[Racal] RE: [R-390] Source for axial caps

[Roy Morgan]

At 11:44 AM 8/1/2004 -0400, you wrote:
>Ooops?!
>
>Bob wrote:
> > Provided the voltage ratings are adequate and you don't get parts that
> > are only rated to 85C (like polystyrene) just about any modern plastic
> > part will do a lot better than the paper parts it's replacing.
>
>I've stocked up on some yellow poly's here and there -- don't remember
>whether they're polystyrene or polyethylene polypropylene or what.
>
>What are the typical temp ratings of the various yellow axials?

Barry,

PolySTYRENE caps are usually clear, you can see the foil inside through the 
stuff.  See the (poor) picture at:
<http://www.industrialnewsroom.com/fullstory/8253>
The common polystyrene cap is the right most one.  (It may be that the 
other ones shown are also styrene.)  Note: polystyrene caps are normally 
rated at 100 volts or less but one outfit seems to have stocks up to 630 
volts: see:
<www.seacorinc.com> or:
<http://www.seacorinc.com/products/capacitors/tables/precision.asp>
Where you will note that the styrene caps have one tenth the dissipation 
factor and three times the leakage resistance of the polypropylene units in 
the same table.

This link is a nice comparison chart showing many common dielectrics and 
their significant characteristics:
<http://www.seacorinc.com/products/capacitors/tables/dielectrics.asp>
I see from this data the styrene caps have a negative temp coefficient. 
This may give us a way to determine if a cap is styrene or not.

The caps that are usually yellow are poly-something-else.  If your caps 
are  recently made, you can check manufacturers specs for the particular 
ones you have for details.  I think you have nothing to worry about.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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