Thanks Bertus
Unfortunately, the plasma speaker has been put on hold by my father, who is convinced i will kill myself
how dangerous would a flyback converter from a monitor be, if it was connected to say 5 turns of 12V wire turning on and off at audio frequencies?

 

It is a good that your father is concerned for your welfare. It is also good to hear that you are not embarking on dangerous projects behind his back.

I would not recommend high voltage experiments to anyone young enough to need his father's permission. People in different countries seem to have different ideas about how old you need to be for this sort of thing, perhaps voting age would be about right.

I do not know whether you would succeed in doing anything useful with that flyback, but if you did get it to work you could be in danger of getting a serious shock. There could also be a significant fire hazard.

 

So then, is a small Tesla coil less dangerous? Like, miniature? Like a little baby Tesla coil maybe 4 inches high?

Obviously I'm not going to be touching it and I will take all precautions I would with any high voltage source, but I don't know how much voltage/current Tesla's put out.

My main goal is simply to make a little plasma source.

 

After reading your introduction, I would say you seem pretty responsible. I never had a flyback, but I did have a neon light transformer and built a Jacob's ladder and Tesla coil using it when I was young. We can't advise on building such things here (I agree with that policy), but if you can get your dad involved, it would make it a lot safer.

One important aspect of safety in laboratory work is not to work alone ever. Your dad could simply be your spotter, so to speak.

There are sites that specialize in Tesla coils.

John

 

I was thinking this design or something similar
http://kaizerpowerelectronics.dk/wp...55irfp250/audiomodulatedflyback555IRFP250.gif
i liked it because it could be driven with a 555

And my father doesn't really inhibit me with anything this is a first.
I have a 180vdc converter i play with, but hes a mechanical engineer and everything he's read about flybacks is how dangerous they are. As it is I really don't know much about how dangerous they are.

 

So then, is a small Tesla coil less dangerous? Like, miniature? Like a little baby Tesla coil maybe 4 inches high?

Obviously I'm not going to be touching it and I will take all precautions I would with any high voltage source, but I don't know how much voltage/current Tesla's put out.

My main goal is simply to make a little plasma source.

I'd say the Tesla coil would take a lot more time to produce as the big problem is in all the wire involved for the secondary. Not hard to do really but I doubt a 4" one is going to be of much fun. Another search will turn up stuff like this http://www.rmcybernetics.com/projects/DIY_Devices/homemade_tesla_coil.htm but a true Tesla coil should be driven with a minimum of 20 KHz - higher the better. I don't agree with that guy's design at all, in the old days we drove them with an RF output tube into a properly wound and tuned coil for the primary and as many turns you could wind on the secondary. Back then we just had cardboard tubes we scavenged from the factories that would shrink wrap pallets of he tubes they sold to make Estes rockets out of, nowadays PVC pipe is your friend. Magnet wire is hard to come by but the best supplier I've found is http://stores.ebay.com/tech-fixx

Far more dangerous but easy to so, just search for "plasma generator microwave" and as we all know people toss these things out constantly - especially at apartment complexes when the move or craigslist always has a ton for free.

You could always make a Van de Graaff generator: http://en.wikipedia.org/wiki/Van_de_Graaff_generator.

In my opinion your first idea will be complicated but more educational. Find an old flyback from a TV repair shop (they'll probably give you the old ones for free since they're rarely replaced anymore) then use a 555 set to the frequency, into a driver transistor capable of driving a common horizontal output transistor. Some will include a damper diode, some won't so if it isn't there you'll need one as well.

Your other option that I doubt your father will complain about is simply getting an old automotive ignition coil from a salvage yard (~$1) and driving it with a 555 > driver transistor > a high power MOSFET. You'll need a good heat sink on the MOSFET and a good 12V source. The driving frequency will vary on there, they weren't designed to take much more than 100 Hz but can probably be pressed well above that - especially if you've got more than 12V driving them. They can always be put in parallel provided you realize they take a lot of current. If the MOSFETs) you choose don't already have or can't take the back EMF (figure on about 10x your supply voltage) you'll want to put one across the D-S. I'd recommend doing this anyway just for safety to your circuitry.

Sounds like something interesting to draw up.

 

Thanks for the idea, dad said i could take the coil off an antique in the garage, where can i scavenge the mosfets though?

 

Ah well, perhaps not the worst I've ever made up but I'm not feeling well so you can get the idea:

http://www.innoengr.com/examples/plasma_generator.jpg
http://www.innoengr.com/examples/plasma_generator2.jpg

I'm not feeling well so there are probably mistakes in these, in the original I included a driver transistor because it will help shut the MOSFET off in shorter order than the 555 alone will, it also won't require a logic level MOSFET for the output.

Logic level FETs vary widely as to what their threshold values are and you'd want solid switching thus you'd want a good transistor in there for a driver so you could put a lower value in for R3. If you go over 100 Hz you'll need a higher voltage to drive the ignition coail anyway but I'd separate it feeding only 12V to the 555 part of the circuit.

When I used to have a good test bench set up I had a couple of 12V car batteries set underneath on a trickle charger with a DPDT switch so I could switch to 24V if I needed it, rare occasion though. If you experiment with stuff it's a highly recommended idea, just run some leads up to a home made plate with decent 5-way binding posts on it.

