun Sept 19 2010, 09:05PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

hi all

I'm new in the field of tesla coils and finally registered here to find some advice and share my work

I'm a laser enthusiast and I've always been interested in HV stuff... during past years I had the opportunity to get my hands on several useful parts, capacitors, NST transformers, pig poles and the like...

I have always wanted to make a tesla coil and I've begun the work on my first prototype

It's very small, one centimeter outer diameter and 4 centimeters tall, wound with 0.3mm magnet wire and a 1mm wire diameter primary

I made calculations about resonant frequency which is near 1.3MHz and have set the primary capacitor accordingly

My power supply simply consists in a CCFL driver fully rectified with small HV diodes, feeding 1.2kV to the primary with a 0.5mm spaced gap...

I have had no streamer, so it doesn't seem to be resonating, but I still had small smarks maybe 0.5mm tall

I just have burnt my CCFL driver by paralleling two identical units thinking the caps would balance the current between them, and I'm now screwed with my current design

I have several BIG CCFL drivers from a big LCD TV and was wondering if anyone here could tell me which would be the best solution

I'd really like to be able to get some streamers, so I'm thinking of building a self resonating driver with feedback... do you think it would be doable?

Just something more to say tonight, a big thank you all for all those nice projects and pictures...

Makes me want to hook a pig pole to mains and use my 50nF 25kV line capacitor, and make some BIG sparks... :D

Dr. Drone

Sun Sept 19 2010, 09:34PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

ScotchTapeLord

Sun Sept 19 2010, 10:14PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

I agree that a neon sign transformer would be the way to go.

By "self resonating driver with feedback" do you mean a solid state coil? That would be very doable but is much more complex than a spark gap coil.

With regards to your 50nF 25kV line capacitor, you should probably read up on its datasheet before using it for anything. Capacitors suitable for tesla coil use need to be pulse rated (which line capacitors are), but also rated for low dissipation, 100% voltage reversal, high RMS current, etc... Polypropylene film/foil are the key words most people look for. "Metalized" is considered a red flag by some.

While on the subject of capacitors, you don't mention any capacitors in your current setup. What kind are you using and what is their value? The value of your primary capacitor must match the inductance of your primary coil in order to resonate at the correct frequency, which is 1.3MHz in this case.

Sulaiman

Mon Sept 20 2010, 09:14AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

First, if you have an NST then I agree that is the best start.
There are very few components in an sgtc
(in theory; NST, spark gap, primary capacitor, primary inductor, secondary inductor, secondary capacitor,
with magnetic coupling between the primary and secondary inductors,
just 6 components)

Find a design on the net that you like
and uses an NST with specs. similar to yours,
post a link here for comments,
start a PROJECT thread
and you should be up and running with little pain.

The 'fun' part will be initial tuning of the sgtc and later enhancements. OH! and impressive arcs & sparks.

A low power miniature sgtc is a harder place to start.

HV Enthusiast

Mon Sept 20 2010, 06:35PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I agree as well. Try finding an old Neon Sign transformer (NST). These are usually rated from 4kV/20mA (small ones) up to 15kV/60mA for the largest. Usually you can luck out at an old sign shop, especially as they are converting old equipment to the newer GFI NSTs. I actually picked up about (20) 12kV/30mA NSTs from a local vendor who just retrofitted with the newer GFI units and got them all for free. I ended up donating most of them to guys on the TCML list as well as a local Venture Crew (Boy Scouts) group.

I may have some others as well. If you live in the Philadelphia area and are willing to pick one up, i probably have 1 i can give you for free. I definitely have some MOTs i can give you.

Shrad

Mon Sept 20 2010, 08:49PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

Hi all,

Thanks for the inputs here!

The main problem to get a decent coil started out with that NST is that I have to rebuild my workshop ^^

I also wanted to go miniature because I really like that, and the challenge was appealing

I use small ceramic type capacitors, the ones used in little PFC circuits for flyback converters (20nF total capacitance for a 75ÂµH average inductance)

They must be worn as I have no result at this time, and they couldn't be so out of tune... I have vintage 1kV 10nF pulse capacitors though, and I think I could have a try with a small 900VAC 10mA transformer I have

What I was really willing was to build the coil with a CCFL flyback though, and I'll keep trying a bit

And for basic designs, there is no problem, I have been reading HV websites for a long time now :)

I have another CCFL flyback which won't run CW as it has internal protections, but it has an enable input which would work nice with a basic 555 interrupter

For tuning purposes, I have left one of the primary leads longer and bend it more or less closer to the base to adjust inductance, do you think it could whack it out of tune?

Also, would it be easier to get RF if I separate RF ground from HVDC ground?

There are still several things I have to learn from experience, as you can see...

TeslaJunky

Tue Sept 21 2010, 02:35AM

Registered Member #3037 Joined: Mon Jul 26 2010, 06:42PM
Location: Los Angeles
Posts: 20

Hey, my first coil was fly back powered, too. My parents wouldn't let me run anything off the mains, so I used a lead acid battery instead and ran a small transistor based power supply. In the dark, I could see small streamers.

You can see a video of it at

Good Luck! :)

Matt Edwards

Tue Sept 21 2010, 09:40PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

An NST is pretty small for the most part. If you are avoiding them due to space concerns it shouldn't be a problem. You should be able to make a small SGTC with a small NST.

803

Tue Sept 21 2010, 11:13PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

If you want small, you could use a couple of old tube tv transformers. they would only give out around 4kv in all (4 0f them) but if you used a small cooled spark gap, they should work.

Shrad

Wed Sept 22 2010, 07:36AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

OK thanks all for your answers

I think most of the concern was current

I hooked up the coil to a small 800V 10mA transformer, added a top load and upped the primary capacitor a bit

I tried to use Visconol capacitors but they seem to be passed out (the pulsed ones, they were used in a flyback multiplier and I have 0.05Âµ 1kV ones and 0.1Âµ 7kV ones) as they didn't even make the spark gap to fire

I then took some more ceramic capacitors with an equivalent value and rating, and then the gap fired

Unfortunately, as I had no time yesterday to machine something and have a decent spark gap, I soldered a copper bolt on a screw, and a copper L bracket with a steel screw pointing to the bolt, as an adjustable spark gap

The result was the gap rolling on the floor (me not laughing my ass out) due to overheating

I'll keep you updated!

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