For those frustrated by spark gaps...

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RobertMendelsohn
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For those frustrated by spark gaps...

Post by RobertMendelsohn »

Like many others, I have been frustrated using spark gaps for Marx generators, and other generic high voltage switching tasks. They are loud and more than a bit particular with respect to distance, electrode material, erosion, etc. I have had more than a few tests ruined because of this and began to look for alternatives. For those looking for a solution, I'd like to report that I've successfully used gas discharge tubes to great effect. They are cheap, many can handle repetitive kiloampere currents, and are precision-made with pretty decent-tolerance breakdown voltages. They are also pretty quiet. I've found them to be an excellent solution, and I just wanted to post this in case anyone is running into similar frustrations.

Attached is a photo of a little high-voltage triggering circuit (inductively charged wave-erection-type Marx generator (~14kV, 10A, 40Hz)). You can see two different types of gas discharge tubes. They have the neat effect of lighting up when they discharge. I paid less than a dollar apiece for them on Arrow and Digikey, respectively. Be sure to read the datasheets as some are designed with a short lifetime. Others are meant to be used quite frequently, although you can extrapolate downwards for lesser current; less current means more life. Hope this helps someone else!

-Robert
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Rich Feldman
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Re: For those frustrated by spark gaps...

Post by Rich Feldman »

Nice idea. How are the two GDT arrester models different, and how do they share the load?
Do you think the traditional optical coupling between spark gaps is still present in a meaningful way?
Do your inductors come with a voltage rating comparable to the charging voltage?
All models are wrong; some models are useful. -- George Box
RobertMendelsohn
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Re: For those frustrated by spark gaps...

Post by RobertMendelsohn »

Thank you Rich. I put the the two different models in parallel to compare GDTs with different characteristics; the larger ones are made for repetitive switching, while the ones with the red bands aren't. The red bands have a slightly lower discharge voltage, and I wanted to test their longevity, but didn’t want to have an issue mid-test, hence the parallel nature. So far, the small GDT’s *not* meant for repetitive use have stood up fine. I’m running only ~10A through them, but they are rated for 1kA, 10 times. They seem to have stood up perfectly against thousands of firings.

I try to make little devices to test various questions/principles before scaling up with respect to size/components/money. The GDTs as replacement for spark gaps was one hypothesis, that so far has been great. The inductor voltage rating issue was another: I had a hard time sourcing high voltage inductors that were of the right inductance and weren’t “contact us”-level-expense items. So one thing I hoped to test is how well these handled moderately high voltage (~2.1kV) So far, these have held up well. Another question was about current rating; in this application, the instantaneous maximum current far exceeds the ratings, but the average current is several orders of magnitude below the maximum. So far, that has not been an issue.

I think the optical coupling between the stages is essentially zero. They give a rather dim flash.
Cai Arcos
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Re: For those frustrated by spark gaps...

Post by Cai Arcos »

That is a very good idea!

I do wonder if you could trigger those spark gaps using a magnetic field, thus creating a cheap and easy-to-make trigatron. Furthermore, it could be triggered using high-current low-voltage pulse, and thus simple solid state circuitry. In theory, you would put a coil around the external body of the GDT and pass current through there, which due to enchanced ionization would decrease the starting voltage. If lower than the present voltage, a glow discharge is created that, if allowed to transition to an arc, will stay on even after the removal of the magnetic field.
I found this paper: Makinson, R. E. B., Somerville, J. M., Makinson, K. R., & Thonemann, P. (1946). Magnetically‐Controlled Gas Discharge Tubes. Journal of Applied Physics, 17(7), 567–572 (that I add).

I've managed to find one GDT in one of my spare boards, labeled EPCOS 800 074. Can anyone provide me with a datasheet?

EDIT: Also this? Capacitive triggering might be interesting too https://www.radiomuseum.org/forum/rf_tr ... _bulb.html
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RobertMendelsohn
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Re: For those frustrated by spark gaps...

Post by RobertMendelsohn »

Good question Cal. I think there are many GDTs that could stand in for a trigatron, at the cost of very limited life. Many are rated for only 10 shots at kA currents. Using several HV GDTs in parallel could probably make you a single/limited shot switch. The GDTs are (generally speaking) very cheap. I think where they really shine is where a plurality of high voltage/moderate current spark gaps would otherwise be used.
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Ed Meserve
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Re: For those frustrated by spark gaps...

Post by Ed Meserve »

Robert,
would you mind re-posting your original .jpg? It seems to have been one of the casualties of the recent website struggles.

I only saved Magnetically-Controlled Gas Discharge Tubes PDF, which I will re-post.

Thanks!
Magnetically-Controlled Gas Discharge Tubes.pdf
Magnetically-Controlled Gas Discharge Tubes.pdf
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Richard Hester
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Re: For those frustrated by spark gaps...

Post by Richard Hester »

One trick used in Marx generators is to mount the spark gaps such that the UV from the one that fires first helps the others to fire.
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Re: For those frustrated by spark gaps...

Post by Richard Hester »

A valuable reference for cranky things like spark gaps, thyratrons, and impulse generators is a book by Sarjeant and Dollinger called "High Power Electronics", published by Tab. It's a survey of high-power, high voltage switching devices and techniques.
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Ed Meserve
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Re: For those frustrated by spark gaps...

Post by Ed Meserve »

Thanks Richard! Just ordered it from Abe Books for $90.
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Richard Hester
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Re: For those frustrated by spark gaps...

Post by Richard Hester »

You got robbed - use bookfinder.com and you can find stuff cheaper...
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Ed Meserve
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Re: For those frustrated by spark gaps...

Post by Ed Meserve »

Welp, that was an expensive lesson. At least I have a new place to look for books now.

I searched Amazon based on title and it didn't return anything. Bookfinder.com had half a dozen amazon links listed. For half the price at Abe Books.
So it goes.

Ed
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Richard Hull
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Re: For those frustrated by spark gaps...

Post by Richard Hull »

If its used, you will most likely find it cheaper at ABE (American Book Exchange) note: This is only if it is no longer in print, but there were tens of thousands printed. Current, in print, stuff is often found in paper back, used, cheap on Amazon.

I outfitted my vast nuclear library of out of print books almost entirely off ABE between 1990 and current. Many of my books are rare early works.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Richard Hester
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Re: For those frustrated by spark gaps...

Post by Richard Hester »

Bookfinder.com include ABE books in the vendors that are searched. I've found tons of used books with bookfinfer.
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Re: For those frustrated by spark gaps...

Post by Richard Hester »

I did a search on Bookfinder for the S&D book, and ABE books was the high-priced spread. They show up in Bookfinder searches as an option.
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