We've got a salvage yard in town called "Pull-A-Part" and they pull the batteries from the junk cars when they arrive. If they're still at 12V and will hit a good amperage on a tester they sell them for $10 - $12. Many are only a month or so old as the vehicle owner tried to fix the car by putting in a new battery only to find they had further problems. You can also find them for the mere cost of asking at a lot of places. If you've got a true load tester it helps but if they read 12V all you chance is a dead cell when they get hot or cold.

 

You can't just use any MOSFET, go rake some leaves then go to http://www.mouser.com and pick up some good ones, give me a minute and I'll make a few suggestions.

 

Take your pick if this will come up. In your application I'd want at least 10A or far better (since the duty cycle will be less that half of that) but I'd also want one that has a high D-S breakdown voltage as well as a decent gate breakdown voltage. A lot of people don't realize that the G:S breakdown voltage also works in the reverse as in if a high voltage pules hit the Drain it can bridge back over. The integral diodes they contain tend to prevent this but ignition coils have a high reverse EMF since you can't put a snubber across them so you may as well get good FETs to start with.

http://www.mouser.com/Semiconductor...Z1z0w94mZ1z0vxq4Z1z0w76vZ1z0w7iu&Ns=Pricing|0

Scavenge? Any out of an old PC power supply or that monitor's power supply could be a good start. They're free that way so if they fail then you just have to try again. Some are, some are not rated for the amperage to handle an automotive ignition coil's demand so go rake some more leafs and work up a small order from Muser, $25 will get you a ton of stuff you should have around.

Also see my post somewhere in here about "I didn't need any more stuff" or whatever and by all means pick up a few surprise boxes from Goldmine. Rake a few more leaves and have a ton of stuff to play around with.

 

It all looks good to me, but that doesn't really mean much.
Thanks for the help, I was hoping to be able to throw this together with stuff i had lying around but oh well, ill order some stuff, Ive got the cash
Hope you get feeling better!

 

I think, with tinkering around with the frequency and duty cycle, (should only need about 30% or so) you'll get what you want but please don't test it out on your cat.

Sorry for the poorly drawn schematics but if they're not accurate I'm sure someone will chime in.

A salvaged ignition coil is good for 25KV or so, the new ones go even higher. It sure beats trying to balance out an old flyback transformer as they're pretty frequency specific and depend on a tuned circuit, whereas a car ignition coil is not.

Get the oscillator up and running, then if you chose logic level MOSFETs you won't need the driver transistor but it is recommended.

I'd certainly like to see some people chip in on this design. From my estimation a plain ignition coil can't handle more that 200 or so Hz unless you hit it with more voltage, too much iron inside them.

Regardless you'll have a really nice HV source that can certainly produce a basic plasma at a current that will please your father as I'm sure he's confident enough with car ignition coils and given the correct combination of coils (and and any necessary MOSFETs or driving voltage to the coils) will achieve the desired results.

I will assume you've researched this, to get a good plasma you need a near vacuum.

 

I not overly concerned with "good" plasma, i just want a basic continuous stream I can modulate to produce sound
no real reason, just for kicks

Ive done some research, enough to be dangerous, know whats dangerous, and know whats going on anyway
Thanks for the info on the ignition coils

Someday in the distant future id like to be able to recreate the scene from sorcerers apprentice with the Tesla coils
without all the Hollywood unrealism of course (pretty sure a laptop in the middle of that would get totally fried), and probably without the blonde

 

Heres something that runs of 12V & drives an Ign coil, should be safe enough & simple to build.

 

Thanks for the pics

Is the -12V on the schematic just the negative terminal of the battery? Because i thought that that was just 0V?

 

Yes -- 12v is the bat neg, just the way i draw it. To drive an Ign coil you need a special transistor such as a MJH10012 which needs 4x 75v 3w Zenner diodes in series & across the C & E of the transistor to protect it from back EMF. Will post pics of 3 suitable transistors that can be salvaged from old cars, Ive managed to get them for free from recycler yards as they crush the cars with them still in there. The Bosch 1012 is from a1984 BMW from its ECU & is the Ign coil driver. The 5401GM is from another car ECU. The one with the T shaped conector was bolted to the side of the distributor af a car. So they are out there for free if you look. They are custom made transistors for Bosch, i think as i havent been able to find any data on them.

 

Marshallf3, would this FET be sufficient? You said a high gate-source voltage i wasnt sure how high you meant.
http://www.fairchildsemi.com/ds/IR/IRFS654B.pdf

Vgss +/- 30V
Id 13-21A
BVdss 250V

@debe, thanks, ill see what I can find

 

Radio shack carries the IRF510 or 540? It's a TO-220 package. 14 amps I think. Should be a decent one to build some test circuits with. Needs more than 5 volts on the gate though(again, I'm not 100% sure on the accuracy)

 

Making sure i got this right,
So the +12 volts goes to the small + terminal, the other small terminal goes to ground, and the center one puts out the high voltage?

 

Making sure i got this right,
So the +12 volts goes to the small + terminal, the other small terminal goes to ground, and the center one puts out the high voltage?

Thats about right, The switching transistor pulls the negative terminal to ground.