Re: Some capacitor questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dragan Stancevic by way of Terry Fritz <twftesla-at-qwest-dot-net>" <visitor-at-xalien-dot-org>

On Fri, Oct 19, 2001 at 07:07:05AM -0600, Tesla list wrote:
> Original poster: "bdsabds sadgsdgsda by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <fdgahbdfhbfdb-at-juno-dot-com>
> 
> What is the difference between pF uF, and the others? If there was just a
> site that tells conversions, that is all I would need, since I know what
> pico, micro and the lot mean. 

pF is pico farad
uF is micro farad

> I see how capacitors do their job, but I don't understand how they are
> charged by weaker power sources and how they get discharged in a circuit.

Capacitors will only get charged to the charging source voltage, if the
voltage that is charging a capacitor is lover than capacitor voltage rating
that is OK the capacitor will only get charged to that voltage. The way the
get discharged into a curcuit can be achived in different ways, one of them
is for e.g. if the voltage in a curcuit drops bellow the voltage in the
capacitor than the cap will release some of it's charge to end up at the
curcuit voltage.

> Also, what makes some capacitors discharge all their energy at once, or
> slowly over time?

It's resistance, if you place a resistor in serias with the capacitor it
will take longer to discharge. Capacitive time constant is t = R x C with
this formula you will get the time required for the cap to loose 63.2% of
it's fully charge voltage. There are five time constants between a fully
charged and fully discharged capacitor. eg.

Capacitor and resistor rating
E = 10V (Cap fully charged voltage)
C = 2 Farad (capacitance, purly examplory)
R = 2ohm (resistor resitance)
-----------------------------------

t = C x R = 2 x 2 = 4 seconds
T1-5 = t x 5 = 20 seconds

This capacitor would take 20 seconds from it's fully charged state to
discharge, if we change resistance to 1 ohm:

E = 10V (Cap fully charged voltage)
C = 2 Farad (capacitance, purly examplory)
R = 1ohm (resistor resitance)
-----------------------------------

t = C x R = 2 x 1 = 2 seconds
T1-5 = t x 5 = 10 seconds

You see this capacitor will loose 63.2% of it's fully charged voltage in 2
seconds as oposed the previous one in 4, so 5th time constant is 10 seconds
which is half of the previous ones time. So time has a direct relation to
Capacitance of the capacitor and resistance of the discharge path. You have
to note though that voltage has no influence on the time of discharge but it
can be calculated nicely, eg voltage in the first cap after t1 is 3.68V.

-- 
Peace can only come as a natural consequence
of universal enlightenment. -Dr. Nikola Tesla