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Polyethylene Dielectric

 * Original msg to: Oldradio-at-cheney-dot-net
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting "James M. Watson" <oldradio-at-cheney-dot-net>:

> I measured the capacitance on a capacitance/inductance analyzer at
> = school today.  I did this using one layer and then three layers of
> .030" = polyethylene.  The results were as follows:

> 1 layer    -at- 100Hz    =3D> 272pF
> "         -at- 100kHz  =3D> 267pF

> 3 layers  -at- 100Hz   =3D> 102pF
> "          -at- 100kHz =3D>   99pF

> I then solved for the dielectric strength (K) and got 1.1995, which 
> would round to 1.20;  I had expected it to be in the neighborhood of
> 2.2 - 2.3.  The instrument that I used to measure the capacitance
> given above is part of the equipment in the EE senior design lab, and
> I would consider it 100% reliable.  I would expect that the
> polyethylene that I have is the source of the discrepancy.  I would
> welcome comments as to whether anyone has had similar results when
> checking the material to be used in a Tesla Coil.

In homemade capacitor construction the polyethylene typically pulls a
"K" or dielectric constant of around 2.0 or a little lower. When stacks
of thinner polyethylene are doubled or trippled up the dielectric con-
stant typically drops some more. One reason for this is that the 
material is not as dense when layers are stacked.

> About neon sigh transformers in parallel;  4 15kV neons in parallel
> would have an impedance of Z=3D15kV/120mA =3D 125k ohms  (close
> enough for ball park figures).  Assuming the circuit is totally
> capacitive, capacitance would be:  C=3D1/(2 x pi x Z x 60) =3D
> 1/[2(3.14159)(125k = ohms)(60Hz)] =3D .021221uF, which is a whole bunch
> of capacitance when building the things yourself.  Do yall see any
> flaws in these calculations?  

No, from a practical standpoint these transformers will match up
pretty well to a .02 uF capacitor. Yes, this is a lot of capacitor
when building things yourself. 

> The only thing that bugs me is that the impedance of the primary 
> winding has not been considered.  Should it be considered, or 
> can it be disregarded?

If you look at the primary impedance in relation to the secondary
impedance, it is not a big deal. If you are referring to matching
your caps, and you are thinking the primary impedance is going to
affect your calculations, I would not worry about it.

Richard Quick


... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12