[sdiy] Fixed filter bank questions

Tom Wiltshire tom at electricdruid.net
Tue May 14 12:27:18 CEST 2019

So if we could repeatably make plastic films of exact thickness, we could have cheap tight-tolerance capacitors? Does that follow?


> On 14 May 2019, at 10:12, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> Capacitance is proportional to plate area, dielectric constant and inversely proportional to dielectric thickness. Everyone wants small components because you can fit more in a product, they weigh less, cheaper to store and ship, etc. So the dielectric is very thin. It's hard to maintain a tight tolerance on a very small dimension. If not throughout a batch, then definitely between subsequent manufacturing batches.
> BTW, it is actually possible to 'trim' high voltage metallised polypropylene capacitors with self healing properties, by slowly raising the applied voltage until the capacitance falls to the target value. The voltage at which this occurs gives additional information about process variation. 
> -Richie,
> -Richie,
> Sent from my Xperia SP on O2
> ---- David G Dixon wrote ----
>> What I don't understand is, in 2019, with everything highly automated, why
>> are capacitors still subject to 10% tolerances, or even 5% tolerances?  If
>> the plates are made exactly the same way by the same machine from the same
>> material, and the coatings are applied in exactly the same way, etc, etc,
>> why aren't they essentially all identical?  I can understand 1 or 2%
>> tolerance, but 10 or 20% (as the standard) should be obsolete.  People
>> aren't hand-making these stupid things!
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