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[SOLVED] Big capacitor accompanied by a small capacitor?

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bobsun

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Hello,

I would like to ask a question on capacitor.

capacitor_small_parallel.png

I have found that frequently when a “big” capacitor is connected to the ground, it is often accompanied by a “small” capacitor in parallel with it. For example, in the attached image, C4 is 100µF and C5, which is connected in parallel, is only 0.1 µF.

In terms of charge storage, there is only 1/1000 of C4 that C% could store at the same voltage level; then why is it put here? I have seen such arrangement quite common: in motor design, boost circuit and elsewhere. What is the principle here?


Bob
 

alexxx

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bobsun said:
In terms of charge storage, there is only 1/1000 of C4 that C% could store at the same voltage level; then why is it put here?
Don't look at it as total capacitance. Big capacitors are used to store energy and give it as current when needed (like when supply voltage is droped for example), and small capacitors are used as decoupling capacitors (provide bigger noise immunity).

So if you see a big and a small capacitor together, each one of them is there for a different reason.

Take a look at the "applications" section of the wikipedia's article:
Capacitor - Wikipedia, the free encyclopedia

And a special article on decoupling capacitors:
Decoupling capacitor - Wikipedia, the free encyclopedia
 
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bobsun

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Alex,

I will study the links.

Bob
 

betwixt

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@bobsun, larger capacitors hold larger charge but because of the additional inductance and resistance in their construction, it is more difficult for them to release it very rapidly. Smaller capacitors suffer less from these problems and can actually work far more efficiently in the presence of high frequencies. By connecting them in parallel you get the best of both characteristics.

Brian.
 
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bobsun

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betwixt,

This answered all my questions, thanks very much.

Bob
 

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