Re: Capacitors 101.
- From: Ryan <quakeserver149@xxxxxxxxx>
- Date: Sun, 30 Oct 2005 02:49:06 GMT
> What do the Farad ratings mean?
>
> What do the Voltage ratings mean?
This may be a terrible analogy, but it is the way I understand it.
I think of a capacitor as analagous to a water pressure tank in a well
water system. If you live in the country and use a well, then you
realize that the pump in the ground supplies water pressure, but only
when the pump is running. When we open a faucet, we don't want to have
to wait on the water pump to get started and develop pressure. To solve
this problem, we insert into the line a holding tank. This tank acts as
a buffer to smooth out the transitions when our source stops and starts
again. This way the output is held in a slower-to-change state rather
than quickly going off and then back on again.
The size of this tank (it's capacitance) can be varied from large to
small. If we make is smaller then it charges more quickly but
discharges more quickly too.
This container is rated for a certain amount of pressure. Too much
pressure and a wall will burst and it will not hold pressure. In a
capacitor, too much voltage will pierce through the insulation between
the many layers inside and it will not hold a charge. My example stops
here as a capacitor is not a big empty tank, it is a bunch of
alternating layers of insulation and electrolytic.
I would say farads and voltage aren't really related, but the two have
an influence on the size of a capacitor. This electrical insulation
takes space. It take mores insulation to prevent an arc when a higher
voltage is used, so this means that a lot of the volume of a capacitor
is occupied by insulation. Conversely, you could have the same volume
of capacitor which has a significantly larger farad rating, but can only
be used at just a few volts, or else it becomes damaged. All else
equal, a low capacitance, low voltage unit will be smaller.
> on the tracks. They suggested a 25v @ 400uF capacitor assuming the maximum
> input voltage to the circuit would be 16v.
As I understand it, you could use a 25v, a 200v, or a 2000v capacitor
and as long as it was 400uF, you should expect the same result in a
circuit. (unless a 2000v cap introduces some non-conforming properties)
Whoever helped you probably calculated the capacitor based on knowing
the voltage which you provided, and knowing how much current an LED will
use (and then using a resistor to limit the current), and then assuming
what duration of time this LED will need to be powered by the capacitor
(thereby deciding how many farads based on the current draw while also
remembering not to let the voltage drop so low that the LED goes out).
.
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