Rotary vs Air Blast gap
crashstudio, Mon Oct 05 2015, 11:25PMI'm currently building an asynchronous rotary spark gap using a grinder and controlling the speed with a variac. I mounted 4 threaded brass rods to a grinding wheel, took it for a spin and it's pretty well balanced. Still have to sand down the edges though. Do I need to add some sort of air flow for quenching this?
Once this is completed I plan to build an air blast gap. After reading a lot on spark gaps I've found many people listing greater results. Some even swear an air blast is by far the best type for an SGTC.
Anyone here have experience with these 2 gaps?
Here's the link to my TeslaMap specs and rotary disk pics.
Re: Rotary vs Air Blast gap
Benjamin, Tue Oct 06 2015, 12:07AM
Hi crashstudio, I have built both types of gaps. I am assuming that you are using an nst. I find that an arsg is better when you are using a power supply with lots of current, like microwave oven transformers. An air blast or vacuum gap will work very well with your setup.A arsg will provide no advantage.
Benjamin, Tue Oct 06 2015, 12:07AM
Hi crashstudio, I have built both types of gaps. I am assuming that you are using an nst. I find that an arsg is better when you are using a power supply with lots of current, like microwave oven transformers. An air blast or vacuum gap will work very well with your setup.A arsg will provide no advantage.
Re: Rotary vs Air Blast gap
crashstudio, Tue Oct 06 2015, 02:35AM
Benjamin; I'm using 2 NST's in parallel. 15kv 60ma, total 120ma. You said an async wont give me any advantage but it should at least do the job right? TeslaMap says my max spark length should be 72.6 in so I'm hoping my rotary will give me at least 48" to 60". Also, once I switch to the air blast do I use the capacitance value suggested for a static gap?
Check the drive link for pics...
crashstudio, Tue Oct 06 2015, 02:35AM
Benjamin; I'm using 2 NST's in parallel. 15kv 60ma, total 120ma. You said an async wont give me any advantage but it should at least do the job right? TeslaMap says my max spark length should be 72.6 in so I'm hoping my rotary will give me at least 48" to 60". Also, once I switch to the air blast do I use the capacitance value suggested for a static gap?
Check the drive link for pics...
Re: Rotary vs Air Blast gap
Benjamin, Tue Oct 06 2015, 04:31AM
For an async or air blast gap you will want to use the suggested value of 34nf. you will not need any air flow for quenching on a rotary. your rotary will work, but not nearly as well as an air blast or vacuum gap. what I have done is used a vacuum cleaner motor connected to a PVC T fitting that splits into two "horns" of 1/2 inch copper pipe ending in 2 brass fittings that face each other, if that makes sense. Check out for a similar gap.
Benjamin, Tue Oct 06 2015, 04:31AM
For an async or air blast gap you will want to use the suggested value of 34nf. you will not need any air flow for quenching on a rotary. your rotary will work, but not nearly as well as an air blast or vacuum gap. what I have done is used a vacuum cleaner motor connected to a PVC T fitting that splits into two "horns" of 1/2 inch copper pipe ending in 2 brass fittings that face each other, if that makes sense. Check out
for a similar gap.Re: Rotary vs Air Blast gap
Graham Armitage, Tue Oct 06 2015, 02:23PM
For what it's worth, I have had much better results from an air gap than RSG, especially when using NSTs. One of the most successful (and easiest) gaps I built was with two thin walled 1" pipes aligned axially and separated by the desired gap. One pipe was close and attached to a handle to adjust it closer or further from the other pipe edge. The second pipe was attached to small vacuum cleaner. This pulled air from outside the pipes and back into the "exhaust" pipe. I enclosed the whole thing in a muffler box to reduce light and noise but allow air flow. Because of the high flow rate of the air the spark was quenched very quickly which is what you want. Hope this helps.
Results can be seen here -
Graham Armitage, Tue Oct 06 2015, 02:23PM
For what it's worth, I have had much better results from an air gap than RSG, especially when using NSTs. One of the most successful (and easiest) gaps I built was with two thin walled 1" pipes aligned axially and separated by the desired gap. One pipe was close and attached to a handle to adjust it closer or further from the other pipe edge. The second pipe was attached to small vacuum cleaner. This pulled air from outside the pipes and back into the "exhaust" pipe. I enclosed the whole thing in a muffler box to reduce light and noise but allow air flow. Because of the high flow rate of the air the spark was quenched very quickly which is what you want. Hope this helps.
