Hi,

My real question is how do you know what size capacitor to use? I am developing an AVR project and the guys over at AVRfreaks recommended 100nf on the VCC/GND pins and 10uf where the power comes in.

I was testing out a 7805 voltage regulator tonight and I put a 200ma load on it and was watching the voltage on my scope. Adding a 10uf capacitor reduced the waveform height by about 2. Adding an additional 100nf capacitor reduced it significantly more. Why?

Wouldn't a 12uf be greater than a 10uf+100nf? I've seen multiple capacitor sizes in parallel before in circuits - how do they work?

Thanks,

Alan

 

Take a look at this section in the AAC ebook.

hgmjr

 

Hi,

I've seen that, but I still don't understand why a larger and then smaller cap filter so much better than just a single larger cap...

Thanks,

Alan

 

The short answer is that large value capacitors do not filter high frequency noise while small value capacitors do not filter low frequency noise that well. By combining a small valued capacitor with a large valued capacitor, the net result is that the overall noise filtering is improved.

hgmjr

 

Hi,

I was wondering that. Is there a specific frequency to capacitor or relationship?

Thanks,

Alan

 

Hi,

I was wondering that. Is there a specific frequency to capacitor or relationship?

Thanks,

Alan

Yes, However is varies with the material out of which the capacitor is constructed. The major manufacturers are very good about including this information in the datasheet for the component.

hgmjr

 

If a capacitor were a pure capacitor, the bigger the better, as far as bypassing is concerned. However, electrolytic capacitors, because of their physical construction, have a lot of parasitic inductance....they look like they have a big choke in series with them. This makes them almost worthless for higher frequencies, so you generally put a smaller, non-intductive capacitor in parallel with them.

Hope this helps.
eric