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Speed of spark-gap ... ?

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john1

Active Member
Hi,

I am at present fiddling around with several projects,
as i suspect many of you are too.
One of my projects wants a very brief sharp rise pulse,
achieved as minimally and easily as possible.
This wants the rising pulse to be as fast as i can make it.

I am considering a very simple circuit of a tiny air gap
across a small capacitor fed from a couple of meg-ohms to
about 100 v or so.

My question is about the speed of air breakdown, are
modern high frequency transistors faster than a tiny spark ?

I'm referring here to the rise time, not the repetition rate
that i may or may not achieve.

If transistors are faster, i will start on a different track
for this project.

Personally i would think that voltage breakdown of air
would be about as fast as it gets, but i don't really know.

Cheers, John :)
 
Look into step-recovery diodes. You can create sub-nanosecond risetimes, and with a short transmission line (a piece of coax or a PC board trace), you can create very short pulses. I did this about 30 years ago.
Also, Google "spark gap risetime".
 
How much current you want to switch, what is your load, what rise time do you need, and/or why does your load need a fast rise time.

It's unlikely your requirements could not easily be met with conventional components.

100v is not a great choice for a spark gap. It would need to be so close together the exact distance will be unpredictable.
 
Thanks Ron,
i have got fed up of trying all sorts of stuff on google trying to find out
if the breakdown of air is faster than electronics
or whether it is not.

I have nothing much to go on but
the breakdown of air and passage of current happens very quickly.
Quick enough for the molecular disturbance to emit light,
and its blue light, which is at the fast end of the spectrum.

So maybe its faster than semiconductors,
or maybe the blue is just the glow given off by air .......
........ no, i think its also blue in a vacuum .....

I will look into a semiconductor solution, if this won't work.
But this possibility is very cheap to knock up,
and i can try it easily, and easily alter the gap ... etcetera.

Regards, John :)
 
Thanks Oznog,

I think i will end up with an electronic solution.
But for now i will be trying the spark gap to produce very high rise times.

!00 v may not be a good choice, its pretty experimental and can be
altered as required.

The unpredictability is not an issue, the fast rise time is an issue.

You say:
It's unlikely your requirements could not easily be met with conventional components.


That is not saying that a small spark gap is faster than electronics,
or that it is not faster.

But i will say that making a little spark is a lot easier than building an
electronic assembly with a soldering iron.
Although i feel that is how this will end up.

Regards, John :)
 
Oznog said:
How much current you want to switch, what is your load, what rise time do you need, and/or why does your load need a fast rise time.
We could offer more suggestions if we had some concrete answers to Oznog's questions.
 
Hi ...

How much current you want to switch, what is your load, what rise time do
you need, and/or why does your load need a fast rise time


We could offer more suggestions if we had some concrete answers
to Oznog's questions


I dunno what current this will want, but it won't be much, i guess between
half an amp to about 5 milli-amps
I don't know what rise times this will want, but they have to be very fast.

Its an experimental set-up for an unusual job.
I will adjust various elements(aspects?) as i go along.

Unfortunately i suffer from the same complaint as many people who try to
get their ideas to work, and that is that they don't like to discuss it.
If i had been able to find out from Google or found any way to determine
for myself, I would not have posted.

Recognising the paranoia in myself, does not negate it. :) :)

None of which has helped me to decide whether or not a small spark will
give me a faster rise time than electronics.
On balance, there may not be much in it.

I'll see how it goes.
Regards, John :)
 
We did not provide a direct answer to the "is a spark faster than other stuff?" because this is not enough information. It's like asking "is a playing card faster than an orange?" This may actually make sense in some context, but you'll have to give the context.

Blue is not "faster". In fact the speed of a spark gap would be a very complicated question. They are usually used for switching very high current at very high voltage which is difficult, complicated, expensive, or even impossible to achieve with traditional components.

Rise time is often not limited by the switching element itself, but other conditions such as capacitance and connection inductance. Any bipolar or MOSFET transistor is unbelievably, out-of-this-world fast in the world of human perception, but when we deal with certain electronics problems we may need switching in terms of milli, micro, nano, pico, or femto seconds.

First we need to figure out if you're looking at needing milli or femto. Since these are a trillion times off from each other, that's where I'm getting at this being a nonsense question without a context. Can't help you without that.
 
the spark gap... do you mean the time it takes to "break down" the air or the time it takes for the electricity to pass through it. electricity travels at the speed of light, thats your time it takes for the electricity to pass through. the breaking down of the air would depend on the size of the gap and the wattage.
 
Hi Y'all,

I have been trying to find out what i can about this, without much
success, there doesn't seem to be much about the actual speeds,
although i seem to recall that the speed of electricity through a
wire is rather less than the speed of light.
It might vary a bit with different conductors, i think its between
a half to two thirds of the speed of light.
There are instruments which can send a brief pulse down an open
ended wire, and listen fo the return pulse from the open end.
These instruments are now pretty accurate, good enough to decide
where a crew should dig a hole in the road.
Such an instrument would rely on knowing the speed of the electricity
in the wire, and if different conductors have an effect, then this
would be part of the adjustments available to the operator.

