Ken said:
Hi,
This is a newbi questions I suppose.
If I take a voltage source say a battery, I put a capacitor with rating of
300volts to it, the charge went up to the maximum charge of the battery
(12.4volts). I thought the charge would accumulate up to 300 volts. Why does
it go to 300 volt ?
thank you
Ken
The voltage rating on a capacitor tells you the maximum voltage you
can safely apply to it and still have it function as a capacitor.
The relation between current and voltage for a capacitor is:
I=C*(dv/dt), where I is in amperes, C in farads, and dv/dt is the rate
of change of the voltage across the capacitor in volts per second.
When you first connect the capacitor across a voltage source, like a
battery, there is a large pulse of current, limited only by the
internal series resistance of the source and inside the capacitor.
But this large current pulse results in a high rate of change of
voltage and the capacitor quickly charges up till its voltage matches
that of the source. At that point, you have two equal voltages
bucking each other, and the current heads toward zero.
This is something like attaching a small air storage tank to a much
larger one. There is a brief blast of air through the connecting
hose, and then, as the small tank pressure approaches that of the
larger tank, the flow through the hose falls toward zero. You can not
get the small tank to reach a pressure higher than that inside the
larger tank that is filling it.