Homemade Feedthrough

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
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Liam David
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Homemade Feedthrough

Post by Liam David » Sat Jun 14, 2014 4:02 pm

After realizing that spark plugs can fail catastrophically over 25kV, and seeing the prices for commercial feedthroughs, I decided to build my own HV feedthrough.

The feedthrough design credit goes to Andrew Seltzman. rtftechnologies.org

It consists of a 1' section of 0.25" OD, 0.156" ID alumina tube capped with a Swagelok 0.25" ID compression cap. A 2.75CF to 0.25" compression fitting compresses an o-ring to the surface of the alumina, forming a seal. A piece of wire electrically connects the Swagelok fitting to the inner grid.
Image

It cost about $45 to construct.

-Liam David

For the compression fitting without nut and o-ring
http://www.bmisurplus.com/products/4588 ... ompression

For the alumina and Swagelok fitting
mcmaster.com
Last edited by Liam David on Sat Jun 14, 2014 11:36 pm, edited 1 time in total.

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Chris Bradley
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Re: Homemade Feedthrough

Post by Chris Bradley » Sat Jun 14, 2014 7:17 pm

Liam David wrote:After realizing that spark plugs can fail catastrophically over 25kV
How did you come to realise this? I've never heard of a spark plug 'failing', let alone 'catastrophically'.

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Liam David
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Re: Homemade Feedthrough

Post by Liam David » Sat Jun 14, 2014 10:54 pm

According to this previous thread:
viewtopic.php?f=18&t=9420

I read somewhere (can't find where) that spark plugs could explode, creating alumina shrapnel. I'll try and find where I got this info.

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Chris Bradley
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Re: Homemade Feedthrough

Post by Chris Bradley » Sun Jun 15, 2014 7:45 am

Liam David wrote:According to this previous thread:
viewtopic.php?f=18&t=9420
I read somewhere (can't find where) that spark plugs could explode, creating alumina shrapnel. I'll try and find where I got this info.
Never heard of any of this. Sounds like a load of cobblers to me.


In regards;
Dennis P Brown wrote:You might want to ask if a spark plug can handle that level of power - I be surprised if it could ( 40,000 V * 0.02 amp/sec = 800 watts!)
from the other thread, the voltage across the spark plug will be essentially zero throughout, assuming you've not bought a resistive type. It will have to handle roughly zero power. This seems to be a misunderstanding of electrical power.

In general, if you want advice then you should trust those who have done these things before. However, it is an open forum and people will comment with only their guesswork and intuition - which at times might be useful or might be misleading. You have to sift what you read for relevance and accuracy. Spark plugs failing and producing dangerous shrapnel is either something seriously lacking in my own knowledge that I need to be aware of, or it is bogus. I very much presume the latter until I've got good reason to think otherwise.

That being said, your feedthrough build looks very well executed and I am sure it will be useful and will serve the purpose you want from it.

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Dennis P Brown
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Re: Homemade Feedthrough

Post by Dennis P Brown » Sun Jun 15, 2014 1:19 pm

Not sure why my "quote" is used here since it has really no relevance to the question at hand; nor did I say anything even remotely incorrect much less implying or in any way reference issues of explosions by spark plugs - in fact, never heard of that happening nor do I believe it could/would happen with most fusor power supplies.

Relative to the power handled by a spark plug Chris is correct only for room temperature/low temperature operation - if there is no significant heating of the spark plug conductor, then essentially zero power is absorbed by the spark plug conductor. That is, so long as the spark plug remains cool then like any good conductor material it has very little resistive power absorption (i.e. it is very conductive.) This is rather obvious and I am surprised I have to mention this well known and understood effect by conductors.

