# Capicitance calculation

1. Dec 14, 2009

### Neo11

I want to build a small circuit which will power a pulsating load. I got a power supply which can provide 3.3V and 2 mA. Now my load requirement is 100 micro amp average and 100 mA for 10 mSec(all at 3.3V). I want to calculate

1. Value of capacitor which will provide 100mA for 10 mS
2. Time duration to sufficiently charge the capacitor(to determine after how much time load current surge can be adjusted)

Can anybody help me to solve this problem?

2. Dec 14, 2009

### Staff: Mentor

The equation you want to use is:

$$i(t) = C \frac{dv(t)}{dt}$$

Or in simpler form (if you haven't studied calculus):

$$i(t) = C \frac{\Delta v(t)}{\Delta t}$$

or,

$$\frac{i(t)}{C} = \frac{\Delta v(t)}{\Delta t}$$

This means that the droop in voltage with respect to time is equal to the current out of the cap, divided by the size of the cap.