Direct current spark

[Neil9327]

Electrical sparks through air are always viewed as transient, or at very least caused by alternating current.

 

Is it possible to have a continuous direct current spark?

 

If so, what is the lowest voltage that would be required to produce one, and then to maintain one, assuming the following parameters:

 

1cm air gap between electrodes

standard air pressure

standard air temperature in the surroundings

no external wind

 

What current would be produced by this, and what temperature in the spark gap. How bright would the spark be?

If the spark is maintained, presumably the air on the spark track would turn to plasma? If so would its electrical resistance reduce, so requiring much less voltage to maintain the same current.

[YT2095]

that`s actualy a Very interesting question, I also have wondered on the DC issue myself.

most of my HT work is indeed DC (several 10`s of Kilovolts) but these require caps charging and when they discharge there is a build-up time involved, so it IS pulsed.

 

However, depending on what you call a "Spark" I can at DC levels create a beautiful Blue/UV Corona discharge between two electrodes that aren`t Quite close enough to together to discharge anything, and so this arc(s) are sustained.

 

the problem lies in Air current and pressure as you pointed out, things are much different here.

in a little neon bulb you`ll have a current pass and light produced as a Plasma even at as low as 90VDC and it`ll be constant.

 

in air, things take on a whole new set of "rules".

air current is the biggest Pain in the a$$ mostly, as the arc tends to rise and break (heat rises) this doesn`t happen in a vacuum.

for a 1mm gap at Low current, 1--> 1.5 KvDC should do it in a sustainable way (providing a still air current is around it).

[woelen]

Have a look at this one. A beautiful arc, with a lot of technical information about it. So, arcs can be maintained quite a long time in air. This one existed for seconds.

 

http://205.243.100.155/frames/longarc.htm#500_kV_Switch

[YT2095]

Nice site!

 

kinda makes my 35Kv kit look a bit Tame though

[Neil9327]

Ditto.

 

I like that exploding transformer. Who says that chemistry is the only source of flashes and bangs.

 

I think its funny the way the sparks stop suddenly.

 

Of course all these videos show AC sparks, not DC. But I expect that the visual effects would be essentially the same in a DC scenario?

 

 

 

[[in air, things take on a whole new set of "rules".

air current is the biggest Pain in the a$$ mostly, as the arc tends to rise and break (heat rises) this doesn`t happen in a vacuum.

for a 1mm gap at Low current, 1--> 1.5 KvDC should do it in a sustainable way (providing a still air current is around it).]]

 

 

YT: Thanks for the above. I suppose one way round the air current problem is to have a DC power supply that detects in microseconds the breakup of the arc, indeed the start of the breakup of the arc, and increases the voltage produced to either maintain the arc before it breaks, or indeed to force the creation of a new arc across the shortest path between the electrodes.

I suppose this would result in a scenario where the arc is initially created, rises as its hot air rises, reaches a height (several meters?) to a point where a new arc forms directly between the electrodes, causing the first to break.

 

It starts to make sense now.

 

What would happen if you placed the electrodes at the top of the room directly below an insulator such that the rising hot air has nowhere to go. Would the arc then take on a semblance of stability? I guess the air would heat up more and more. What happens to the resistivity of air as it gets hotter (into the thousands of degrees)?

[YT2095]

another good question, and one I have a definitive answer to as I`ve done it

 

depending entirely upon the insulator material of course, it gets frakin HOT!

ceramics are the best and then you get little lightening "Fingers" that move about on it`s surface, one or 2 will remain on the same track(s) the others will dance about seeking new ones, the thing gets Mega Hot though.

as for plastics, they last seconds until a a carbonised track is formed and then you get whiteish sparkles like little stars form along it`s length, even tualy it breaks through and the insulator catches fire.

then if you leave it, the arc will even jump the fire making the flame shape change and also it`s color, and you`ll hear a srong Hissing sound that can become a ROARING when the space inreases.

 

don`t take my word for it though, try it yourself and experience the same

[Rocket Man]

a spark in air.

this is basically the same principle as a flouro tube.

 

an ac spark is easier to make, as you can step the voltage up easier.

the main problem with ac though is the inductance of the plasma.

as current travels it builds a magnetic feild. it takes a lot of energy to reverse it again so an ac spark is doomed to a maximum current. (still high tho)

dc however, if you manage to supply enough voltage at a theoretically infinite potential curent, the current is not limited by an oscillating magnetic feild so the current will rise exponentially as the current path widens with the plasma.

 

if you intend to make a high density plasma, you could try a noble gas, (low breakdown voltage so you can get higher currents)

 

also, to direct a plasma, you can try a magnetic feild, it uses the lorentz force to make the plasma deflect from one side of a magnet, some people use it to make rectifiers in tesla coils.

