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Physical size of capacitors?

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Andy1845c

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Why is it that a capacitors physical size dosn't reflect its ratings? I have in front of me a 50v 10uF and a 50v 1000uF electrolytic cap, and the 10uF is almosts twice as big as the 1000uF. I had to fix a broken wire on a start cap for an AC motor on a tool today, and that thing was rated at 125v and 300 some odd MFD (microfarads?) and was 1/3 the size of a soup can. Is it a quality differance?
 
It is because not all capacitors are fabricated in the same way.
 
What is the differance though? I know there are many types of caps, ceramic, poly, tantalum, ect. But they all look completly different from electrolytic types. What makes a 10uF electrolytic so much bigger then a 1000uF? Are there data sheets for caps like there are for IC and semi conductors that would show how they are different inside?
 
Is the 1000µF capactitor also a motor cap ?
which i doubt.
Motor rated caps have different types of insulation materials between the cap plates, as these have to deal with starting spikes as well hence the size difference.

For example:
If both were standard electrolytic caps for circuit boards the 50 Volts 10µF is definitely a lot smaller than the 50 Volts 1000µF. The 1000µF requires a larger surface area to store the charge on, for the internal plates (foil), hence bigger size.

Same applies the other way too, a 10 µF 16 Volts cap is a lot smaller than a 10µF 400 Volts. In this case the insulating material between the plates (foil) is thicker to allowe for the higher working voltage level.
 
RODALCO- The 1000µF is not a motor cap, I can't remember what i took it out of, a tv mabey..

I understand what you are saying about surface area and insulating materials, but where I am confused is why my 10µF is bigger then my 1000µF, both rated at 50v.

I'll post a picture when I get home from work.
 
Yes, a picture will help. It seems you have a 10uF motor cap and 1000uF electrolytic cap. Motor caps are normally plastic film, and that construction type is far larger for equivalent values. It is required sometimes though, for one it is bi-polar/non-polarised unlike electrolytic. I believe they usually have lower ESR too and are also better for audio applications (irrelevant here though).
 
Generally older capacitors will be bigger and so will non-polarised versions.

Is your 10µF capacitor from a really old appliance on the 1000µF unit brand new?

Your motor run capacitor is probably rated to 125VAC, which is 175V DC, so it's probably rated to 200V DC to give a margin of safety. Here in the UK mains capacitors must be able to withstand 2kV for short periods so I'd assume 1kV in the US.
 
Here is a pic.
I would guess the age of the two smaller ones to be about the same. withen a few years anyway, mid 80's to early 90's. The motor cap could be a little older. The box for it looks kinda old.
 

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Given identical manufactoring methods the size of a cap will be related to it's voltage and farrad rateing. So the difference in size of your caps the manufactring methods are different. With Electrolytics you have the dielectrical material, the conductor and the electrolyte, all can be chosen for different reasons.
 
I too noticed the word "BP" on the cap and the missing of the familiar white stripe polarity marking. This showed that the capacitor is definitely a bipolar.

Other factors that could affect the capacitor size would be the max. operating temperature and ESR characteristic.
 
Hey! I never even noticed that, Cool, thanks for your help everyone!
 
Bipolar, thats the difference in size.

The caps look like similar age < 15 y old anyway
 
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