Re: [TCML] Burnt Joint (Cap Bank)

[Jim Lux <jimlux@xxxxxxxxxxxxx>]

On 5/21/13 11:34 AM, Mike Gray wrote:
> For the same money I can get roughly 250% more PVCu tube from here:
>
> http://www.pipestock.com/pvc/pvc-pipe/pvc-pipe-10-bar-metric/
>
> So is there any real difference between acrylic and PVCu? What's the
> electrical property of primary concern here, relative permittivity?


you don't care about epsilon in this application. And you don't really 
care about dissipation either (unless you're using it in a capacitor as 
dielectric).  Terry Fritz made some measurements a few years ago with 
secondaries wound on various materials, and there's really not much 
difference.


What you really care about is breakdown voltage.
And that depends a lot on things that aren't controlled in, e.g. PVC 
pipe. For instance, PVC pipe can have some amount of metal fragments in 
it and still meet spec for plumbing. Not that it's very common, but it's 
something to bear in mind.

And along the breakdown voltage subject.. Some kinds of tubing have a 
foam core with solid skins (black ABS drain,waste, vent DWV pipe is like 
this, as is some sewer pipe).  The foam breaks down easier than the 
solid plastic (less "stuff" in the way, and the bubbles create field 
concentrations).

the concern about carbon black as a pigment has more to do with surface 
resistivity. If you stick an ohmmeter probes on the surface and it reads 
infinity, you're probably good to go.



> I'm still wondering about that volume resistivity figure I was given for
> the PVCu... anyone able to shed some light on that one?
>
> Or am I over-engineering this whole decision...? Is it just a case that any
> old (pure) plastic will do a good job?

Any old plastic that doesn't have a lot of inclusions and debris in it 
will do.

Fiberglass tubing works quite well, for instance.

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