Math of spark gaps - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: June 05, 2018, 01:57:14 pm
content: Formula is Voltage = (3000 x Pressure(Atm) x Distance (mm) )+1350Does this formula work in a vacuum or does it break down like the ideal gas law PV=nRT because at the extremes things are not ideal point like particles?V= (3000x0Atmx1mm) + 1350V= (0)+1350V= 1350 so the minimum to make a spark in a vacuum is 1350? Sort of though... That sounds reasonable but when you brush things you can spark at much shorter distances unless that's hot metal sputtering off. But what the max distance per volt?

author: EEVblog
author_rank: Unknown
timestamp: June 05, 2018, 03:20:16 pm
content: Are you talking PCB spark gaps of true air spark gaps?

author: JimRemington
author_rank: Regular Contributor
timestamp: June 05, 2018, 04:42:32 pm
content: For spark gaps in air, the minimum distance is determined by the geometry of the electrodes, the voltage difference between them and the electric field strength that leads to dielectric breakdown, which in turn depends on air pressure and composition.No simple formula could possibly capture all those variables.

author: floobydust
author_rank: Super Contributor
timestamp: June 05, 2018, 10:33:40 pm
content: Spark voltages are a necessary part of safety standards. You have to know a safe creepage/clearance distance. Look atPaschen's Law.I know other factors to consider, but spark gap math is imprecise.Time - if there's any ionization beforehand, ozone build up lowers sparkover voltage.Light - Ambient light influences breakdown voltage.

author: CopperCone
author_rank: Super Contributor
timestamp: June 06, 2018, 01:26:21 am
content: you might want to look at data regarding certain tubes like a krytron, triggatronThere is detailed information about gas mixtures and what not regarding to the design of certain tubes that also pertains to simple spark gaps.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 06, 2018, 01:40:57 am
content: Isn't thathttps://en.wikipedia.org/wiki/Paschen%27s_law

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: June 06, 2018, 02:43:11 am
content: Quote from: Beamin on June 05, 2018, 01:57:14 pmYou can't have a spark in a vacuum. A spark is the emission of photons due to excitation of electrons which requires matter.You can, however, pass electrons through a vacuum (obviously - vacuum tubes).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2018, 08:35:01 pm
content: Quote from: JohnnyMalaria on June 06, 2018, 02:43:11 amVacuum tubes certainly spark when they break down; initially emission rises sharply due to field emission, then the impact kicks out ions and the cascade builds exponentially.A vacuum spark doesn't begin the same way as a gas spark does, due to the lack of matter; but once it starts, the electrodes are eroded to provide the material.Beamin: the spark rule only applies above the minima. Namely, if pressure = 0, you get a constant offset, which is wrong. As it happens, the breakdown voltage goes back up as the vacuum becomes harder and harder, a phenomenon not captured by this overly simplified formula.The bigger lesson learned is this: never trust a rule-of-thumb, always research the underlying mechanics. Thumb-rules are rarely repeated with their assumptions intact (in this case, the assumption that only ~atmospheric pressures are seen). Any time you see an overly simple formula (just multiplication and addition, pfft!), for something far more complicated (like spark phenomena), immediately doubt its veracity.Tim

author: Beamin
author_rank: Super Contributor
timestamp: June 07, 2018, 04:10:28 am
content: Quote from: EEVblog on June 05, 2018, 03:20:16 pmIn air I guess. I just realized that formula might be for a PCB: a flat plain where one side has a dieelectric on it or a ground plane under the PCB spark gap. I'm wondering more about vacuum tubes. When does a flow of electrons become a spark? Does it need to have a gas to heat up to plasma and an ion path to follow? Seems like you can have bright blue sparks in a vacuum if so what is giving off the blue light? Residual left over gas or vaporized electrodes or glass molecules?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 07, 2018, 04:27:44 am
content: A corollary of what T3sl4co1l is saying is that anytime you use any approximation method you must account for the inaccuracies and errors. Some folks call this the factor of safety, and it is a fudge factor to compensate for lack of knowledge. The larger the lack, and the larger the consequences of being wrong the larger the safety factor.If you are using the simple spark gap formula to set the starting distance for a Jacob's ladder for your private entertainment you might as well start with the exact result from the formula and adjust as required based on initial results.If you are using high voltage near conductive equipment that people may touch you should consider a huge safety factor, like ten or more, depending how far your specific application varies from what the formula was intended to represent. You may find those details in old texts or handbooks, or perhaps if there are really large consequences from being wrong you need to get some experienced and capable help.