The Below is a H-Bridge Schematic from this site: http://www.societyofrobots.com/schematics_h-bridgedes.shtml. Wouldn't the 10-100nF cap be rendered useless by the 10000 uf cap? Wouldn't the caps just have a equivalent capacitance of 10000 uf + 10-100nf

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    \$\begingroup\$ electronics.stackexchange.com/questions/3879/… \$\endgroup\$ – endolith Dec 17 '10 at 15:37
  • \$\begingroup\$ @endolith, If you think it is a duplicate, vote to close. In this case I think no-one else would vote and it would go away without an issue, if other people thought so, they would vote also. You will not hurt anything by voting to close, if it gets closed, but everyone disagrees there can be a vote to reopen. No harm done. \$\endgroup\$ – Kortuk Dec 17 '10 at 16:04
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    \$\begingroup\$ I don't. Just linking to related topic \$\endgroup\$ – endolith Dec 17 '10 at 16:11

Large capacity electrolytic capacitors are very bad at high frequencies, where their inductive behavior becomes stronger than their capacitance. This means that due to the capacitance the impedance decreases to a certain frequency, from where it starts to rise again. For those higher frequencies you need a smaller capacitor, which doesn't show this inductive behavior.
The impedance curve below is for a typical 1000uF electrolytic capacitor.

alt text

  • \$\begingroup\$ didn't mean to steal your answer. I didn't see it until just now. \$\endgroup\$ – Kellenjb Dec 17 '10 at 16:01
  • \$\begingroup\$ @Kellenjb: No problem. I think you were writing it the moment I posted it. \$\endgroup\$ – stevenvh Dec 17 '10 at 18:58
  • \$\begingroup\$ I was asked this in an interview today, and I was able to draw this plot! :D Anyone know what the inflection point in the curve is called? \$\endgroup\$ – user7994 Mar 16 '17 at 23:44
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    \$\begingroup\$ It's called the resonant frequency of the capacitor and is due to the esr and esl of the inductor, more info and detailed explanation at : youtu.be/BcJ6UdDx1vg \$\endgroup\$ – user7994 Apr 10 '17 at 23:33

The smaller capacitor is able to filter out higher frequency noise that the large capacitor is unable to.

This has to do with the non-ideal aspects of capacitor, mostly their internal inductance and resistance.

  • \$\begingroup\$ I was using an ideal model. This makes sense. \$\endgroup\$ – Richard Dec 17 '10 at 13:18

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