**Table of Contents**

In the picture above you see that the discharge of buffer capacitor
C1 starts at t=T1. Now assume that T1=0. The rectified sine wave will now
be a rectified cosine. At t=T2
V_{C1}=-V_{top}cos(2∙π∙f∙T2).
(Note the minus sign. At t=T2, the original transformer output is
negative.)

So
cos(2∙π∙f∙T2)=-V_{C1}/V_{top}.
This means

When you discharge a capacitor with current I,
V_{C}(t) = V_{C}(0)-I∙t/C. In our
case:

V_{C1}(t) =
V_{C1}(0)-I∙t/C1, so at t=T2:
V_{C1} = V_{top}-I∙T2/C1. Since
V_{C1}=V_{top}-V_{r},
we can also say V_{top}-V_{r} =
V_{top}-I∙T2/C1. So V_{r} =
I∙T2/C1. This means

If f=50Hz and V_{top}=20V and we want a 2V
ripple voltage, we need a 4.3mF capacitor per ampere load current.

**Note: Make sure your calculator uses
radians!**