Results can be seen here -
Re: Rotary vs Air Blast gap
crashstudio, Tue Oct 06 2015, 03:33PM
Someone else told me to go with the 55nf suggested by teslamap for sync rotary. This won't work? I literally just finished rebuilding the mmc from 34 to 55. Now if I just rewire it each string will have a different length of wire connecting them causing a difference in resistance right?
My rotary will be adjustable up to 10,000 rpm. 55nf won't work within that range?
crashstudio, Tue Oct 06 2015, 03:33PM
Benjamin wrote ...
For an async or air blast gap you will want to use the suggested value of 34nf.
For an async or air blast gap you will want to use the suggested value of 34nf.
Someone else told me to go with the 55nf suggested by teslamap for sync rotary. This won't work? I literally just finished rebuilding the mmc from 34 to 55. Now if I just rewire it each string will have a different length of wire connecting them causing a difference in resistance right?
My rotary will be adjustable up to 10,000 rpm. 55nf won't work within that range?
Re: Rotary vs Air Blast gap
Benjamin, Tue Oct 06 2015, 04:32PM
55nf will only work with a sync rotary not async! Async gaps in general work best at a high break rate, above 240 bps. if you try to run them at a lower break rate you will have surging due to the gap speed going in and out of phase with the charging frequency. A sync gap allow the nst bank to charge a much larger cap bank. With such a large cap size your nst's wont be able to charge them quickly enough each cycle and the result will be sporadic firing and a much reduced spark length. My advice is to switch back to 34nf and build an air blast gap, they're not that hard to build.
Benjamin, Tue Oct 06 2015, 04:32PM
55nf will only work with a sync rotary not async! Async gaps in general work best at a high break rate, above 240 bps. if you try to run them at a lower break rate you will have surging due to the gap speed going in and out of phase with the charging frequency. A sync gap allow the nst bank to charge a much larger cap bank. With such a large cap size your nst's wont be able to charge them quickly enough each cycle and the result will be sporadic firing and a much reduced spark length. My advice is to switch back to 34nf and build an air blast gap, they're not that hard to build.
Re: Rotary vs Air Blast gap
crashstudio, Tue Oct 06 2015, 06:01PM
If I just rewire the mmc with an equal length of wire per string that should eliminate the difference in resistance right? I really don't want to take it all apart and rearrange everything again.
crashstudio, Tue Oct 06 2015, 06:01PM
If I just rewire the mmc with an equal length of wire per string that should eliminate the difference in resistance right? I really don't want to take it all apart and rearrange everything again.
Re: Rotary vs Air Blast gap
Benjamin, Tue Oct 06 2015, 06:40PM
All of the strings should have the same number of capacitors but the length of the wires connecting them are not that important as the difference in resistance between them is very small. the only way you can use that configuration is with a sync rotary gap or a sync triggered gap. 3 strings of 13 caps each will give you 34.62nf at 26 kv which is more than enough voltage rating at the right capacitance. I'm sorry but It looks like you have to rearrange it, I know how much of a pain it is as I have had to do that several times before too :).
edit: I just realized that If you simply remove 2 of your strings that will give you 27.2nf which is about 1.3 times your resonant value of 21.2nf which is perfect!
Benjamin, Tue Oct 06 2015, 06:40PM
All of the strings should have the same number of capacitors but the length of the wires connecting them are not that important as the difference in resistance between them is very small. the only way you can use that configuration is with a sync rotary gap or a sync triggered gap. 3 strings of 13 caps each will give you 34.62nf at 26 kv which is more than enough voltage rating at the right capacitance. I'm sorry but It looks like you have to rearrange it, I know how much of a pain it is as I have had to do that several times before too :).
edit: I just realized that If you simply remove 2 of your strings that will give you 27.2nf which is about 1.3 times your resonant value of 21.2nf which is perfect!