I feel sure someone on this board would know more precisely :)

However, back to the spark.
Sparks are generally weird shapes. They travel from point to point
in a haphazard sort of way, i would guess that this could vary the
distance involved up to about a third more than the direct route
would be.
Also, i think that (breakdown) sparks start from both points, then
sort of meet somewhere in between.

There is another type of spark, a drawn type of spark. These do not
rely on an extra-high voltage to cause a breakdown of ionised air (if
in air), because they occur when conductors disconnect and move apart.
These type of sparks can be quite destructive to the surfaces
involved, and much work goes into making sufaces which withstand
this treatment fairly well. This type of spark is a study of itself
and is not the type of spark which i would be trying to use.

If the spark is considered as a conductor, then the speed would be
relative to the distance involved.

I haven't got as far as that yet, but the distance i will be trying
is likely to be quite small, i would guess at around a half of a
millimetre, but i really don't know how far a couple of hundred volts
can jump, so i will have to fiddle around.

Maybe someone on this board could put some numbers to it ?

So i really can only guess at the speed of the rise time involved.
Other factors affect the rise time too, as has already been said,
but these factors are generally understood and known.

As i have said, i don't think there will be much to choose between
the speed of electronics and a small spark.
But i think i will end up with an electronic arrangement because of
the possibility of interference. Sparks seem to be notoriously
difficult to screen compared to electronics.
And also for controlling the repetition rate. This may turn out to
be important, i dunno yet.

Thank you all for your interest and comments, John :)
 
The propagation velocity in conductors (not including inductively-wound ones) is dependent solely on the dielectric constant of the insulator which surrounds the conductors. It is inversely proportional to the square root of the dielectric constant. The relative dielectric constant of air (and vacuum) is 1.0. Signals in conductors surrounded by air propagate at the speed of light. For stripline (buried conductors) in FR-4 (epoxy-fiberglass board material), the relative dielectric constant is around 4 - 5 (depending on the fiberglass/epoxy ratio), so the propagation velocity is slightly less than half the speed of light. For microstrip (surface trace over a ground or power plane) in FR-4, the effective dielectric constant depends on trace geometry, but typically is between 3 and 3.5.

Regarding the breakdown voltage of air, I found this statement **broken link removed**:
Figure 6, known as Paschen's curve, shows the breakdown voltage for plane electrodes in atmospheric air as a function of the product of the atmospheric pressure and the electrode distance. The product is given in units of mmHg • cm. Given a pressure of 1 atm (760 mmHg) and an electrode distance of 0.1 cm, we find at an abscissa of 76 mmHg • cm a breakdown voltage of 3000 V, corresponding to the breakdown field strength of 3 • 106 V • m–1 for plane electrodes.

The curve also shows that there is a minimum breakdown voltage of about 330–340 V, corresponding at 1 atm to a distance of 5–6 µm and a field strength of approximately 6 • 107 V • m–1. It means that it is not possible to get a discharge between parallel electrodes at a voltage less than approximately 300 V.
I have no idea what happens if the electrodes are sharp points or spheres.
 
Hi Ron,

Thanks for that interesting passage, i will have to read the full
article several times, and copy it for reference.

It seems there are more variables than enough, and an electrical
assembly would have to be fiddled with regularly to make it work, if
it could be made to work at all (for this purpose).

It is starting to look like an electronic solution is definately the
most likely one.

Don't think he mentioned humidity, which i would think might also be
another variable, which would be avoided using an all electronic
set-up.

I will probably spend a little time to see how plausible a spark is,
and i will try it with pointed bits, or one pointy bit anyway.

I would have tried pointy bits anyway, but after reading that article
i won't enen bother with bits that aren't pointed. I had thought of
dome headed mirror screws, but after reading that article i won't do
that.

Cheers, John :)
 
And,
i did not know that conductors in air would carry signals at the
speed of light, thank you for that.
Nor that conductors could be that affected by their immediate surroundings.

So, if a Yagi assy were dipped into a solution of glass-fibre type insulator,
then withdrawn and dried, presumably its response frequency would drop to
a lower frequency?

What about when they get covered in rain? my telly often seems poorer when
its raining, but that could be my imagination :)

Cheers, John :)
 
isn't your t.v rf not cable. Rf siganals get weaker when they pass through somthing, if at all. so the rain drops affect it.
 
john1 said:
And,
i did not know that conductors in air would carry signals at the
speed of light, thank you for that.
Nor that conductors could be that affected by their immediate surroundings.

So, if a Yagi assy were dipped into a solution of glass-fibre type insulator,
then withdrawn and dried, presumably its response frequency would drop to
a lower frequency?

What about when they get covered in rain? my telly often seems poorer when
its raining, but that could be my imagination :)
Cheers, John :)
I don't know much about antennas. The wires I was talking about always have an identifiable current return path - either two parallel wires, such stripline or TV twin lead, or a wire over ground, such as microstrip.
I don't know what the effective dielectric constant is for a coated wire in free space. If you potted the entire Yagi in a dielectric, then I'm guessing you could predict the change in frequency.
 
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