[sdiy] Fixed filter bank questions

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Tue May 14 13:02:21 CEST 2019


I think so, at least for metal/poly type capacitors.  You'd have to get 
any air voids out too, as these would also alter the capacitance.

I don't know what the manufacturing processes are for ceramic dielectric 
caps, but I'd guess the dielectric thicknesses in something like a 
modern 10uF 0603 multi-layer ceramic cap is phenomenally small!

-Richie,


On 2019-05-14 11:27, Tom Wiltshire wrote:
> So if we could repeatably make plastic films of exact thickness, we
> could have cheap tight-tolerance capacitors? Does that follow?
> 
> Tom
> 
>> 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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