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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
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--
Bert Hickman
Stoneridge Engineering LLC
Woodridge, Illinois, USA
http://www.capturedlightning.com
+1 630-964-2699
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