So for fusor operation this type of feed through doesn't necessarily operate at room temperature and, in fact, can be forced to handle a large heat load - the electrodes within a fusor often "glow" and can even melt. So for fusor electrode operation a spark plug isn't necessarily a good feed through because those power levels, combined with heating of the electrode within the fusor could cause the plug to over heat and fail. That is, resistance by the spark plug can rapidly increase and hence the power absorbed by the plug from the current induced heating effect can then get serious and even lead to failure of the conductor within the plug or worse, possibly cause the ceramic to "crack" due to expansion of the conductor - this effect is seen / happens in cars leading to plug failure. Still, this is an issue and one that needs to be addressed rather than ignored. So my concern for the total power being handled by the plug is relevant and these devices may not be a good option for those levels of power if the plug will also have to handle a heat load created within the fusor (a common occurrence for many applications.)

Another point: I am not sure the safety factor that a standard plug has built into it for high voltage operation allows for 40 kV operation; maybe the plug can sustain such a high voltage but than again, they were not designed for that high a voltage so might not be a useful high range for operation.

Bottom line - Chris statement:
In general, if you want advice then you should trust those who have done these things before. However, it is an open forum and people will comment with only their guesswork and intuition -


While good advice on any forum it is positioned to somehow imply my post isn't correct or somehow related to this advice - if so, I find that both impolite and factually incorrect.

As for this statement:
This seems to be a misunderstanding of electrical power.
I feel is neither polite nor was needed.

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Chris Bradley
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Re: Homemade Feedthrough

Post by Chris Bradley » Sun Jun 15, 2014 3:14 pm

Please explain how your quote demonstrates an understanding of electrical power, and I can reconsider my comment. At the moment, I cannot see a way to interpret it in a meaningful way, maybe it is my lack of comprehension.

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Richard Hull
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Re: Homemade Feedthrough

Post by Richard Hull » Sun Jun 15, 2014 3:37 pm

The spark plug can do fusion. I did it in fusor III. I replaced the spark plug in fusor III with a more substainial insulator as I went over 25 kv. (It never failed!) The big issue was possible external, in air, arcing. The spark plug is a ceramic refractory material and capable of huge heat loads. No issue there. Even at 40kv+ it is a rare fusor that will run over 500 watts. The ceramic will handle its fraction of that with ease.

I personally would not recommend a spark plug over 30kv or perhaps even to that level without special heroics. Not due to running failure, structurally, but due to electrical breakdown or arcing issues. Non-resistive plugs and ones with a long tip reach like the one shown recently can do amazing things, especially in the hands of an amazing builder.

Ricahrd 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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Homemade Feedthrough

Post by Dennis P Brown » Mon Jun 16, 2014 10:17 am

Chris Bradley wrote:Please explain how your quote demonstrates an understanding of electrical power, and I can reconsider my comment. At the moment, I cannot see a way to interpret it in a meaningful way, maybe it is my lack of comprehension.
Since my simple calculation is both correct and appropriate your statement about my electrical knowledge - which you know nothing about my undergraduate degree nor graduate degree - was both wrong and insulting. Also, you still haven't answered my question of why you place my post next to that ridiculous claim about exploding spark plugs? Exactly what does one have to do with the other?

I am not aware that standard car spark plugs are designed for 800 watt continuous load at 40 kV - if you have a source for that claim I'll agree and stand corrected. While my first post did not address this issue - resistive heating - the total power through a device is critical parameter for the onset of this phenomena. So, as I pointed out in a follow up post the issue of resistive heating - which you apparently overlooked - makes your claim that these devices can handle such a load with no significant resistance both misleading, and for steady state conditions, which most people with fusors require, very likely, wrong. Despite this I do not consider your electrical knowledge lacking - just the opposite.

However, it was YOU who made a claim (based on what?) that my knowledge is lacking relative to electrical physics- either prove that statement or at least demonstrate common professional behavior by withdraw your insulting claim.

Since this thread is hardly an appropriate forum to continue this discussion, I'd suggest a PM - considering you made such an issue out of one of my double posts - one of which had an ebay line - as being excessive waste of a thread, I'd think you'd have followed your own advice on that policy.
Last edited by Dennis P Brown on Mon Jun 16, 2014 11:05 am, edited 1 time in total.

