Adjustable spark gap - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: November 13, 2020, 10:44:20 pm
content: Hi,I need to make an adjustable spark gap. Two screws (connected to two wires) that I can screw in or out to adjust the distance between their tips to change the gap precisely. So the spark occurs between the tips of the screws.1) is it better for the screws to have their tips flat, half-sphere or sharp point?2) to avoid having it release ozone where I operate it, I am thinking of enclosing it inside a glass tube and seal it completely. Glass so I can see through it. After operating it for few minutes, I assume the whole glass container would be filled with ozone. At that point would the breakdown voltage always be the same? Will it increase or decrease compared to the initial condition? After a while will the ozone inside the sealed glass tube go back to normal oxygen like it was at the beginning?Thank you

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 13, 2020, 10:59:29 pm
content: Ozone will decompose over time but it will likely react with your electrodes. And I'm pretty sure the ozone and maybe more critically nitrogen dioxide will affect the dielectric strength of your gas mix. I'd say either make it vented, open air, or make it vacuum.What are the voltages, currents and times we are talking here?Electrode geometry is important, points will initiate an arc more rapidly but spheres, I believe but could be wrong, are more predictable in breakdown voltage.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 13, 2020, 11:15:40 pm
content: Thank you Christoffer,Voltages are anywhere between 2Kv to 20Kv.Currents I have no idea. Whatever will result from the discharge. The idea is that it just discharges to ground.Frequency of discharge is up to 100Hz continuous repetitive discharge.1) So what you are saying is that the breakdown voltage will change in time, right? But will it reach a steady state? If so, considering an enclosed volume of 10 to 20 cm cubes how long would it take to reach steady state, seconds, minutes, hours? Just a very rough estimate/feel...2) would the tube be better plastic or glass? I think it makes no difference but maybe that's not the case?Thank you

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2020, 11:31:44 pm
content: Keep it simple.A couple of standard (=cheap) automotive or motorcycle spark plugs will do the job.Grind off the lower/bent electrode with a Dremel kind of tool, then you have your spark electrode, in an alloy that's designed for this job.Automotive spark plugs have a thread of 14 x 1.25 mm, motorcycles 10 x 1.0 mm.This will allow you to adjust the spark distance.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 13, 2020, 11:38:16 pm
content: Thank you,that's a good idea!The only thing is that I want to change the gap very finely and spark plug has a relatively coarse thread (for fine adjustment). And don't want to design a complex jig around it. Any suggestion how I could do that?Thank you

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2020, 11:53:52 pm
content: Actually, 14 x 1.25 mm or 10 x 1 mm are fine threads.But that's not my problem. How did you plan to do it before my suggestion? You must have had an idea. Or not? You mentioned "two screws" in your original post. So?

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2020, 12:00:14 am
content: Plastics will be damaged by ozone and UV radiation and can burn, so are a bad idea. Use a ceramic.If you're going to seal it, you're better off avoiding oxygen. Fill it with argon, which is unreactive and should prolong the electrode life. Oh and the breakdown voltage vs spark gap distance is non-linear. Look up Paschen's Law.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 14, 2020, 12:26:20 am
content: Thank you Benta and Zero999,Benta,Yes, I was thinking of using fine thread adjustment screws like these:https://www.newport.com/p/AJS100-2They have very long travel and also very fine pitch.Zero999,no idea of how I could fill it with argon or any other gasI was just thinking of sealing it inside a glass tube. But if you have any suggestion, they are more than welcome!

author: sandalcandal
author_rank: Unknown
timestamp: November 14, 2020, 12:56:04 am
content: Quote from: ricko_uk on November 14, 2020, 12:26:20 amArgon is available at welding supply stores. You might be able to buy a small disposable bottle. Just crack the valve a touch open and let it displace the air. Breakdown in argon is MUCH lower than air however.Should be worthwhile also asking onhttps://highvoltageforum.net/index.phpHV hobbyists have loads of experience with DIY spark gaps.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 14, 2020, 01:02:30 am
content: You might find a tungsten welding electrode, cut it in half and grind a point on each piece. Mount them through the ends of some insulating bars that pivot, so you can change the distance easily and with precision. If you want something loud and annoying, build a rotary gap!

