[Prev][Next][Index][Thread]

Dielectrics




----------
From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Thursday, March 26, 1998 6:39 AM
To:  Tesla List
Subject:  Re: Dielectrics

Tesla List wrote:
> 
> ----------
> From:  Justin Oliver [SMTP:joelec1-at-ozemail-dot-com.au]
> Sent:  Wednesday, March 25, 1998 4:09 PM
> To:  tesla-at-pupman-dot-com
> Subject:  Dielectrics
> 
> Would it be okay to use some polystyrene as a dielectric in a stacked plate
> cap as I can get a heap of sheets of this stuff for free. It is about 1cm thick.
> Also, does it matter how big the dielectric is. Can you make a 100Kv cap and
> then use it on 10Kv?
> Thanks in advance,
> Justin

Justin,

Polystyrene (PS) has excellent voltage breakdown and dielectric loss
characteristics. Unfortunately, its also brittle and has poor solvent
resistance. Because of the latter properties, it can't be submerged in
mineral oil, and thin sheets can't be "rolled". However, it may be
possible to make a flat plate cap from the material and use another type
of synthetic/silicone oil (perhaps refrigeration oil?) to prevent corona
as long as it didn't dissolve or swell the PS. You can always overdesign
a cap to have more breakdown voltage than required - it'll just be
physically larger and tend to cost more. 

The formula for capacitance can be expressed as:
    
     C = 0.2248kA/d    

   Where:
     C = capacitance in pF
     k = dielectric constant (for PS about 2.5)
     A = Effective plate area ( square inches)
     d = overall dielectric thikness between plates (Inches)
 
A 15 kV TC capy uses about 2 mm of total dielectric thickness. Ignoring
that k is a bit higher than for LDPE or PP, in order to get the same
amount of capacitance as a typical cap made from 2 mm material, you'll
need to have about 5 times the number of plates in parallel or 5 times
the effective area. Since the plates are 5 times thicker, the total
volume of the cap will be about 5 times larger for the same capacitance
as one made from 2 mm material. However, it WOULD be bulletproof!

Safe coilin' to you, Justin!

-- Bert --