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#1
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JB wrote:
enough to avoid problems with the transmitter. Multiple series gaps only raises the firing voltage, but the breakdown voltage will be set by the largest gap in the series. A gap at .035 will fire at about 2-10kv depending on humidity. You only want it wide enough so that it won't arc Humidity doesn't change the breakdown voltage very much. In fact, increasing humidity increases the breakdown voltage. One correction table for a "rod gap" with 1/2" square electrodes has a 10% correction going from 15 torr to 30 torr water vapor pressure and a -16% correction going from 15 to 2.5 torr, which is nowhere near the 5:1 variation cited above. Interestingly, on this basis water vapor is a better insulator than nitrogen, since density of the humid air is actually less. Typical breakdown voltage tolerance on a rod gap is +/- 8%. |
#2
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enough to avoid problems with the transmitter. Multiple series gaps
only raises the firing voltage, but the breakdown voltage will be set by the largest gap in the series. A gap at .035 will fire at about 2-10kv depending on humidity. You only want it wide enough so that it won't arc Humidity doesn't change the breakdown voltage very much. In fact, increasing humidity increases the breakdown voltage. One correction table for a "rod gap" with 1/2" square electrodes has a 10% correction going from 15 torr to 30 torr water vapor pressure and a -16% correction going from 15 to 2.5 torr, which is nowhere near the 5:1 variation cited above. Interestingly, on this basis water vapor is a better insulator than nitrogen, since density of the humid air is actually less. Typical breakdown voltage tolerance on a rod gap is +/- 8%. That was off the top of my head so I stand corrected but there ARE variations based on other real world stuff like air pressure and dirt on the insulators. 5-20kv for inside an engine is real world but that depends on other stuff too, like what kind of spark-plug, compression, resistors, mixture, timing, but I digress. The point is the gap may have to be adjusted for best results and might be too wide for protection. I invited him to read up on the subject because I have other projects I have my head into So what are your experiences with spark gap transmission line arresters? Perhaps you have the precise gap he should be using? My dim recollection is of a chart for the transmitter power in use. |
#3
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![]()
JB wrote:
enough to avoid problems with the transmitter. Multiple series gaps only raises the firing voltage, but the breakdown voltage will be set by the largest gap in the series. A gap at .035 will fire at about 2-10kv depending on humidity. You only want it wide enough so that it won't arc Humidity doesn't change the breakdown voltage very much. In fact, increasing humidity increases the breakdown voltage. One correction table for a "rod gap" with 1/2" square electrodes has a 10% correction going from 15 torr to 30 torr water vapor pressure and a -16% correction going from 15 to 2.5 torr, which is nowhere near the 5:1 variation cited above. Interestingly, on this basis water vapor is a better insulator than nitrogen, since density of the humid air is actually less. Typical breakdown voltage tolerance on a rod gap is +/- 8%. That was off the top of my head so I stand corrected but there ARE variations based on other real world stuff like air pressure and dirt on the insulators. 5-20kv for inside an engine is real world but that depends on other stuff too, like what kind of spark-plug, compression, resistors, mixture, timing, but I digress. The point is the gap may have to be adjusted for best results and might be too wide for protection. I invited him to read up on the subject because I have other projects I have my head into So what are your experiences with spark gap transmission line arresters? Perhaps you have the precise gap he should be using? My dim recollection is of a chart for the transmitter power in use. A good rule of thumb for small gaps with large radius of curvature electrodes is 30kV/cm (e.g. a 1cm gap between 10cm diameter spheres). In strongly non uniform field gaps (e.g. needle gaps), the breakdown is typically 1/3 or less.. that is, 10kV/cm 70kV/inch or 25 kV/inch, respectively. A sparkplug with a 0.035inch gap would breakdown around 1-2kV in normal air. In an engine, where the density at firing is probably 5-10 times higher, the voltage is 10-20kV as you've given. The usual way to set spark gaps for this kind of thing would be to make it adjustable, fire up the transmitter at worst case max power and reflection, and set the gap to be somewhat bigger than causes the gap to break down. A good worst case would be 2x peak RF voltage (i.e. 2.8 Vrms), since that's what you'd get with a 100% reflection at just the wrong phase. (unless the source can supply reactive power.. then it's a lot more complex, because of the potential for resonant rise) You could use a current limited DC power supply or something like a neon sign transformer to set the spacing. But making things practical.. say 2 kW and a 450 ohm transmission line. Erms = sqrt(2000*450) = about 950Vrms.. call it 1350V peak. With sharp ended electrodes (like the sheet metal V shapes shown in old ARRL handbooks) one might want a spacing of 0.050 inches? |
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