author: LaserSteve
author_rank: Super Contributor
timestamp: November 14, 2020, 03:23:54 am
content: Tungsten-Copper alloys are often available cheap on Ebay as rod stock.This is one of the alloys favored for atmospheric pressure spark gaps for things like nitrogen lasers. Adding copper to tungsten let's you work it with normal tools. It also makes it far more resistant to oxidation processes. Any erosion comes off as a fine dust easily removed by a tiny flow of gas through the switch.Big gas gaps in accelerators and Marx generators often use tellurium copper as the secret sauce.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 14, 2020, 04:28:28 am
content: Sealing the tube while having an adjustable screw is going to be just impossible for you, this requieres specialzed equipmnet and methods. Variable vacuum capacitors worked like this and they are no easy to build.My advice is to foget about the sealing and answering to your questin use 2 round shaped surfaces that you can adjust their spacings, this helps create a stable breakdown voltage. Also depending on the temperature and humidity and ambient light breakdown will change.

author: jogri
author_rank: Frequent Contributor
timestamp: November 14, 2020, 08:45:44 am
content: Quote from: Zero999 on November 14, 2020, 12:00:14 amWell, Teflon could work... Just get a block/tube and use squeeze-type gaskets (or other methods of sealing that don't require another material). A glas tube will be a PITA as it requires a glass-metal seal, and those are rather difficult as a DIY project (most metalls don't stick that well to glass). Also, glass has the not-so-great tendency of getting soft when you heat it, so your container has to be big enough to dissipate the heat. And don't forget, hot gases expand so you are building a potential bomb if your gas mixture gets too hot and shatters the glass...But from a more realistic point of view, just don't bother with some sort of fancy sealed apparatus... Too much work, just use it outdoors or build yourself a scrubber for the ozone. You could have an enclosed cell (with your spark gaps) in which you continuously pump fresh air that then gets scrubbed by a bucket of water. Cheap, and you will always have the same gas mix in your cell (fresh air).

author: ricko_uk
author_rank: Super Contributor
timestamp: November 14, 2020, 01:19:39 pm
content: Thank you all!! Very interesting suggestions and ideas!Jogri, when you say "You could have an enclosed cell (with your spark gaps) in which you continuously pump fresh air that then gets scrubbed by a bucket of water." do you mean that the extracted air is pumped into water? Would it not then bubble out of the water and back into the room?Also out of curiosity, what would the gases generated by the spark gap "transform" into when running through water?Thank you

author: jogri
author_rank: Frequent Contributor
timestamp: November 14, 2020, 01:38:46 pm
content: Yup, that's exactly how a water based scrubber works: You pump the gas in at the bottom, and the gases get absorbed into the water. The smaller the bubbles/the greater the total surface the better. With ozone, you'll probably just get a saturated solution of ozone that slowly decomposes and attacks contaminants (that's how water treatment works btw). The other stuff (mostly nitrous oxides) will get converted to some form of nitric/nitrous acids, so you shouldn't drink it but it shouldn't be toxic.An aquarium bubbler works best as it creates a lot of really small bubbles-> big surface area.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 14, 2020, 03:40:09 pm
content: Thank you jogri!

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 03:54:01 pm
content: for mine I got 2 ball bearings, heated them red hot, polished them, put a flat on them, drilled blind holes in them, tapped the holes then attached it to threaded rod and connected the threaded rod to the circuit with a copper bushing, (anneal copper sheet and hammer it around the thread with a hammer then tension it around the thread with a bolt on the bottom and solder a wire to this. Then use a mechanial thread support to hold it up and use the bushing to transfer electricity

author: ricko_uk
author_rank: Super Contributor
timestamp: November 14, 2020, 03:56:30 pm
content: Thank you coppercone2that's a good idea.Why did you heat them red hot?Also how did you polish them?Thank you

