Capacitor Calculations

[skmetal7]

hey guys, i have a question about a cap. calc. lets say i have a small dc generator, that outputs about 4-5 vdc to charge a capacitor, and i have a LED that i want to run off the capacitor with a know voltage and current. what equation would i use to determine what farad capacitor i should use to have the light run for a given amout of minutes?

and if i then find that capacitance, what equation would give me the time to charge the cap.?

[Al_The_Man]

The time for charge/discharge is not linear, rather exponential, the Time Constant or usefull time to indicate a for a capacitor to discharge is calculated for 63% of total charge/discharge time.
where T sec = Rmeg x Cµfd.
I am assuming you want the LED to be kept lit after the generator stops?
Al.

[niclatrique]

Tha capacitor voltage at a known amount of time is determined by the following equation:

Vc = Vcc * 1 - e^(-t/RC)
where:
Vc = instantanneous capacitor voltage
Vcc = Supply voltage or souce voltage
e^ = constant e = 2.718281828 at the power of...
t = time
R = total resistance in the capacitor network
C = Capacitor value

The discharge equation is almost the same

Vc = 1 - e^(-t/RC)

As you know the voltage drop across your led and the resistor you can find the moment where the voltageat the capactor won't be high enough to polarize the LED.