# how to select capacitor values

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.

• So if I knew what effect I would like the capacitor to have, there are not ways to calculate values? Similar to the mentioned calculation for a power supply filter. – rob j loranger May 2 '13 at 0:26
• I understand that, which is why I am asking to be directed to a resource about selecting capacitors in circuit design. I'm open to book suggestions as well, I like books. – rob j loranger May 2 '13 at 0:30
• Any textbook on circuit design should tell you how to select the capacitor values for any kind of circuit they cover where choosing the capacitor value is important to the operation of the circuit. – The Photon May 2 '13 at 0:32
• I think you answered your own question with "It is beginning to look as though there are many different formula depending [...]". There is no one size fits all rule for caps. I think it would be better if you understood more of what a capacitor was and how it worked so you knew how to choose the values necessary. – Kurt E. Clothier May 2 '13 at 0:40
• A capacitor is used to present a frequency-variable impedance to the flow of AC, to bring about filtering or a phase shift, or to block DC while coupling through AC. The formula for capacitive reactance is $X_c = \frac{1}{2\pi fC}$. For example, a 0.1uF capacitor presents an impedance to 1000Hz AC of: $1/(2\pi 1000 * 0.0000001) = 1592\Omega$. Once we know this, other frequencies are easy: $10Khz \rightarrow 159.2, 100Khz \rightarrow 15.9; 1Hz \rightarrow 1.59 M\Omega$. Other capacitances are similarly easy. Twice the C, half the Z. But you can't treat these Ohms like resistance. – Kaz May 2 '13 at 2:01

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.

• Thank you :) Still searching for a good learning resource or book, but this gives me a few more topics to include in my research. – rob j loranger May 2 '13 at 4:00

Look for Application Report #SLTA055, "Input and Output Capacitor Selection" from TI that gives a pretty good explanation about capacitors.

• For referencing application notes, at least give the number and name for future reference. (this one is Application Report #SLTA055, entitled "Input and Output Capacitor Selection"). – markrages Jul 1 '13 at 3:14