*small* spark generator

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I need to generate sparks between two pieces of brass, about a half
inch apart or slightly more. I want each spark to be triggered by a
logic line going high (going low would work, too.) The voltage supply
is 6v or 12v from a battery. The problem is that I need the circuit to
fit inside a 1.5" inside diameter PVC pipe, and not take up more than,
say, 3" of the pipe length, discounting battery. I was looking at
autotransformers, but I'm not finding anything that will fit in that
space. I don't need more than 1-2 sparks a second, but I want the
circuit to be reliable. Can anyone suggest a circuit design that fits
in a small space like this?

I'm aware of the safety risks in generating a voltage high enough to
cross 1/2" of air, and the spark won't be anywhere that a finger can
get into it. If it wasn't for the voltages involved, I'd tinker
something up by trial and error, but in this case I don't like the
possible consequences of "error", so I'd rather get expert advice. I'm
willing to wind my own transformer if that's needed. Thanks!

Re: *small* spark generator

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May I suggest an electronic gas stove igniter?

The most common type runs fro a i.5V cell using a single transistor blocking
oscillator to generate about 250V - that charges a capacitor which is dumped
into a HV pulse transformer a few times a second by aa gas discharge tube -
replace the tube with a thyristor and you can control it.

If you're lucky, you might find a handheld igniter, alternatively go to
recycling and look on the backs of scrap gas stoves.

Alternatively; look for a tiny SMPSU (DVB-T tet top box type thing)  with a
little re-arrangement of the blocking oscillator wiring you can make a 5V or
12V inverter - you should be able to produce a continuous ark between the
pins that were the primary at about 20kHz or more.



Re: *small* spark generator
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Have you looked at a Tazer?



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Re: *small* spark generator
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Potato gun?

Re: *small* spark generator
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It's usually a good if the OP states the objective. For a potato gun
most use a BBQ igniter.



Re: *small* spark generator

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Fair enough. It's to make a prop for a play involving mad science. The
spark will be created inside a small brass funnel, and I'd like it to
be bright enough and loud enough to be noticeable at a distance, which
is why I want a longish spark. I'm not igniting anything with it. I'd
prefer not to use a tazer circuit, on the grounds that those are meant
to knock people over; I want something less likely to cause lawsuits
if there's an accidental shocking.

I hadn't considered a continuous arc, on the grounds that I figured
that would eat a lot of energy, but I have to admit that would look
impressive, if possible...

Re: *small* spark generator
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I don't think the arc will be heard any distance in the magnitudes that
you are describing.
If it's for "Mad Science" why not incorporate a Jacob's ladder, much
more impressive?

Tom

Re: *small* spark generator

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With a motorcycle ign coil (twin HT lead "wasted spark" type is best) and a
555 driving a reasonably "fat" MOSFET its pretty easy to get fast enough
sparks for persistence of vision make it seem continuous.

After playing with the experiment for a while, my ears were left ringing!



Re: *small* spark generator

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This sounds promising, but I've been digging into this, and I need
30kV per cm of air gap. Or about 45kV to get what I want. I'm not
finding motorcycle coils that talk about those voltages. But I think
I've found a pulse transformer that's rated to that voltage, and fits
in a 1.5" tube:
http://www.amazing1.com/Graphics/CD45.jpg. From the
scanty description elsewhere on the site, I think it wants a 300v
pulse for input, so now the problem reduces to generating a 300v pulse
from a 12v battery, at a rate set by a CMOS logic circuit (1/sec would
be the absolute minimum, 10/sec would rock.) Suggestions appreciated!

Re: *small* spark generator
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ScottM wrote:

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Ah, but the gap doesn't start out being a cm wide. It starts off tiny,
with the two wires diverging as they go upwards in a 'V'. The air gets
ionized or plasma-ized by the spark, making it a great conductor. The
warm air around the spark makes it rise up the wires.

I once got the spark four or five inches wide before it went out. That
was with a sixteen thousand volt neon light transformer and four feet
of 'rabbit ear' shaped wires, tapering from about 3/8 of an inch to
about six inches wide at the top.

Once the spark extinguishes itself, another one automatically starts
at the narrow gap and the process repeats.

You may need a glass tube to shield the arc if there are any strong
drafts in the vicinity.

Here's one using ten thousand volts:


http://www.youtube.com/watch?v=P5KRE1nlgTc


The power consumption is very low. My transformer drew under two amps
from the 120 volt wall outlet.

mike












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Re: *small* spark generator
I probably should clarify the problem, since people are talking about
AC power sources and jacob's ladders.

The prop is a handheld device, about 6-8" long, made mostly of 1.5" ID
PVC pipe. Maybe 3" inside that pipe are available for electronics. On
the front is a small brass funnel, with an insulated wire inside it. I
want sparks to jump from the wire tip, to a point on the inside
surface of the funnel. The funnel will point at people (the audience
when possible) and I want the spark to be plainly visible at a
distance. (I figure the acoustics of the funnel will make sure it is
audible.) The power source is a small 12v battery, probably in a
backpack or pocket. (A 9v battery would be great, but I figure if I
want a few thousand sparks, that's impossible.)

