[sdiy] Help with Timing Capacitor types.

[Tim Parkhurst]

On 3/3/07, Mike Beauchamp <mikebeauchamp at gmail.com> wrote:
> There must be as many opinions on what type of capacitor to use for
> timing caps as there are types of capacitors! The general consensus I
> get is to not use mylar, but to use Polystyrene (as stated in the
> guide that has been posted on here several times according to the
> Archives). Digikey and a few other places don't seem to have
> Polystyrene capacitors at all, let alone .047uF ones. Several VCO
> circuits online suggest Silver Mica as an alternative, but I can't
> find anything over .01uF on Digikey (and that thing is $6!).
>
snip
> --
> Mike Beauchamp
> http://mikebeauchamp.com
>

Hi Mike,

.047uF sounds like a fairly large value for a VCO. Are you sure that's
right? I only checked a few schemos, but it seems like the timing cap
values range from about 300pF to 1000pF. Just for reference:

1000pF  =   1nF  =  .001uF
300pF   =   .3nF  =  .0003uF
47000pF = 47nF =  .047uF

However, I see on the schematic for the Thomas Henry 566 VCO that
there is a .047uF timing cap. In this case, I might recommend trying
the polyester film or polypropelene film caps shown on this page
http://www.mouser.com/catalog/629/666.pdf

Since these have a conformal epoxy coating, they're meant to withstand
moisture and environmental changes better than most film caps. And
they're certainly cheaper than silver mica!


Tim (let the "cheaper than" jokes begin!) Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein