Discussion in 'General Electronics Chat' started by danielb33, Sep 3, 2012.

1. ### danielb33 Thread Starter Member

Aug 20, 2012
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0
I am studying cap's right now. I understand the premise of blocking most of DC (minus very small leakage) and AC passing through caps, and passing more as frequency increases. My confusion is why not always choose a large cap? If a high frequency always passes capacitors better, whether the cap be big or small (micro f to milli f), why not always choose a large cap, even for filtering problems?

Because as frequency lowers, reactance increases, wouldn't a large cap be the ideal fit always in order to get rid of AC? A large cap would get rid of really high frequencies and the lower AC signals, correct?

I was told by someone today that the reactance equation is not really true, that reactance actually reflects a bell curve when dealing with very wide ranges of frequencies and people have to find the right fit for the frequencies they need to reduce, is this true?

2. ### #12 Expert

Nov 30, 2010
16,247
6,739
Size and price. A \$500 capacitor the size of a football wouldn't fit well in an Ipod.

and yes, capacitors with large capacitance also have associated resistance and inductance so they don't work well in the megahertz range. In practical circuits you often end up putting a small ceramic capacitor in parallel with an aluminum electrolytic capacitor.

3. ### danielb33 Thread Starter Member

Aug 20, 2012
105
0
Really??? I was getting that impression from equations, but it seemed hard to believe with so many very specific caps out there. Looking on digi-key, datasheets show so many specs that details such as size must be the least of concerns right?

Thanks for the response.

4. ### #12 Expert

Nov 30, 2010
16,247
6,739
As far as size, the manufacturers are doing amazing things. 30 years ago, a half a farad capacitor was the size of a small car. Now, you can fit one in your hand.

But look again at the idea that there are "so many very specific caps". Doesn't that tell you that none of them are "universal"? Nobody has created the capacitor technology that doesn't have any weaknesses.

5. ### MrChips Moderator

Oct 2, 2009
12,414
3,353
Why not drive the biggest car available? Or live alone in a 24-room house?
Or use the biggest hammer on every size nail?

Bigger is not always better.

There are always compromising factors to consider. Choose the right tool or component for the job.

absf likes this.