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 03:59:07 pm
content: Quote from: ricko_uk on November 14, 2020, 03:56:30 pmbecause their extremely hard and I don't know if you can drill them naturally. I used black polish (emery I think) on a stitched wheel (sisel might work better) then used another finer polish (this one needs to be a stitched wheel, I think sisal would be too aggressive for the fine polish). I think it was green rogue. I heat them red hot when they are against the corner of 2 fire bricks with a torch (oxy-acetlyene works faster but you can do it with mapp gas, this makes them oxidized).Now I think mixing the black aggressive polish and the stitched wheel was a problem, because they were hard to polish, I imagine it would work much better with the sisal wheel (the stuff you make ropes out of), I believe thats the proper wheel to use with aggressive black polish.Polishing done on a bench grinder refitted with buffs, obviously a buffing wheel would be better, if you can afford it. The polishing process for metal has to do with the wheel hardness and the compound.The three wheels are sisal, stitched and loose buff. I think you could go through more steps if you want, i.e. use black polish and sisal then brown polish (medium) and stitched and then green with the loose buff. The loose buff looks like the thing you find on 1600's style politicians/royalty, like curtains. I never experimented with it because I would need a real polishing wheel machine so I can switch the buffs and store them in bags, my shop will basically trash the polishing wheel so its only a gesture as far as polishing goes for someone like a jeweler. I have to undo bolts to change the wheels, so I have a bench grinder for metal and one with a wire wheel and a buff on it. Now, if you know anything about polishing, having a wire wheel, welders and grinders next to a buffing wheel in a professional environment would be akin to putting a giant skillet and BBQ in a electronics lab.It works but I don't think I could sell things I made to aficionados.

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: November 14, 2020, 04:06:53 pm
content: Consider using two spark plugs. I leave the details to your imagination, because that is the fun part.

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 04:08:06 pm
content: Quote from: Pulsepowerguy on November 14, 2020, 04:06:53 pmNot sure that the spark plug is the best, the material is, but its geometry is designed to ignite fuel, the exact characteristics of the spark is designed with a mechanical system and timing in mind. The specification is literary 'ignite fuel reliably'.If you can get those balls indium plated or whatever they use, that would be better for a lab apparatus I think.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 14, 2020, 04:09:45 pm
content: Thank you coppercone2How big were your ball bearings? And how did you hold them while buffing?Did you drill before buffing and screw a rod in to hold them during buffing?Thank you

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 04:10:05 pm
content: obviously if you get a soft metal ball or are able to cut into a hardened ball bearing somehow and thread it, the finish it has is superior to anything you can do I believe. I was just trying to 'fix it'.

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: November 14, 2020, 04:10:47 pm
content: Quote from: coppercone2 on November 14, 2020, 04:08:06 pmI deliberately left the details out. They can work with mods.

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 04:13:09 pm
content: Oh boy, that was a ghetto operation. What I would recommend is using a 3 jaw chuck on a milling machine to cut a flat and drill it.What I did was different. The hardware store around here sold something like ghetto-sharpaners, which were round disk stones drilled to hex shaft (they fit in a drill or screw driver). Now, I wedged the ball in a machinists vise, and ground the top flat by gently lowering the corner of a fixed stone on it, in a drill press, flat enough to start drilling. Then I drilled a off centered hole (my mistake), threaded it with the blind tap, put a threaded rod in there with loctite (after cleaning with alcohol) and attempted to polish it like holding food in a camp fire. It worked OK but basically doing anything is an improvement.You can probably make it flat with sand paper once its softened from the heat.

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 04:13:40 pm
content: Quote from: Pulsepowerguy on November 14, 2020, 04:10:47 pmI thought they were plated and you cannot modify their geometry in any significant way.

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 04:17:52 pm
content: also, with my design one of the balls was totally drilled through so you can put a piece of coaxial hard line in there connected to a BBQ starter to make a peizotriggered spark gap , which was a precursor to a krytron that I gave up on.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 14, 2020, 04:18:37 pm
content: coppercone2,what diameter were the ball bearings you used?

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: November 14, 2020, 04:24:37 pm
content: Cut off the part that faces the tip. Use the threads for distance adjustment between the two tips...

