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The microwave tower next door has a lightning arrester attachment. The charge probably goes harmlessly to ground. If it were possible to siphon part of the charge as it went to ground, how would one choose the capacitor (capacitance rating, punch through voltage, etc)?

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    \$\begingroup\$ A flux capacitor, of course ;-) \$\endgroup\$ – Chris Stratton Jul 13 '12 at 4:43
  • \$\begingroup\$ @Chris Stratton: In my native language dubbing of BTTF the word "capacitor" from "flux capacitor" was translated to mean "storage" in most generic sense, so until I saw the "flux capacitor" phrasing I believed it just was "flux storage"... \$\endgroup\$ – sharptooth Jul 13 '12 at 9:12
  • \$\begingroup\$ ...and I believe that using the word "capacitor" was a bad idea and the net result is that every other time someone asks about using a lightning energy he is reminded of the flux capacitor just because of the "capacitor" wording. \$\endgroup\$ – sharptooth Jul 13 '12 at 9:13
  • \$\begingroup\$ The contradiction in specs for a storage device are many as limited by the surge ratings of 100KA. This is produced by "tunnelling" a negative resistance when the air dielectric breaks down from a gas to a plasma in the air similar but different mat'ls to gas tubes, SCR's but the cloud surface area is huge. The energy lost in ESR of any man made cap with a conductor would burn up and otherwise becomes an insulator. TO match impedances one would have to create the 1MV and connect it to a cloud with a wire. which wont work since it will arc before it gets there. and No man made cap works \$\endgroup\$ – Sunnyskyguy EE75 Jul 13 '12 at 12:28
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    \$\begingroup\$ The ideal storage device would be another cloud which is impossible to interface with. \$\endgroup\$ – Sunnyskyguy EE75 Jul 13 '12 at 13:06
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It IS possible to store high voltage energy, and probably possible to built a store that would not be destroyed by the first lightning strike. BUT even the biggest cap easily available will store less energy than a 2500 mAh NimH AA cell. Many such capacitors would allow significant energy to be stored "at a strike" but the store cost would be large.

It would be possible to construct capacitors capable of say 10's of kV rating which were protected [tm} by spark gaps so that the gap broke down well below capacitor destruction voltage. You would then potentially (pun noted) have a problem with back conduction of the capacitor via the ionisation path, but could very probably build a system that used a spark gap from input to capacitor to initialise charging and then another spark gap from input to ground such that it reduced the available potential of the charging spark gap and extinguished the arc. While this sounds rather "Heath Robinson" it is similar to the sort of things that are really done at extra high potentials.

As an example of spark gaps being used as switches, a "Marx Generator" uses arc breakdown to cause spark gaps to act as switches and allow construction of a voltage multiplier witj DC input. Good trick if you can do it. You can ! :-)
See "Marx generators below.


Capacitors:

You can make DIY multi kV capacitors aka "Leyden Jars - with glass jars or bottles - or even plastic "pop" bottles. From here

enter image description here


Here's a 19 nF 10 kV cap made with foil and mylar shopping bags

You can buy capacitors in the nF range rated at 10's of kV.

Many offered here - Alibaba

Example from above - 40 kV, 10,000 pF disk ceramic.
At 40 kV 10 nF will store 8 Joule. About the same as a 0.64F cap at 5 Volt.

Many very high voltage caps here - very worth looking at

1.3 uF at 100 kV !!!!!!!!!! :-)
6500 Joule.
Less than the energy in a 2500 mAh AA NimH cell (= about 10,000 J)

enter image description here


Marx Generators - as examples of spark gap switching.

This video of a 180 kV output Marx Generator operating is worthwhile just for the sound :-).

I am NOT suggesting that a Marx generator is what you need to meet your suggested requirement - but offering it as an example of how switching may be achieved by quite unusual but eminently practical means.

Wikipedia - Marx Generator

The following is far from the most impressive spark-gap-switch unit image available - I chose it because it gives a good view of the spark gap switches working (small arcs along the centre line) plus the resultant overall multiplied voltage arc (you should be able to find that without my guidance :-) ). To make this work you only need a voltage able to break down the small spark gaps.

enter image description here

Worthwhile

LOPT in, 200 kV out](http://c4r0.elektroda.eu/_hv/index.php?page=hv/marx) Polish language.

