Hello,

I just had a quick question. I've seen several different places people put two caps of different sizes in parallel to ground. like a 0.1uF and a 10uF. why is that?? why not just put one? is it to block a certain range of frequencies?

here's an example:
http://electroschematics.com/wp-content/uploads/2009/01/cell-phone-jammer-schematic.gif

in that circuit there are several places where they have 2 caps in parallel to ground.

thanks

 

You are close, different caps have different frequency responses. A small ceramic cap generally is viable much higher in the spectrum than an electolytic, while the much larger electrolytic can provide a stable voltage for a longer duration.

Bypass caps don't block frequencies, they shunt them to ground. A wall blocks, a pipe redirects (shunts).

 

You'll often see pairs like this, like on the output side of a voltage regulator. My ASCII art skills are atrocious, but a schematic involving the ubiquitous 7805 should help

 

Sometimes specific values are needed.

Capacitors in parallel: Ct = C1 + C2 + C3 + C4 etc...
Three or more Capacitors in series: Ct = 1/(1/C1)+(1/C2)+(1/C3)
Two capacitors in series: Ct = C1 x C2 / C1 + C2

 

great. thanks for all the input.

 

Hello,

For some applications the use of capacitors of different values are used to have a better decoupling over a larger frequency range.
See this PDF for more info:
http://www.engr.uky.edu/~melham01/ee499/appnotes/decouple.pdf

Bertus

 

Gotta love this forum. everyone's knowledgeable and respond quickly to questions.

thanks again for all the inputs.