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 05:18:22 pm
content: Quote from: ricko_uk on November 14, 2020, 04:18:37 pmI think I used 5/8 inch or something, I would need to find it. I mothballed it. It was with 3/8 inch rod. I have NO IDEA about the electrical properties of this, its just some crap I came up with. I would not recommend following my dimensions only the manufacturing process of something you have a bigger interest in then i.Bigger is probobly better? Also, you should use the hardened drills for this (like Milwaukee Cobalt, if you rely on the hardware store), otherwise like Norseman or something. You can get them in single quantity in the hardware store without buying the kit.

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 10:35:06 pm
content: Quote from: Pulsepowerguy on November 14, 2020, 04:24:37 pmI don't think a spark plug has a spherical geometry for electrical stability, it has a cone/rod geometry to facilitate arc at the lowest breakdown voltage possible (to decrease need for stability of the pulse circuit), since the idea is to ignite the gas. So long the chemical reaction is started, it does not matter.If its solid iridium you might be able to machine a ball out of it with some sort of grinding wheel. But I don't know how safe the dust would be either.

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 10:38:53 pm
content: I think a bigger ball is better because when it gets worn and you repolish it, you get more close to the original sphere, and it is a bigger heat sink, so it cools more before sparks and wears less (less metal is vaporized when its colder).Also, I want to say what could be a good source for a soft ball is a old mouse ball with the rubber removed, but they are usually rusty and require polish, but being soft it should polish easier. I don't think they will use a expensive precision ball bearing if they are coating it in rubber. Or you can polish it, or you can use a threaded rod that is big (1/2 inch) and grind a sphere on the end yourself carefully. Or buy a metal ball online that is not hardened (cheap). I just thought it was barely worth doing so I used what I had.

author: floobydust
author_rank: Super Contributor
timestamp: November 14, 2020, 10:42:36 pm
content: Look at airplane magneto testers that use a third "teaser" electrode:https://www.eevblog.com/forum/projects/high-voltage-vintage-equipment-question-for-the-physicists/msg1286145/#msg1286145With ozone build-up the firing voltage will never be consistent, as well as ambient light also affecting things. That thread has a research paper url about spark gap testing, as it was critical for aircraft.A bit here:https://www.eevblog.com/forum/projects/creating-an-electronically-controlled-spark-gap/

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 11:10:15 pm
content: I imagine if its slow you can be consistent, the ozone should dissipate quickly. The magneto tester is basically a krytron type thing

author: LaserSteve
author_rank: Super Contributor
timestamp: November 15, 2020, 03:56:35 am
content: Grind a flat on the ball and braze a rod to them. So much easier then trying to drill s hardened ball.Orcutt order brass ones from McMaster Carr.Spherical gaps are almost never used in triggered gaps anyways, because the pre-ionization from the trigger negates geometry induced jitter. Most designers just chamfer or slightly radius the edges of the gap facings which are otherwise flat.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: November 15, 2020, 04:22:09 am
content: Yea I did the ball hole thing because I thought it would be more modular / replaceable once its inserted into something but I think cutting a new rod and brazing it together is still easier. The only problem is you likely need a brazing torch for this, not sure if you can do it with mapp gas. I would guess probably if you use silver solder, but keep in mind you need to buy a tub of flux and silver solder rods, I don't know if you can use solder <30% silver for steel to steel in a reliable way. The low silver alloys are listed as 'copper only' on the tables.The first alloy listed as ferrous to ferrous metal is 30% silver, with 25% being copper to ferrous, and everything under being non ferrous (non aluminum obviously). Then you need a tub of black flux and I don't like the idea of soldering it with a air torch, unless it was acetylene air. Does this work? I don't want to waste silver and gas to find out. It would be good to know. I try to make it noob friendly and a small hardware store torch and a drill press is way more useful then a acetylene setup for anyone but the die hard. I think these prices will be unpleasant for electronics people. I also notice alot of people seem to have drill presses in their apartments.