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Chris Bradley
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Re: Homemade Feedthrough

Post by Chris Bradley » Mon Jun 16, 2014 10:59 am

Dennis P Brown wrote:you still haven't answered my question of why you place my post next to that ridiculous claim about exploding spark plugs? Exactly what does one have to do with the other?
Because the OP quoted the thread which that came from, and even if he doesn't hold much credence to the 'exploding spark plugs' he might still interpret what you have put to think that a spark plug may experience electrical heating of 100's of Watts, which is nonsense.
Dennis P Brown wrote:I am not aware that standard car spark plugs are designed for 800 watt continuous load at 40 kV
What does the voltage have to do with the electrical load on the spark plug? P=I^2.R. At 20mA, a spark plug of some 0.1 ohm will experience an electrical stress of 40uW. A copper cored spark plug, for which copper has a resistance coefficient of around 0.003/degK, even if it was at 1000C, would only increase its resistance by a factor of 3. 3 times nearly zero resistance is still nearly zero resistance.
Dennis P Brown wrote: if you have a source for that claim I'll agree and stand corrected but that in no way support's your baseless and insulting claim I have no electrical knowledge.
I didn't. I said that a suggestion that the spark plug would experience an 800W electrical load seems to be a misunderstanding. I believe that to be a statement of fact and it is immaterial who said it as I did not make any personal reference to the state-er of that statement.

I think it is significant enough to leave the discussion open to show newbs that high voltage electrics can, at times, confound the best of us. High voltage but low currents will pass through relatively high resistances with barely an effect. You could use a resistive-type spark plug of 2000 ohms if you like and 20mA would still only generate 0.8 watts of electrical heating. Resistances of less than 10,000 ohms are barely noticed by HV low current, but they are very useful for ballasting a sudden short-circuit as all circuits will have some intrinsic capacitance. Just a few meters of coax at 50kV has enough capacitance that the stored energy can upset your heart and induce VF. It is a Very Good Thing to have a few 1,000 ohms resistance in any mA level circuit to mitigate this sort of thing. mA worth of current have almost no 'electrical power' thermal interaction with resistances of less than a few hundred ohms.

I'll just add that spark plugs are also designed to be robust to thermal effects at the ends of their electrodes too. If you recall, their purpose is to ignite burning fuel, which gets pretty hot at the centre of ignition. The thermal conduction from the electrode to the body of the spark plug is therefore a designed-for characteristic and spark plugs come with a statement of their heat capacity (in the lettering code). Whereas I have never seen a statement of thermal tolerance in a commercial feedthrough, so it might even be the case that spark plugs are more suitable for fusor application than commercial ones for which thermal conductance has not been taken into account. I do not know but a suggestion that spark plugs cannot tolerate high thermal loads at the electrode is surely erroneous - they are designed to be immersed in a chamber that sees 10's of kW of thermal energy coming and going, not just a few 100's!

Dan Tibbets
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Re: Homemade Feedthrough

Post by Dan Tibbets » Thu Jun 26, 2014 10:02 pm

My two cents worth. The conductivity of materials can change substantially if heated. The tungsten in incandescent light bulbs is the best example. But the conductors in spark plugs do not change so drastically (moderately). Also, a few mA of current does not make the wires glow red hot in a Fuser. In an incandescent light bulb a 5 watt bulb glows only feebly at ~50 mA, and this is in a designed high temp sensitive conductor (tungsten) and with a fairly narrow wire (30th gauge?). This heating of the exposed electrode in a fusor is an entirely different process (sort of). the heating in this case is from high energy ion bombardment hitting the wire. This is nonexistent in the internal metal of the electrode/ wire, and even is absent at the surface if insulated. The insulating material is also possibly bombarded, but it doesn't heat as much and create a visible blackbody radiation glow, because it has more mass to heat up, and also more surface area to radiate off the heat (it radiates off more heat than the low density plasma/ ion impacts impart once some equilibrium temperature is reached (well below incandescence)). The wire with a relatively smaller surface area for radiative cooling will reach a higher temperature before an equilibrium is reached. The same 5 watt bulb example above would not visibly glow if the wire was thicker (even if it was designed to have the same overall resistance). Note that it would glow, but only in the infared, not the visible light spectrum.

Dan Tibbets

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