Hi, I have been thinking about building a "MARX" with these caps. Can
someone recommend a suitable Resister for a 10 stage Marx using these caps?
Doug
On Tue, Nov 6, 2018 at 9:10 PM Steve White <steve.white1@xxxxxxxxx> wrote:
> Hi Bert,
>
> Obvious cheap Chinese stuff. I found a Chinese vendor that will sell 20 of
> these at $1.50 each including shipping! My suspicion goes up when I can get
> one of these 2000 pF 30 KV capacitors for just $1.50. How good can they be?
> They advertise them as being used for DC filtering, electrostatic air
> fresheners, Cockroft-Walton voltage multipliers, and the like. I think they
> would be OK to use for these applications. They also appear to work well
> for Marx generators even though that is a pulsed power application.
>
> I did look at the capacitors a little closer today because I have one from
> my Marx generator. As well as I can see through the translucent plastic
> case, it does appear that the ends of the foil roll have been sprayed with
> some kind of metallic substance which is in line with the way these
> capacitors are supposed to be made. The mystery is why the tiny wires are
> there along with the much larger axial leads. The tiny wires appear to be
> attached to the foil at the outer circumference of the roll and the inner
> circumference as you described. However it also appears that the large
> axial leads are also attached to the metallic coating end caps at the
> center. So it looks like they attached large and small leads to the end
> caps. Weird.
>
> Steve White
> Cedar Rapids, Iowa
>
> ----- Original Message -----
> From: "Bert Hickman via Tesla" <tesla@xxxxxxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Monday, November 5, 2018 12:09:03 PM
> Subject: Re: [TCML] Mystery HV capacitors
>
> Hi Steve,
>
> Most metalized-film capacitors spray a layer of zinc onto the ends of
> the capacitor. The zinc makes a gas-tight connection with the evaporated
> aluminum metalization at the end of the capacitor roll. The zinc cover
> layer forms a base that's then soldered to the axial copper end lead.
> This construction technique creates a mechanically and electrically
> robust, low inductance capacitor capable of handling high pulse
> currents. However, high-current stress testing often shows that the
> zinc-aluminum interface is the weakest link during repetitive pulse
> discharge testing.
>
> In the images shown for the eBay capacitor, it looks like this
> particular manufacturer may have taken a shortcut by using small wires
> to make connection to the ends of the capacitor roll instead of using
> sprayed metal layers. The wire appears to be physically pressed against
> the bare end of the capacitor roll and perhaps held in place by the
> molded outer case. Although this might work for low-current
> (Cockcroft-Walton or DC filter) applications (at least for a while), it
> will likely arc, and prematurely fail, under repetitive high-current
> discharges seen in Marx or TC use.
>
> If you can't identify the manufacturer, and can't get any spec sheets,
> that's not a good sign. You get what you pay for... and with no-name
> Chinese sources, less.
>
> Bert
>
> Steve White wrote:
> > Does anyone know anything about these capacitors? They are unbranded
> shipped from China. The only markings are:
> >
> > CB80
> > 2000 pF
> > 30 KV
> >
> > I have searched all over the internet and I can't find out anything
> else. I know they are plastic film capacitors. I have a Marx generator made
> from these capacitors and they seem to work well for that purpose. They
> have an odd construction feature. They have nice solid axial leads and the
> leads seem to go all the way into the foil. But if you look closely, each
> lead has a tiny wire, much smaller than the axial lead, connected to it
> leading back into the capacitor. You can't really see it on the picture. I
> am wondering if the large leads are just for good mechanical support and
> the tiny leads are what is actually carrying the current into the foil. If
> so, I can't imagine these tiny leads are going to carry much current. I am
> wondering how well these might work in a small tesla coil. My hunch is not
> very well. My interest in them is because the price is inexpensive.
> >
> > Here is a link to some for sale on E-Bay.
> >
> >
> https://www.ebay.com/itm/2000pF-30KV-DC-High-Voltage-Condenser-Capacitor-for-Marx-generator-HV-Ham-Radio/173570926164?epid=9020951115&hash=item2869a22654:g:4goAAOSw1Sdbtexc:rk:46:pf:0
> >
> > Steve White
> > Cedar Rapids, Iowa
> > _______________________________________________
> > Tesla mailing list
> > Tesla@xxxxxxxxxxxxxxxxxx
> > https://www.pupman.com/mailman/listinfo/tesla
> >
> >
>
>
> --
> Bert Hickman
> Stoneridge Engineering LLC
> Woodridge, Illinois, USA
> http://www.capturedlightning.com
> +1 630-964-2699
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