capacitor/voltage spike question?

just wondering if it matters what sort of cap is used to smooth out voltage spikes, couldnt i use a cap of a higher value than recommended?

a caps discharge is done by the load, so if i dont mind my application running a little longer after the power is turned off, what does it matter what cap i use, as long as it is higher than it recommended value shouldn't it work just as good?

The most important is capacitors ESR (Equivalent Series Resistance). It should be as small as possible. It is better to use few paralleled capacitors than only one for the same capacitance. Low ESR capacitors are ceramic capacitors and tantalum electrolytics. Aluminium electrolytic capacitors should be special low ESR types.

bloki, hi
why its better to use few parallel capacitors ?

ahmedragia21 said:

bloki, hi
why its better to use few parallel capacitors ?

Click to expand...

A real capacitor is like a small parasitic ideal resistor, small parasitic ideal inductor, and ideal capacitor that is in series with each other.

If you put this RCL series in parallel, the R and L decrease (since resistance and inductance decrease when paralleled) and the C increases making it closer to an ideal capacitor. THat is why (I assume you know that the resistance limits current while the inductance limits rate of current change both of which are do not help in smoothing out the voltage spike).

THis is also the reason why you have to use small capacitors to pass or filter higher frequencies in real life- because larger capacitors have larger parasitic inductance, and at high frequencies this inductance blocks the signal from ever getting to the capacitance in the first place making the capacitor useless at that frequency regardless of how big it is. An ideal capacitor would always pass higher frequencies better than lower frequencies, but this is not the case in real life.

So the trick is to know the main frequencies of the spike you are trying to filter and pick a capacitor that is as large as possible, while balancing out the parasitic inductance and resistance (after all, a capacitor with very little parasitic inductance and resistance that is has too small a capacitance for the job won't do too much good either)

Take a look at some impedance graphs for real capacitors. The parasitic inductance and capacitance create a frequnecy point where the impedance is lowest, and the impedance increases on either side of this frequency/

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So the effect is that a a larger capacitor will have lower impedance at low frequencies than a smaller capacitor but the smaller capacitor will actually have a point at a higher frequency where it's impedance dips below the large capacitor (and in many cases, this is the frequency you are trying to filter out, rather than all the lower frequencies that the large capacitor is better at).

really nice explanation dknguyen