[woelen]

I've made a 3.5kV/100 μF Villard cascade array, which stores, when fully charged appr. 60 J of energy (Ceff = 10 μF). I, however, could not make a spark with this with a distance of a few mm . I used a series resistor of 100 kΩ and hoped to make a spark with this, which then could be sustained for a while. Nothing happens at all, not even when I have a few mm between the end-points. I dare not handle any of the wires, while the thing is charged. I also dare not make a spark without the 100 kΩ resistor. I once had an accidental discharge of 3 caps in series and that was quite a nasty bang already. Any idea what I should do to get a decent spark with this, which can be sustained for a while?

 

The device is great, however, for lighting large neon tubes. I also have a strange old glass tube (size appr. 2 cm), which when connected to the Villard cascade in series with a resistor gives a weak purple light. I think that most of the light of this tube is UV. A series resistor of 100 kΩ becomes quite hot, when connected to the tube. So, I'm quite sure that the cascade is working properly.

[woelen]

I discovered that the device also supports a kind of "arc" in a xenon flash tube. Normally these things require a large capacitor (e.g. 300 uF / 350 V) and a trigger voltage of a few kV to get an arc. With such a trigger voltage and a capacitor, charged to 350V or so, the arc only is maintained for milliseconds at most. I, however, could maintain a low current arc with a series resistor of around 1 MΩ. The result is really impressive. The arc is flashing around in the tube, it sometimes narrows and becomes sharp, sometimes it becomes more fuzzy. I made some pictures of it.

 

 

 

The two pictures below are low-exposure pictures, showing more details of the arc. In reality, the background had normal light conditions. The first picture was made, while the arc was fuzzy, the second while it was sharper.

 

 

 

The tube becomes quite hot. After disconnecting the power supply, I felt the tube and it was so hot, that I could not bear it on my skin.

 

 

I also made a little movie of this. I'll see how I can compress that somewhat. Right now it's over 40 MByte, which is too much for my website. But I already want to show these nice "arcs".

[YT2095]

I`m surprised at your 100k resistor not working? My smallest Voltage multipler is the Cocroft Walton type, using 10x 0.1uF at 1000volts each, so it`s a 5 stage multipler and I get a nice steady 5kv out of it, when I used a 100k resistor in line it fired up my HeNe gas laser perfectly, until the resistor burned out about 3 seconds later. it is hard to find the charge/discharge ballance in Time, I found that a direct connection made the laser Flash on/off about twice a second and get very hot and give a Pinkish/white light inside instead of the normal pink/orange.

 

woelen, for more interesting light effects, try the inside quartz tube in a sodium light and a mercury light, it looks especialy strange if you can alter the freqency in DC pulses across them, you get Zebra stripe paterns that dance up and down the tube like little bands of light and dark.

with the sodium tube you`ll get a bright purple to UV glow from it, but if you heat the middle of the tube with a burner Then you`ll see the classic sodium Yellow come out

 

btw, Nice Pics!

[Rocket Man]

I`m surprised at your 100k resistor not working? My smallest Voltage multipler is the Cocroft Walton type' date=' using 10x 0.1uF at 1000volts each, so it`s a 5 stage multipler and I get a nice steady 5kv out of it, when I used a 100k resistor in line it fired up my HeNe gas laser perfectly, until the resistor burned out about 3 seconds later. it is hard to find the charge/discharge ballance in Time, I found that a direct connection made the laser Flash on/off about twice a second and get very hot and give a Pinkish/white light inside instead of the normal pink/orange.

 

woelen, for more interesting light effects, try the inside quartz tube in a sodium light and a mercury light, it looks especialy strange if you can alter the freqency in DC pulses across them, you get Zebra stripe paterns that dance up and down the tube like little bands of light and dark.

with the sodium tube you`ll get a bright purple to UV glow from it, but if you heat the middle of the tube with a burner Then you`ll see the classic sodium Yellow come out

 

btw, Nice Pics! [/quote']

 

 

i've burned a resistor before, stinks like nothing else. for the 100k ohm, use 3 33k resistors and a heat sink, even coat them in sealant in dunk them in water, works a charm.

[woelen]

I`m surprised at your 100k resistor not working? My smallest Voltage multipler is the Cocroft Walton type, using 10x 0.1uF at 1000volts each, so it`s a 5 stage multipler and I get a nice steady 5kv out of it, when I used a 100k resistor in line it fired up my HeNe gas laser perfectly, until the resistor burned out about 3 seconds later. it is hard to find the charge/discharge ballance in Time, I found that a direct connection made the laser Flash on/off about twice a second and get very hot and give a Pinkish/white light inside instead of the normal pink/orange.