My guess is, this is a hard problem, which is why I'm here. If I
wanted a jacob's ladder, I can find designs for those myself, online.
If I had AC power available, I could probably tinker up something for
myself and eventually get it right. It's the combination of small
space, long spark and small battery that makes this something I don't
want to just tinker up. It needs to work, it needs to be efficient,
and for safety reasons I want the circuit to fit entirely within the
PVC. So please, think SMALL. Thanks!

Re: *small* spark generator
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  Not to be rude but, I think it maybe just out side your abilities?

    You can use a Cockcroft Walton voltage multiplier..

http://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generator

  Follow the guide lines on the actual rectifier chain, you can keep
adding to the string..

   but for your circuit to start off with.. You can get a HV Transistor
  of a MOSFET or NPN and what you do is use a high frequency pulse source
like a 555 timer to switch this on/off. This transistor will pull
current through an inductor and when the transistor switches off, the
release energy is many times higher in voltage. This output is connected
to the first stage of the Cockcroft Walton multiplier.

  P.S.

   Look up a MARX generator.

    http://www.electronixandmore.com/project/4.html
   You can use a basic boost single coil switcher to get the voltage up
  to some point where a MARX circuit can the start to work.


  Hope that helps.






Re: *small* spark generator

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It might. I'd ruled out a Marx generator because it looked like the
size of the spark gaps was critical, and it didn't look easy to pack
it into a small space without distorting the wiring and messing up the
gaps. But the CW multiplier looks interesting.

I do have a couple questions. Since I need to get into the
neighborhood of 45kV, the last few stages are going to be handling
serious voltage. I've never worked with diodes that can handle that
voltage - is this likely to fit into my space constraints? Can you
recommend specific parts?

Secondly, I was hoping to get sparks at a regular, controlled
interval. (The goal is to have a 555 drive a 4017 at 10-20Hz, with
each output of the 4017 controlling an LED, except for the last, which
triggers a spark. Hence flash-flash-flash-flash-flash-flash-flash-
flash-flash-SPARK!) The CW circuit is clearly going to have some
recharge time, and if it's a few milliseconds that's fine, but I
suspect it's a lot longer. How is it calculated?

(At some point I'm expecting someone to look at this, and say
something like "12v to 45kV, 2 sparks a second, in 5.25 cubic inches -
give up, it's not possible".)

Re: *small* spark generator
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The sparkfun thing is small but needs three
amps of input so you'll have to be careful
with the batteries - probably need at least
a couple of D cell sets in parallel to prevent
them from getting hot.

You'll also have to take into account that
battery voltage varies over time and also
with temperature and current draw. Getting
exactly five volts from a battery without
extra hardware isn't easy.

Battery voltage varies over time like this:

http://www.powerstream.com/z/AA-100mA.png

eg. You might have to use four D cells which
have been partially discharged to work with
that spark gap igniter from Sparkfun (ie. get
them down to about 1.3 volts each before using
them otherwise you're looking at 6V of input
and it might burn out).


How long does it have to run for? If it's only
short bursts then it might not matter much if
the batteries warm up a bit. For continuous use
it can be a problem.

The sparkfun thing definitely looks like a good
starting point. Get hold of a couple and play...

Re: *small* spark generator

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Then look for a very small SMPSU transformer as I've alreadyy suggested -
you can run the blocking oscillator from 4 or 5 AA NiMh.

Anything over about 20kHz you can get an apparently continuous arc with way
less than the voltages you're on about.

If you want individual sparks the BO is easy to control with a single
additional transistor.



Re: *small* spark generator
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  [...]

ScottM,

You might take a look at this item and its associated video:

  Spark Gap Igniter - 4.8V -- $15
  http://www.sparkfun.com/products/11218

It's not completely clear whether their "gap <0.5inch" is compatible
with your "about a half an inch or more".  Also, it looks like a close
fit (see the photo with a quarter-dollar for size comparison).

Hope this helps...


Frank McKenney
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  another successful campaign against violence, the century-long
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Re: *small* spark generator

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The text say 3A draw, the vid says 20A draw.  ?????

Also, be aware Sparkfun, being a small company, doesn't have the
financial/legal horsepower to fight for yer privacy.  They recently
got into trouble cuz some dirtbags used one of their items to make
illegal credit card readers and the govt came down on them, demanding
lists of all their customers.  A deal was worked out and sparkfun
released only the customers that had recently purchased the offending
device, not the entire customer list.  Not saying you shouldn't deal
with sparkfun, jes that you should know what's what.

http://tinyurl.com/7co6lnp

nb



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Re: *small* spark generator
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There's seven of them in the video.



Re: *small* spark generator
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Just a follow up:

I was ordering some Arduino stuff from
Sparkfun so I slipped a couple of their
spark gap generators into the order.

They're perfect for any kind of mad
scientist machine. You can get decent
sparks from a single AA battery.

This is with three AA batteries:


http://www.youtube.com/watch?v=kQN3Xys1Ilc


Disclaimer: 3xAA is not the way to do
it for more than a couple of seconds.
The batteries are overloaded, they'll
heat up, bad things will happen...

Re: *small* spark generator
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Can you point to a non-private video? I'd like
to have a way to generate single sparks cheaply
to collect audio impulse responses.

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Les Cargill

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