Electronic – how to select capacitor values

capacitancecapacitordesignmath

I am trying to find information on formulas used to calculate capacitance required for a given circuit. Not a circuit in particular, just in general how to figure out what value should be used. I know about selecting voltage ratings at least 20% higher than the max possible voltage, or even more cautious selecting at least double the expected voltage. But have not found much on finding the value in capacitance that will be needed.

It is beginning to look as though there are many different formula depending the the specific capacitors application in a circuit. For example I found to calculate a filter capacitor in a power supply, C = I / (Vripple x 100).

So as I said, I am just trying to find a good resource (or a few) about calculating values of capacitors for various applications. Thanks for checking out my question, I am excited to get reading and figure this out.

Best Answer

In order to how much cap to use where, you need to know a fair bit about capacitors in general:

  • The different types (electrolytic, film, ceramic, tantalum, OS-CON, metalized film, etc.)
  • Their characteristics (impedance, ESR, ESL, polarity, temperature rise, dielectric, etc.)
  • Their failure modes (aging, over voltage, reverse voltage, thermal runaway, etc.)

You also really need to know why a cap is being used for a particular purpose, such as:

  • Power supply output caps: less to do with holding the voltage up, more to do with ensuring the ripple voltage isn't excessive
  • Power supply input bulk capacitors: mainly to do with hold-up time (keeping the output up for a short time after the input is removed)
  • EMI filter: providing specific frequency attenuation, in coordination with other filter components
  • Decoupling: providing instantaneous current to a circuit; a function of the circuit and the physical layout

Good luck with your knowledge quest. It will take some time but as you gain experience, things will get clearer.

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