What attribute of a capacitor is linked to physical size?

Thread Starter

strantor

Joined Oct 3, 2010
5,322
Most of the capacitors I have encountered have been in the pico-, nano- or micro- farad range & are small in size. The exception being those used in car audio systems, which are common to be 1F and about the size of 3lbs of ground beef. The other day my friend was telling me about a 3F capacitor 1" long by .5" round. I didn't believe him; I thought it would have to be at least the size of a trash can, he was right. So now I'm puzzled. I though the farad rating determined the size, so what is it actually?
 

wmodavis

Joined Oct 23, 2010
739
The main factors that determine capacitance are area of the plates and the dielectric used.
More area means larger capacitance and size.
Higher dielectric constant means more capacitance and smaller size.
 

KJ6EAD

Joined Apr 30, 2011
1,569
Physical size increases with capacitance as others have noted but also significantly with voltage. As you can see, those ultracapacitors are rated for less than 3V.
 

ErnieM

Joined Apr 24, 2011
8,053
Generally for a given style of cap and a given case size you can make a rough correlation to the product of voltage and capacitance: You can get a lot of capacitance but at a low voltage, or you can get a high voltage rating of a small capacitance.

I've never tried to predict a cap size with this taking it more as a rule of thumb so I know when to stop looking for the cap I want in the case I think it will fit.
 

Adjuster

Joined Dec 26, 2010
2,148
For a given technology, maximum energy storage capacity may give a better fit with volume. This is given by C*V\(^{2}\)/2, where C is the capacitance and V is the maximum working voltage.

Note that the very compact capacitor technologies tend to pay a price in terms of higher esr (parasitic series resistance) and lower current capability. The big capacitors used with audio power amplifiers require low esr and high peak current capability. Miniature ultracapacitors are more suited to low current applications such as memory supply backup, where a relatively high esr and limited current capability is acceptable.
 
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