Analog Electronics
A normal diode blocks in reverse; a zener is a dam deliberately built "weak": once the reverse voltage reaches Vz, it overflows in a controlled way and holds the voltage across itself near Vz no matter how hard you push. That makes it a cheap voltage reference.
A series resistor must always sit in front of the zener: it absorbs the excess voltage between the input and Vz and limits the current. The resistor is like a tap — open enough to feed both the zener and the load, throttled enough not to burn the zener: R = (Vin−Vz)/(Iz+Iload).
Careful: if the load draws too much current, none is left for the zener and the dam "runs dry" — regulation collapses. That is why the zener solution is for small currents; for serious loads you move to integrated regulators like the 78xx family or to switching converters.
⚡ Open this lesson in the simulator — free