author: jonpaul
author_rank: Super Contributor
timestamp: November 15, 2020, 09:43:54 am
content: Bonjour:I Worked in HV ignitor and PSU design in 1970s...1990. We used multiple gaps to solve the problems mentioned, material like carbon rods, tungsten, etc.Tesla coils and early radio TX used quenched spark gaps, stacked cylindrical discs.Nowadays, Commercial HF welders have replaceable long life gaps, seehttps://forum.weldingtipsandtricks.com/viewtopic.php?f=5&t=13452These are easily available at welding supplier.To give best solutions You must first specify the load switched eg Tesla coil capacitor, neon sign transformer, arc lamp,What stored energy? What is peak current?Ball gap: Higher breakdown per gap sizeneedle gap: lower breakdownsealed gap: Commercially available and calibratedAdjustable gap: Calibrated gaps use air, ~ 25..50 mm spheres, micrometer adjust and 257 nm UV lamp for consistent breakdown.Can send a photo if interested.Triggered gaps are another design and have very different uses, properties and requirements.What is your objective, eg safety gap to limit V? Accurate measurement of V? Discharge of cap?Beware :1/ Open gaps can generate dangerous amounts of Ozone!2/ Krytrons (developed by EGG) may be used as nuclear device trigger switches and may be military restricted items for export and import in most countries with severe penalties.https://en.wikipedia.org/wiki/Krytron3/ You can be injured or killed by HV especiall stored energy. Nickola Tesla wore rubber soled shoes and kept one hand in the pocket to avoid making a closed circuit!Just the ramblings of an old retired EEJon

author: jonpaul
author_rank: Super Contributor
timestamp: November 15, 2020, 12:36:08 pm
content: Just took a variety of gap photos used over the decades.PM me for file too big to upload.Last photo is Calibrated micrometer sphere gap with cal chart and UV lamp.From BAG Turgi, Switzerland.Jon

author: ricko_uk
author_rank: Super Contributor
timestamp: November 15, 2020, 04:32:21 pm
content: thank you all!!Paul, very useful infos. I did not know welders used spark gaps. How do they solve the issue of ozone generation?The photos on the post did not upload

author: sandalcandal
author_rank: Unknown
timestamp: November 15, 2020, 05:05:45 pm
content: Quote from: jonpaul on November 15, 2020, 12:36:08 pmThat sounds really interesting. Maybe the photos need to be downscaled a bit then they can be uploaded?

author: jonpaul
author_rank: Super Contributor
timestamp: November 15, 2020, 08:06:20 pm
content: Rebonjour a tous:To Riko UK: Many thanks for the kind note!I first built Tesla coils with spark coils and gaps in mid 1950s.1/ Sparkgap and gas tube properties were researched about 1870s...1920s, and are very well characterized.2/ Search "airgap breakdown" "quenched gap" "Sphere gap" and "spark gap Tesla coil"3/ Many fine physics texts and electrical power engineering cover the topics.4/ Vbrk is funct of geometry of conductors, light color and intensity, material surfaces, gas pressure and temperature, etc.5/ See also rotary gaps and use a fan on air gaps in continuous use.6/ If you are interested in HV and sparks and history suggest to visit Science Museum on Exhibition Rd in London and Faraday Rooms/Museum at Royal Society, near center of London (if they are open!)After a lot of reduction I try to attached a decimated version of 3 photos. Hope the 5M bit image robots do not again reject my attachment and waste my timePS: TO Moderator/IOgroups..its 20209, .time to allow more than a 5M attachment?Jon

author: jonpaul
author_rank: Super Contributor
timestamp: November 16, 2020, 10:31:13 am
content: Hello again Rikko UKThe welding supply gaps are used in HF welders to start the arc. They are low cost, tungsten multigap, air cooled and very robust. Adjustable as well.Designed for rough use in welders, but they work fine in other applications and at lower energy and currents.Mass produced, they are easily obtainable!In the PDF photo 2 is a Miller 206502, with 2 series adjustable gaps.Photo 3 has a custom made calibrated gap, 50mm spheres and UV light, micrometer, and measures up to 100 kV.Used for measurement of arc lamp ignitors, HV PS, works at any frequency and wave to detect the peak voltage.Finally, standard sphere gap sparking voltage data: from 1936!https://authors.library.caltech.edu/81213/1/06539527.pdfCheck Fig 4More recent ref for smaller sphere gap:http://downloads.hindawi.com/archive/2014/980913.pdfEnjoy and happy sparking!Jon

author: ricko_uk
author_rank: Super Contributor
timestamp: November 16, 2020, 01:13:44 pm
content: Thank you Jon Paul,very useful!!