Electronics capacitors voltage question

i know capacitors are measures en Farads (more often microfarads, mF).. but there is another thing you can see in capactiros, and thats the voltage it can handle.. ie. 220mF 25V ........

my question is, is the voltage thing really important? since the 25v is the max. voltage it can handle, if i am using a 3v power supply, it is the same if i use a 10,16,25,50,63,100v capacitor, right? am i right?

or is the voltage thing something different than the max. voltage it can handle?

 

The voltage indication on a cap is its max voltage. IF you charge a 100v cap with 3v, it won't go past that 3v mark. As long as your cap voltage is a bit higher than 12v, say 16v, you'll be fine. The only real difference it makes to us is the size of the darn thing

 

and the farads are just a measure of how long it takes the capacitor to discharge, right?

 

Farads actually measure the amount of charge in the capacitor. I think 1 farad has 1 coulomb of electrons which is enough for 1 amp for 1 second.

But charged up to the same voltage, and discharging into the same load, yes a larger cap (µF wise) will take longer to discharge.

 

All previous posts are right, but I would like to add a few bits...

In theory, a 1 uF cap rated for 25 V will take the same time to discharge through a given resistor as a 1 uF cap rated for 250 V. So, in general it is recommended to use a capacitor rated for at least 1.5 times the highest voltage it will actually see (for example, a capacitor working at 12 Vdc will have to have at least a 16 Vdc rating, otherwise it will blow up)

But, higher voltage rated capacitors also have several disatvantages:

- the higher the voltage rating, the more expensive the cap
- the higher the voltage rating, the larger the cap (physical size)
- higher voltage caps have a higher ESR (equivalent series resistance), and this is a bad thing... An ideal a cap will have 0 ESR.

Of course, this all depends on the application, so if I were doing an RC oscillator powered at 12 Vdc I would use a 16 Vdc rating cap, because if the voltage rating is too high, the cap's ESR will influence the oscillating frequency of the circuit.

 

I made a pulsing LEd circuit a while back, I used 330uF caps for that. But they were rated at 450v so the thing was HUGE It was pretty nice eyecandy though it faded two leds from one to another very smoothly. The uber-huge caps gave it a nice 'dirty' look.