Ripple in full wave rectifier
A rectifier is non-linear, which means that a Thevenin model must be used very carefully.
Ripple in rectifiers is generally carried out as follows:

1) Assume that, at peak input voltage, the filter capacitor is charged to the peak value. This is a good first-order assumption, since the dynamic resistance of diodes is small for large difference voltages.

2) Assume that, once the peak has passed, the rectifier will not conduct until the next rectified peak. In the case of a full-wave bridge and 50 Hz, this is 10 msec.

3) Between 1 and 2 there is no current into the filter cap, and it provides all of the current to the load. Using the load characteristics, you can calculate the droop on the filter cap over the designated time period. This is the ripple voltage.