Re: Rotary vs Air Blast gap
crashstudio, Tue Oct 06 2015, 09:37PM
I thought about that but wouldn't the mmc be happier with more caps? It would mean less of a load per cap right? I would prefer to stretch the life of these as long as possible. I might just remove 5 and rewire the whole thing to 3 strings of 13 instead of removing all of them and rearranging.
crashstudio, Tue Oct 06 2015, 09:37PM
Benjamin wrote ...
edit: I just realized that If you simply remove 2 of your strings that will give you 27.2nf which is about 1.3 times your resonant value of 21.2nf which is perfect!
edit: I just realized that If you simply remove 2 of your strings that will give you 27.2nf which is about 1.3 times your resonant value of 21.2nf which is perfect!
I thought about that but wouldn't the mmc be happier with more caps? It would mean less of a load per cap right? I would prefer to stretch the life of these as long as possible. I might just remove 5 and rewire the whole thing to 3 strings of 13 instead of removing all of them and rearranging.
Re: Rotary vs Air Blast gap
Benjamin, Tue Oct 06 2015, 10:28PM
I have a magnifier running on 2 mots at about 400-500 bps async and the cap bank is only 2 strings of 6 each and it works fine producing 5 foot bright streamers. These caps are tough and will have no problem at all with only 2 strings. Also I have found that 1.3 times the resonant value works the best for nst's, 34nf would be 1.6 times resonant and most people recommend not going higher then 1.5 times. So even with only 2 strings the lifetime of your caps will be way more then you will ever need. You should only start to worry if the caps get hot.
Benjamin, Tue Oct 06 2015, 10:28PM
I have a magnifier running on 2 mots at about 400-500 bps async and the cap bank is only 2 strings of 6 each and it works fine producing 5 foot bright streamers. These caps are tough and will have no problem at all with only 2 strings. Also I have found that 1.3 times the resonant value works the best for nst's, 34nf would be 1.6 times resonant and most people recommend not going higher then 1.5 times. So even with only 2 strings the lifetime of your caps will be way more then you will ever need. You should only start to worry if the caps get hot.
Re: Rotary vs Air Blast gap
crashstudio, Wed Oct 07 2015, 12:57AM
Thank you. For some reason I didn't even think of using a vacuum motor. I have an old Kirby with a very strong motor in it. I plan to build one similar to this one.
crashstudio, Wed Oct 07 2015, 12:57AM
Graham Armitage wrote ...
For what it's worth, I have had much better results from an air gap than RSG, especially when using NSTs. One of the most successful (and easiest) gaps I built was with two thin walled 1" pipes aligned axially and separated by the desired gap. One pipe was close and attached to a handle to adjust it closer or further from the other pipe edge. The second pipe was attached to small vacuum cleaner. This pulled air from outside the pipes and back into the "exhaust" pipe. I enclosed the whole thing in a muffler box to reduce light and noise but allow air flow. Because of the high flow rate of the air the spark was quenched very quickly which is what you want. Hope this helps.
Results can be seen here -
For what it's worth, I have had much better results from an air gap than RSG, especially when using NSTs. One of the most successful (and easiest) gaps I built was with two thin walled 1" pipes aligned axially and separated by the desired gap. One pipe was close and attached to a handle to adjust it closer or further from the other pipe edge. The second pipe was attached to small vacuum cleaner. This pulled air from outside the pipes and back into the "exhaust" pipe. I enclosed the whole thing in a muffler box to reduce light and noise but allow air flow. Because of the high flow rate of the air the spark was quenched very quickly which is what you want. Hope this helps.
Results can be seen here -
Thank you. For some reason I didn't even think of using a vacuum motor. I have an old Kirby with a very strong motor in it. I plan to build one similar to this one.
Re: Rotary vs Air Blast gap
crashstudio, Wed Oct 07 2015, 01:00AM
Awesome thank you! I'll just keep it the way it is and kill 2 strings. That way if some caps blow all I have to do is switch to the other set. Cool
crashstudio, Wed Oct 07 2015, 01:00AM
Benjamin wrote ...
I have a magnifier running on 2 mots at about 400-500 bps async and the cap bank is only 2 strings of 6 each and it works fine producing 5 foot bright streamers. These caps are tough and will have no problem at all with only 2 strings. Also I have found that 1.3 times the resonant value works the best for nst's, 34nf would be 1.6 times resonant and most people recommend not going higher then 1.5 times. So even with only 2 strings the lifetime of your caps will be way more then you will ever need. You should only start to worry if the caps get hot.
