Guidance on HV power and feedthroughs

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Guidance on HV power and feedthroughs

Postby Ryan Oddie » Thu Feb 16, 2017 2:30 pm

Hi Guys,

Just started on the forums and thought this may be the place to put forward some basic questions.

1.) I have a physics background with some university electronics thrown in but I am very rusty. Can someone point me in the direction of a good book / website on high voltage power (supplies / transformers / wiring etc.)

2.) I am trying to get hold of a HV feed through flange for my vacuum chamber can someone point me in the direction of one that is capable of handling voltages from 10kV to maybe 80Kv (with low current of max 20mA). Every manufacturer website / catalogue seems to have feedthroughs that handle up to 30/40kV but they seem a bit overkill as their current ratings are 5Amps +.

Your advice would be very much welcomed.

Regards,

Ryan
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Re: Guidance on HV power and feedthroughs

Postby Rich Feldman » Thu Feb 16, 2017 3:41 pm

Welcome, Ryan.

Don't fret about those current ratings.
It's hard to imagine a tens-of-kV-rated feedthrough _not_ able to handle 5 amps. A 20 AWG (0.8 mm diameter) wire would do for that current, I bet even if made of good-for-sealing-to-ceramic metal. HV designers want large-radius conductors, to keep surface E-field strengths from getting too high.
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Re: Guidance on HV power and feedthroughs

Postby Ryan Oddie » Thu Feb 16, 2017 4:05 pm

Thanks Richard,

If I wanted to go above the 40kV that the feed throughs are rated for (say 60kV or 80KV) but still only using a low current would that be plausible or do I need to find a feedthrough rated for higher voltages?

My concern is that a feed through rated at 40kV will have limited insulation and even at lower currents I may get arching at the higher voltages (I am assuming as they are rated for higher currents heating would be less of an issue)

I should point out that I have no intention of using such high voltages initially but as these feed throughs are pretty hard to come by and relatively expensive I would rather get on that I can "grow into".

Regards,

Ryan
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Re: Guidance on HV power and feedthroughs

Postby Sarvesh Sadana » Thu Feb 16, 2017 5:15 pm

The Spellman HV reference manual is a pretty decent reference on high voltage supplies.

http://www.spellmanhv.com/-/media/Files ... manual.pdf

Also, I find that KJL has good technical information on vacuum products and feedthroughs, though they do tend to be rather expensive. This may be of service: https://www.lesker.com/feedthroughs-vie ... -resources
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Re: Guidance on HV power and feedthroughs

Postby Dennis P Brown » Thu Feb 16, 2017 6:50 pm

In all likely hood, getting a good deal on a 40 kV feed-thru on ebay would not be too difficult and allows you to get a working fusor up faster. A 60-80 kV feed-thru isn't likely to appear on e-bay and new ones, I understand, are very expensive (as is cabling.) Also, dealing with voltages above 35 to 40 kV get's very difficult for cables and other parts. Not a good idea to build your first fusor for 80 kV if you will just be trying to get a unit operational and more to the point - obtaining a fusor grade 30 - 40 kV power supply is a task that many fail at. I built my own 35 kV feed-thru and frankly, it was easy; further, I'd never attempt to try the same for a 80 kV unit mainly because such a cable is far beyond my means ... .
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Re: Guidance on HV power and feedthroughs

Postby Peter Schmelcher » Thu Feb 16, 2017 7:21 pm

If you have access to welding equipment and a lathe you might consider making your own HV feedthrough out of a spark plug.

A champion N3C spark plug when cut open has a 5/8" (15.66mm) OD alumina core. The core is easily good for 50KV, been there done that. In hindsight next time I would cast the HV electrical connector using a 2 part silicone rubber.
viewtopic.php?f=10&t=4237

The alumina core diameter should be fine for 80KV if the plug was instead sealed on the OD. I have been thinking of clamping and sealing a plug using a standard vacuum SS tube fitting welded to a conflat for my -70KV Spellman DXM.

You need copper between the plug body and the SS surface for a vacuum seal. A 5/8" SS tube fitting would require the fabrication of a copper olive. Alternatively with a 3/4" SS tube fitting a thick walled copper pipe should work. I believe this eBay item 310578064543 is an electrical wire crushing splice connector, 18.86mm OD and 14.89mm ID, unfortunately it is slightly undersized for the spark plug so a bit of lathe work is needed.

-Peter
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Re: Guidance on HV power and feedthroughs

Postby Richard Hull » Thu Feb 16, 2017 7:53 pm

It is purely the voltage that determines the break down point of an insulator or insulation once the surface area of the conductor is figured into the mix. Current, at any voltage, determines the damage doa8 once a breakdown arc occurs. Current handling capability, be it 5 nano amps or 5,000,000 amps, means nothing until a "closed circuit" is established, (arc breakdown).

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Re: Guidance on HV power and feedthroughs

Postby John Futter » Fri Feb 17, 2017 2:57 am

I bought a FA18576 from solidsealing.com only US$850
and I have pushed it to 120kV with no problem
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Re: Guidance on HV power and feedthroughs

Postby Ryan Oddie » Fri Feb 17, 2017 9:41 am

Thanks for all your advice.

Bottom line - I think I will start with a 30-40kV feed through as Dennis suggests. Once I have a bit more experience (and money) the FA18576 that John referenced looks like a great option.

I am sure Peters explanation of how to make a feedthrough will be really useful to someone on this forum but unfortunately I don't have the skillset or equipment to follow his advice (right now) - I would love to get to the stage that I can build the parts myself.

@Sarvesh - your link to the Spellman documentation was just what I was looking for and answers many of my questions.

@ Both Rich and Richard - you both seemed to provide the explanations I needed even before I completely understood what questions I needed to ask. Much appreciated.
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Re: Guidance on HV power and feedthroughs

Postby Rex Allers » Fri Feb 17, 2017 10:43 am

John, Your mention of solidsealing.com for feedthroughs was interesting. I didn't know of them. I'm not sure if your comment of "only" $850 for the FA18576 had any tongue-in-cheek to it, but probably is a reasonable price for a new feedthrough of those specs.

I looked at their site. They have a parameter-based search for feedthroughs that didn't find any matches at all for me (?). But I scanned through their full list. The FA18576 is rated at 70 kV. You say you pushed it to 125 kV with no badness. That's a factor of about 1.79 times rated voltage.

In their list, I saw an FA12140 rated at 30 kV for $200. Probably more in line with what most fusor builders might afford. If it can be pushed to the same ratio over the rated voltage, it would be good to about 50 kV.

I see they both only have about 15 mm of insulator projecting on their inside end and that's from the CF mating surface. I guess they depend on the effect of Paschen's law and a vacuum to not arc on that end, although their data sheets doesn't seem to mention anything about that. Any comments about the inside dimensions from those who know?
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