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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