Not sure what kind of transfomer I should use

[Fischer777]

Not sure what kind of transfomer I should use...

Hello. I am trying to build a high-voltage DC transformer for a project (using a disruptor). I will be using a battery for the power (probably a 9v or 12v). I have done some reading about the different kinds of transformers used, and I am confused as to which one would be best for this. I was originally going to use a toroid-core transformer, however, because I'm not using AC I feel that there will be some residual magnetic field trapped in the core, or rather the core will become magnetized. I then considered an air-core transformer, but I'm not sure if this will severely reduce efficiency. Would it be theoretically possible to combine both by wrapping wire around a flexible non-metal tube, and then bending that into a torus?

I'm not really looking for a stable source of high voltage output, but a high peak, if that helps.

 

[sophiecentaur]

A 'transformer' works only with AC. A DC voltage converter would be what you need. This cannot be as simple as a wound transformer, which relies on a constantly changing input current to produce its output.

 

[jim hardy]

i'm somewhat of an experimenter and applaud your practical research.
as you and Centaur observed, traditional transformers don't handle DC very well, you need an air gap to prevent that magnetization.

disruptor? as in interrupter?
read about "flyback " converters
and Lenz's law.

for something to tinker with i'd try a automobile ignition coil
shouldn't cost over a dollar at junkyard

or the magneto from an old lawnmower points-style ignition which should be free at any junkpile.
Read up on the magneto, it's a fascinating device. you'll need to put some iron over the airgap . aside from that it should lend itself well to your battery application.
I suggest a 6 volt lantern battery rather than the little 9v it'll give you more current. Experiment with various airgaps - ordinary paper will do for shims.

 

[Fischer777]

Yeah. When I said disruptor I meant some sort of switch that would rapidly turn the circuit on and off. I did some reading about the lawn mower magneto but I'm still unsure about how it can be used in this application. I also did some research on flyback converters, buck-boost converters, boost converters, and oscillator circuits, and I'm thinking that I'll probably try and build some sort of flyback or boost converter, and see how that goes.

I ordered about 3,200 ft. of 30 gauge wire to use as an inductor, so I'm curious, is it okay if the wire overlaps itself? I was thinking that since this is a huge amount of wire it would be easiest if i just left it on the spool, rather than taking it off and coiling it elsewhere. But that also means it will double back over itself.

 

[jim hardy]

automobile ignition coil is a step - up transformer
set a couple amps through its primary and interrupt that, you get from secondary a pulse intense enough to jump a sparkplug.

lawnmower magneto is similar except the primary current comes not from a battery , but from the flywheel magnets moving past the pole pieces. You can inject that current from a battery instead... but i believe you'll have to put a piece of iron across the pole pieces to get enough flux to make that high voltage pulse.. inject your current at the wire going to points/killswitch.

 

[jim hardy]

""" is it okay if the wire overlaps itself? I was thinking that since this is a huge amount of wire it would be easiest if i just left it on the spool, rather than taking it off and coiling it elsewhere. But that also means it will double back over itself.""

that's fine. Just be sure the ends don't cross over. The insulation will withstand only so many volts.
Your flux will make so many volts -per-turn.
Between adjacent turns there's hardly any voltage,
between adjacent layers there's more,
but across entire coil there's a lot

so don't subject one layer of insulation to full coil voltage.

have fun!

 

[Fischer777]

Okay, thanks for the help!

 

[NascentOxygen]

I scrolled down to add my suggestion that you try a car ignition coil, but I see someone has already proposed that. Why not play with a car coil first? It's so much neater, all sealed and ready to go. Just be careful. A high voltage is a high voltage, and no respecter of person. You can economize with your battery, though. The car coil will deliver a high voltage spark when you break the DC, so if you leave the primary current flowing for even a few seconds, you are flattening the battery without producing HT. You might get sparks where you connect the battery, too, so don't think there will be only 12 volts there. May be wise to have something in series with the battery to limit the current: a length of nichrome wire of 6 ohms would be good to start with. It is easy to solder to. Good luck with your experimenting.

 

[sophiecentaur]

If you want to maximise the induced voltage then you should use an appropriate Capacitor to 'tune' the system and to allow more peak current to flow. I remember we were told that the capacitor in a lab induction coil was to cut down on the erosion of the contacts by sparks but it does much more than that.
I seem to remember that, before they were produced with ferrite (before it was available / invented) they experimented a lot and found that bundles of soft iron wire made the most effective core for car ignition coils. This is presumably an advance on the normal laminations you find in ordinary transformers to cut down on eddy currents.

I actually possesses a large, impressive Rumkorf Induction Coil. It's all brass and wood and makes a fearful racket when it is vibrating. The sparks are really deadly! Much worse than the safety conscious EHT generators that people use these days with a couple of mA output.

 

[NascentOxygen]

When I went through the EHT stage, I read somewhere that you could get the argon in an ordinary incandescent light globe to glow like neon (though in a different colour) if you held it by the glass and touched just the base to the EHT source.

I think it was some miscreant's idea of an April fool's joke. : : :

Don't try it!

 

[Fischer777]

Well, when my physics teacher held a light bulb over a Van de Graaff machine so that it arced through the bulb (very high voltage but virtually zero amps) it did glow a purplish color and create some sort of "plasma globe" effect. He would also hold small tubes containing different gasses over it, so that the class could observe colors all the different gasses produced.

That's kind of what I'm trying to do here; create a machine that will produce high voltages at virtually zero amps. I figured that if I build a modified flyback converter, and ground one end of the output, I can induce a coronal discharge from the other. By placing that end close to (but not touching) a metal sphere, I can get the sphere to accumulate a charge.

 

[jim hardy]

search the hobby sites for a kit

somebody makes a plasma globe kit based on TV flyback transformer

old jim

 

[Fischer777]

Oops, sorry, what I mean to say was I'm trying to replicate the Van de Graaff machine without the moving parts; not build a plasma globe.

 

[NascentOxygen]

Oops, sorry, what I mean to say was I'm trying to replicate the Van de Graaff machine without the moving parts; not build a plasma globe.

Is that getting close to a Tesla coil?

Google for Kelvin's electrostatic generator, it may be fun to build. There is plenty of scope for experimentation here. It has its own "built-in" accumulators of charge. Sure to be videos online, too.

 

[Fischer777]

In a sense it's like a tesla coil, but I indent to 'trap' the charge.

One more question as of now. In order for me to construct the high voltage converter, I need some sort of electronic switch that can rapidly turn the system on an off, like a spark gap; only the initial voltage coming out of the battery will not be enough to create a spark gap with. I was wondering if there was any simple method one could use to switch a circuit on and off many times a second, or if such a 'switch' requires another circuit of capacitors, resistors, and transistors.

 

[sophiecentaur]

It's not the 12V from the battery that produces the high voltage in the primary, it's the back emf when you break the primary circuit that does the biz. You can easily get 400V spikes in the primary if you build it and tune it right.

Please read up about induction coils. It will be well worth your while.

 

[Fischer777]

I know. I'm looking for a way to break the primary. Because it's a modified flyback converter, the primary is isolated from the secondary. In order for me to generate a changing magnetic field, I need to turn the primary off and on quickly.

 

[sophiecentaur]

You need a crude mechanical buzzer / bell circuit that will take a bit of abuse from sparks without the contacts eroding. A hefty relay (surplus job) would do what you want, at least to prove the point.