How to actually specify the capacitor needed?

[Migs]

Friends:

This may sound like a stupid question, but how does one actually specify the capacitor needed for an application? I can see where it's voltage will be the voltage of the circuit, but what about its Capacitance?
Let me guess: The energy stored will be converted to power at the needed voltage. So if I need so many seconds of amps at that voltage, then my capacitance must be C. Does it go something like that?
Are there any rules for this selection?

Migs

 

[Nigel Goodwin]

You don't give any clue as to the application, try telling us what you're wanting to do.

 

[Migs]

Well that's exactly the gist of this post. I don't really have an intended use. What got me thinking is that I was told to put a Capacitor in a micro controller circuit, but the value specified was not exact. It was like this: "...use at least a .1microF cap..." Which lead me to thinking: Why wasn't the value as determined as say resistor? Lets say I'm making a simple power supply for 5vdc at 0.5 amps. What caps should go across the outputs? Why that value?
Migs

 

[Migs]

I read that:
# The formula to calculate the amount of energy is: W = V2 * C / 2 where:
# W = Energy in Joules
# V = Voltage in Volts
# C = Capacitance in Farads
Maybe this is related to the selection of the Cap...

 

[audioguru]

The datasheet for a uA7805 voltage regulator shows a 0.1uF capacitor at its output to ground to stop it from oscillating and to improve its transient response.

Logic ICs draw a high current spike when they switch from high to low or from low to high. A 0.1uF capacitor at the supply pins of the IC stops the supply voltage from sagging during these very short duration spikes.

Ceramic disc capacitors with short leads are usually used because they are excellent at high frequencies.

 

[Migs]

Hi Audioguru

Good to know, is this similar in concept to "debouncing"? The cap goes between the voltage input to the IC and ground, right? -Migs

 

[audioguru]

Hi Audioguru

Good to know, is this similar in concept to "debouncing"? The cap goes between the voltage input to the IC and ground, right? -Migs

No.
This is connecting capacitors to filter the power supply voltage.
Debouncing is filtering the many bounces made by a pushbutton so that the signal to the input of a logic IC is only a single pulse instead of many pulses caused by the contact bouncing.

 

[Migs]

Oh... Thanks Audioguru. -Migs

 

[stevez]

In my amateur radio handbook are several RF circuits with instructions on how to adjust the circuits over a faily wide range of frequencies.

For many of the capacitor values the reactance presented by the capacitor would determine the intended current or voltage. The author would indicate the value of reactance - all the user would need to do is select the capacitor that offered the desired reactance at the particular frequency.

For some other parts of the circuit the capacitor would be combined with an inductor with an intended resonant frequency in mind. An LC oscillator is an example - you' establish the L then back your way into the C required for the pair to be resonant.

I hope this helps.