If I want to run a device with that capacitor... How can I calculate the time?
(Device: 250 Watt)
I know the time is in MiliSecs, but I don't know how to calculate it.
Second question:
if I have 3 capacitors like the first one (connected in serie)
A) What is tht total voltage?
B) Total uF?
The time constant (tau) capacitor circuit is resistance * capacitance. This is the amount of time it takes the voltage to fall to 1/e (37%) of the original value.
in your case ..
P = V^2 / R
R = 400^2 / 250 = 640 Ohms
tau = 640 X 14 e-6 = 9 ms = .009 seconds
so after 9 miliseconds the voltage will be 400/e = 147 V
after 18 miliseconds .. 147/ e = 54 ms
Question 2 :
The total voltage will still be whatever voltage you apply to the circuit. The voltage across each capacitor will be 1/3 of this.
The total capacitance will be1/( 1/C1 + 1/C2 + 1/C3)