Ron H said:When you make a simple shunt regulator with a zener, you generally have a series resistor from a higher voltage to supply current to the zener and to the load ( the circuit that uses the stabilized voltage). A zener does not have zero resistance. What this means is that a change in current through the zener will still cause a small change in voltage. This means that if there is ripple on the unregulated supply, some of that ripple will still show up on the zener.
Think of a real zener as an ideal zener in series witha low-value resistor. The value of this resistor depends on the type of zener and the current through it, but it will generally be in the range of a few ohms to a few tens of ohms.
The bottom line is, you still have a voltage divider between the current-limiting resistor and the zener resistance. For example, if you have a 1k resistor for current limiting, and the zener's resistance is 10 ohms, about 1% of the ripple on the unregulated supply will show up on the zener.