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posible plastic for HV capacitors

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brandon3055

Wed May 16 2012, 08:08AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Hi all I would like to build some descent HV caps I now the best thigh for this is overhead projector sheets but i can't find anywhere that sells them so i have got some 100um industrial grade polyethylene film dose anyone now how well this will work and how many kv per sheet it can handle or anything else that will work alright.

Also if anyone has a chart with the dielectric strength of different materials that would be grate
Thanks

Mads Barnkob

Wed May 16 2012, 08:55AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

You can get an idea from reading this

and here is a lot of information and different materials dielectric strengths

Sulaiman

Wed May 16 2012, 05:52PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Between everyone using 'Bottle Caps' and now pp foil/film MMC
the 'in' thing to do was 'roll your own'
There is a lot of info in the Pupman archives

In short....
Polythene is excellent, preferably a few thin layers as opposed to one thick layer.
(due to minute imperfections that may allow punch-through of the dielectric)
If rolling, leave at least 1/2" dielectric at the sides (what will become the ends)
Preferably vacuum out the air before filling with oil, permanent vacuum not necessary.
If making flat-plate caps as above and leave 1" at the connection ends.

Mylar/polyester,PET and pvc are not good.

For a flat plate cap HDPE is good,
now polypropylene sheets are available on eBay cheaply, e.g.

The two main 'killers' of diy caps are corona and surface-tracking,
followed by dielectric failure.

A pp foil/film mmc is much more reliable and cheaper,
but it's good to have a go at diy hv capacitors, you learn a lot.

P.S. A single high voltage capacitor is best made from a few lower voltage capacitors in series,
if possible keep the voltage per capacitor below about 3kV
this is due to corona problems.

brandon3055

Thu May 17 2012, 10:11AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Any thoughts on polyethylene? I cant seem to find anything about it

Pinky's Brain

Thu May 17 2012, 11:50AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

I can see how mylar is bad for Tesla coil tank capacitors ... but why would it be bad for say a pulse capacitor?

Plain polypropylene is very different from the PP they use in commercial film capacitors, they use BOPP which has undergone a similar treatment as Mylar ... and has an order of magnitude higher breakdown strength. As far as breakdown strength is concerned you have BOPP and Mylar ... and then almost everything else (including polyethylene) is an order of magnitude or more weaker.

Of course this is true for capacitor grade film, to know whether a particular packing grade of BOPP has useful dielectric strength you'd have to go ahead and test it (the advantage of Mylar is that you can easily get the real thing with known properties, with BOPP it's going to be a crap shoot).

Ash Small

Thu May 17 2012, 07:36PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Destroyer of mosfets wrote ...

Any thoughts on polyethylene? I cant seem to find anything about it

As Sulaiman said, polyethylene/polythene is excellent. Don't use re-cycled polythene, as it has impurities/more imperfections. Use clear polythene if possible, as black/other colours contain dyes, etc. (black polythene possibly contains carbon black.)

I'm looking for some myself and found some reasonably thick, clear polythene in a garden centre, but it wasn't smooth. It felt quite coarse, which means it wasn't an even thickness.

As Suliaman points out, a few thinner layers may be better than one thick layer.

brandon3055

Thu May 17 2012, 10:30PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Ash Small wrote ...

Destroyer of mosfets wrote ...

Any thoughts on polyethylene? I cant seem to find anything about it

I did not now that polyethylene and polythene where the same thing thanks for that

I got a 5 by 2 meter sheet from my local hardware for $5

edit: i just built my first cap its very neat comared to some of the others i have built it uses a total of 4 strips roled tight taped and sealed on a hot-plate it reades as 16nf and i will test it to the limit tomorow i hope to get at least a good 20kv for 2 laers of plastic.

Sulaiman

Fri May 18 2012, 05:47PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Don't quickly test it to the limit then assume that
a similar unit at a slightly lower voltage will be reliable,
make your test setup safe enough that you can work in the dark
and look for corona whilst SLOWLY increasing the voltage.
ANY corona production will quickly eat away at the dielectric and must be avoided.

Be prepared for failure in the dark !

brandon3055

Fri May 18 2012, 10:53PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Sulaiman wrote ...

Don't quickly test it to the limit then assume that
a similar unit at a slightly lower voltage will be reliable,
make your test setup safe enough that you can work in the dark
and look for corona whilst SLOWLY increasing the voltage.
ANY corona production will quickly eat away at the dielectric and must be avoided.

Be prepared for failure in the dark !

Thanks I nether thought of that

brandon3055

Tue Jul 03 2012, 12:48AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

lasat night i decided to make an oil filled version of this capacior for the tc im rebuilding and i had a question about removing the air from the cap after i submerged the cap in kooking oil i hooked it up to my refrigerator pump to remove all the air but after about 15mins there was still lots of bubbles coming from the bottom of the cap so i want to know how logn it shuold take.
after about 15mins of removeing air i atached it to my tc and it seened to work alright i coldnt see any carona but after a fuw runs it faild so tonight im plan to build a new one with more plastic and kerocein instead of oil (that worked well in the original plate stack cap)

Dose anyone have any advice befor i start?

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