I have a magnifier running on 2 mots at about 400-500 bps async and the cap bank is only 2 strings of 6 each and it works fine producing 5 foot bright streamers. These caps are tough and will have no problem at all with only 2 strings. Also I have found that 1.3 times the resonant value works the best for nst's, 34nf would be 1.6 times resonant and most people recommend not going higher then 1.5 times. So even with only 2 strings the lifetime of your caps will be way more then you will ever need. You should only start to worry if the caps get hot.
Awesome thank you! I'll just keep it the way it is and kill 2 strings. That way if some caps blow all I have to do is switch to the other set. Cool
Re: Rotary vs Air Blast gap
crashstudio, Sun Oct 11 2015, 07:59PM
Thank you everyone! Here's my new Sucker Gap. Got to fine tune but it seems to be working great.
crashstudio, Sun Oct 11 2015, 07:59PM
Thank you everyone! Here's my new Sucker Gap. Got to fine tune but it seems to be working great.
Re: Rotary vs Air Blast gap
Graham Armitage, Mon Oct 12 2015, 01:33AM
That looks cool. How do you tune the gap distance? It looks like air is being sucked through both tubes. Have you tried blocking one so that the force of air moving through the gap is greater? Could change how quickly the spark is quenched.
Graham Armitage, Mon Oct 12 2015, 01:33AM
That looks cool. How do you tune the gap distance? It looks like air is being sucked through both tubes. Have you tried blocking one so that the force of air moving through the gap is greater? Could change how quickly the spark is quenched.
Re: Rotary vs Air Blast gap
crashstudio, Mon Oct 12 2015, 09:30PM
I cut slots in the pipe for adjustment. You can see in the pic below. I'll probably end up wrapping tape around the opening until can figure out a different method. I just wanted to be sure the electrodes would remain secure once the gap was set.
As far as plugging one of the holes, I'll try that after I test it with the coil. I was just thinking there would be a more even flow if both were open.
crashstudio, Mon Oct 12 2015, 09:30PM
I cut slots in the pipe for adjustment. You can see in the pic below. I'll probably end up wrapping tape around the opening until can figure out a different method. I just wanted to be sure the electrodes would remain secure once the gap was set.
As far as plugging one of the holes, I'll try that after I test it with the coil. I was just thinking there would be a more even flow if both were open.
Re: Rotary vs Air Blast gap
Benjamin, Mon Oct 12 2015, 11:43PM
I would love to hear how it works when you test it.
Benjamin, Mon Oct 12 2015, 11:43PM
I would love to hear how it works when you test it.
Re: Rotary vs Air Blast gap
crashstudio, Tue Oct 13 2015, 12:31AM
Just tested it. took me a couple minutes to figure it out but it was arcing "inside" the secondary, there's trace marks everywhere. My top load connection is outside and the ground is inside. Apparently neither should be inside so I'm going to change the ground connection to outside.
The only reason I had it inside was for aesthetics, whatever, at this point I don't care. I'm also going to try and figure a way to seal the inside of the secondary at the top and bottom.
crashstudio, Tue Oct 13 2015, 12:31AM
Just tested it. took me a couple minutes to figure it out but it was arcing "inside" the secondary, there's trace marks everywhere. My top load connection is outside and the ground is inside. Apparently neither should be inside so I'm going to change the ground connection to outside.
The only reason I had it inside was for aesthetics, whatever, at this point I don't care. I'm also going to try and figure a way to seal the inside of the secondary at the top and bottom.
Re: Rotary vs Air Blast gap
crashstudio, Thu Oct 15 2015, 10:43PM
Here it is. Still got to fine tune but its already too powerful for my little room. Plus there's too much stuff all around it. Check out the video link below and thank you very much everyone for your help! I never even went above 90 volts on the variac.
crashstudio, Thu Oct 15 2015, 10:43PM
Here it is. Still got to fine tune but its already too powerful for my little room. Plus there's too much stuff all around it. Check out the video link below and thank you very much everyone for your help! I never even went above 90 volts on the variac.
Re: Rotary vs Air Blast gap
crashstudio, Fri Oct 16 2015, 12:19AM
crashstudio, Fri Oct 16 2015, 12:19AM
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