Project with image


Very good project description.
"Quick & Dirty" Marx generator

enter image description here


Karl would have been proud!

enter image description here

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    \$\begingroup\$ "1.3 uF at 100 kV". We're getting close! \$\endgroup\$ – stevenvh Jul 13 '12 at 8:54
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Yes, a capacitor can be made to store some of the charge from a lightning strike. Early experimenters with electricity back when both capacitance and lightning were poorly understood used something called a "Leyden jar". It was meant to be a bottle for electric charge. These were glass bottles usually with mercury inside and another conductor outside. Back then they were thinking of storing the charge in a jar, just like you can store other things in a jar. Actually the two electrodes and the glass of the jar formed a capacitor.

Look more closely at Ben Franklin's famous lightning experiment and you will see he was actually trying to do exactly what you are proposing. He wasn't just flying a kite in a thunderstorm for no good reason. He had a key attached to the kite string partway up, and a wire going from the key to a Leyden jar. He was trying to prove that lightning was electricity by using it to "fill" the Leyden jar. It worked and he made his case.

One problem with capturing lightning energy is that the voltages are extremely high. The final voltage on the capacitor is only going to be a small fraction of the lightning. Think of it this way. How far apart can you keep the two leads of the capacitor? Now consider that the lightning voltage was high enough to arc thru several miles of air. If the capacitor was charged up to that level, you'd need the equivalent of a few miles of air insulation to keep it from arcing and discharging.

I vaguely remember that Tesla also did some experiments attempting to capture energy from lightning during his Colorado days. It might be worth a little poking around to see if this is true and find some more information about it if it is.

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What kind of capacitor could build up, and maintain a charge acquired from lightning?

None.






How about this one? This capacitor is the highest rated capacitor I could find, 100 nF rated at 100 kV. That's good for 0.01 C. An average lightning strike has a 5 C charge, so the capacitor won't survive it; the voltage will rise to 50 MV, destroying the capacitor.

In the same article it says that a positive lightning bolt has 100 times more charge, and then you're better not near the capacitor when it gets hit.

Russell and Olin mention the Leyden jar, but several sources mention 1 nF as a typical capacitance. Then a 1 µs, 100 kA bolt will charge this to 100 MV, or up to 1 GV if it lasts longer. That will flash through any kind of insulation.

If the 100 kV capacitor has an internal resistance of 1 mΩ then a 100 kA bolt will cause a 10 MW dissipation. That looks bad, but if it lasts for 10 µs then it only represents 100 J. (I have 500 Ws light strobes for my photography, which discharge this 500 J in 1/2000 of a second, which is also 1 MW during a jiffy.)

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    \$\begingroup\$ Early experimenters did successfully charge capacitors from lightning. The capacitors were called Leyden jars back then. Also, high voltage need not destroy a capacitor, just arc around it. \$\endgroup\$ – Olin Lathrop Jul 13 '12 at 11:03
  • \$\begingroup\$ @Olin - Yes, Russell mentions them. He also has a 100 kV/1.3 uF cap, though it doesn't look like it will be SMD :-). Have you ever seen anything in that range? \$\endgroup\$ – stevenvh Jul 13 '12 at 11:06
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Well like stevenvh's answer. There is probably nothing commercial that you could buy to hold the full power of a lighting strike. Putting high current limiting resistors, in your discharge path. Then trying to siphon a portion of the strike off will most times blow the path as lightning strikes are unpredictable in their duration, max voltage and current.

However. In the realm of experimental science and a Bill Gates budget. You could dabble with a Hindenburg dirigible size hanger. Then recreate a cloud inside the hanger. A cloud of moisture can hold a charge under the right conditions. You may be able to recreate those conditions then transfer a strike from an atmosphere cloud to your home corralled cloud. Again. Even if you succeed in transferring the charge. The problem of tapping this energy still remains. And the practically of the project puts it into the Science Fiction arena for now. Perhaps as Chris Stratton's comment suggests. Flux capacitor research should now be a national priority.

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