The breakdown field strength of air is around 3 kV/mm, so that explains why I did not get any sparks. I expected the breakdown field strength to be much lower than this. I've done experiments with much smaller gaps and a series resistor of 1 M and a series connection of capacitors of 22 nF/1000V. With this, I can make nice small sparks, but I did not yet have sustained arcs in air. That of course requires much lower than 1 M resistor values, but I do not want vaporized resistors or other components.

 

The flash tube, shown in the pictures above, contains xenon gas. Apparently, with xenon gas (at low pressure???) the breakdown field strength is much lower. The total length along the tube is appr. 40 mm, so I estimate the breakdown field strength to be only 80 V/mm or something like that inside the tube. Why is it so low inside the tube and do I need 3 kV/mm in air?

 

My circuit is capable of triggering a miniature flash tube from a small camera. Apply a voltage of 300 V on the main leads of the tube and connect the trigger to the capacitor string, then a flash is produced and the 300V capacitor is discharged as well. This of course also an be done with a trigger transformer and a TIC106D, but being able to do without this, shows that the voltage must be at least a few kV. Unfortunately I have no means of actually measuring such high voltages.

[Rocket Man]

xenon, helium and argon, theyre all noble gasses, and all turn to plasma very easily.

also, the breakdown drops when pressure decreases because theres less particles to heat and strip to the nucleus on the way between electrodes.

 

air doesnt like ionising. apparently carbon dioxide ionises easily, they use it in lazers a lot, also you can lower the breakdown of air by adding impurities, humididy will drop the breakdown substantially

 

(i may have a confusion of terms in there, i dont know much about the ionisation energies of certain gasses, correct me if im wrong)

 

if you have an AC HV rig, my suggestion would be to rectify it, a diode bridge will give you dc, and a cap in parralel with the gap will smooth the voltage output.

 

i should have though of it earlier, but dismantle a large-ish fuse and assemble the spark gap inside just for the first stages of the experiment.

you'll have less detrimental factors to deal with.

make sure you dont get any cathode rays coming off it,

cathode rays + glass = Xrays

 

you're talking about unsustained sparks, current might be a problem, if youre not delivering enough energy, the plasma will cool faster the the HV will heat it.

an inductor may also help, it'll stop the caps draining too suddenly and dropping the voltage below an acceptable limit, it'll still need resistors though.

if you put an inductor on it, please remember to put multiple diodes in series to prevent the kickback from kicking too hard

[YT2095]

Woelen, as a Rough Guide, use the 10Kv per Inch "rule". it`s not a Rule as such but in a "Normal" lab condition, it`s fairly acurate

 

the electrode Shape is important too, Needles will work great! spheres will not.

[woelen]

YT, I'll use your rule as thumb. Rocket Man's idea also sounds very nice. I have some fuses and I'll open up one of these. In that way I have the needle-like structure and I have a rigid mount for the spark gap. Right now, I'm hassling around with wires, which constantly move and which make it hard to experiment.

 

If I have some sparks in air, which can be sustained, then definitely I'll make pictures of that and show them over here.

 

I also made a real 240 V-AC / 350V-DC power supply, capable of delivering 200 mA at the AC output and 150 mA at the DC output. This power supply allows me to do less dangerous experimenting. I use a center-tapped 1 : 1 transformer. In this way, I have all voltage floating and touching a single wire does not lead to electric shock anymore. Before this, I did all experiments directly from the wall-outlet, with just a 250 mA fuse in between. Now, things are safer .

[YT2095]

it may be worth getting yourself some Lexan/polycarb sheeting, a few matching nuts and bolts and some pre-drilled `L` brackets, you can bolt these brackets to the lexan and then with a washer bolt the needles to the other side of the L brackets and adjust the gap accordingliy

that`s what I do, and also do the same method with brass spherical doorknobs as spark gaps but I use the Ls facing each other fo that type of config.

 

why don`t you save your broken test Tubes and make your OWN gaps also?

I do

 

all you need is a good flame to polish the glass, some cork and epoxy resin then a bleed tube.

assemble the device making sure ALL is sealed with the resin, add Lithium metal down the bleed pipe as a getter, heat the tube while evacuating it, carefully melt the bleed pipe to a seal and leave it for a few months and let the Li do all the work

OR you can get the Li in the middle of the tube after you vac seal it and then heat it (I avoided doing this so I didn`t melt any epoxy), but if your tube is Long enough, that wouldn`t be an issue

[woelen]

Here is a little AVI movie of the xenon arc. Unfortunately it still is 7 MByte and it also is quite noisy (while the original isn't). On compression it becomes really noisy and that is a pity. But the movie shows the effect sufficiently well.

 

http://www.oelen.net/pics/xenon-arc.avi

 

Please, first download the file to your PC and then play it locally. On my PC the file is not played correcly directly off the site. I do not know why. When it is put on a local disk, then it plays OK.