# Capacitor current

Discussion in 'General Electronics' started by Patrick Cobb, Jul 3, 2003.

1. ### Patrick CobbGuest

I need to know how to find the number of amperes a .3 farad capacitor can
put out if it is charged to 7 volts.

2. ### Robert BaerGuest

Use I = E / R for initial current, if the resistance is large compared
to the ESR of the capacitor.
Remember that the discharge current drops exponentially.

3. ### Robert BaerGuest

Sorry, that is incorrect.
Connect a resistor of 7 meghoms across the capacitor, and read an
initial current of 1 microampere.
Use a 7K resistor, same initial charge, and read 1 milliampere.
You have something missing.

4. ### Animesh MauryaGuest

Yes you are correct, but I think time factor for both the resistances
will remain the same so same amount of current will flow. What do you

5. ### Animesh MauryaGuest

Yes you are totally correct I was somehow misleaded. I simulated the
circuit and verified that you are right also my above discussion was
wrong as time of discharge (exponential current decay) is directly
proportional to the value of the resistance.
Although this is a discussion one can not always be right, but I
apologize for my wrong discussion.

Animesh Maurya

6. ### Robert BaerGuest

No; 1uA is *not* 1mA.
However, the total *charge* is the same (Q=c*v) but after that
everything else changes.
The 7Meg load will take 1000 times longer to reduce the voltage to 37%
(one time constant, T=R*C) of initial as compared to using a 7K load.

7. ### unitronGuest

Since the formula for a capacitor's time constant is T=RC any change
in resistance (R) will change the the time constant. As with any
other circuit, the more resistance the smaller the current. The
smaller the current the longer it can flow at that smaller rate before
the capacitor is discharged. Decreasing the resistance will allow
more current to flow, but, since the capacitor holds a finite amount
of charge, the time over which that greater current will flow before
the capacitor is discharged will be shorter.

8. ### Robert BaerGuest

Aie-Yup!  