In the breakdown region, zener voltage remain constant even though the current through the diode changes. The output voltage is equal to the zener voltage of the diode and therefore it is fixed at one of the available voltage levels.The input voltage must be higher than the output voltage to allow regulation to take place. It should not be too high however, as this will result in more power being dissipated by the diode.
Zener diode is known as a voltage regulator because it converts an unregulated dc voltage into a constant dc voltage.
Here, the unregulated voltage is connected to the diode through a series resistance such that the diode is reverse biased. In case of high input voltage, the current through the resistance and diode increases. Thus, the voltage drop across the resistance increases without any change in the voltage drop across the diode. It is so because in the breakdown region, zener voltage remain constant even though the current through the diode changes.
Similarly,in the condition of low voltage, the current through resistance and diode decreases. The voltage drop across the resistance decreases without any change in the voltage across the zener diode.
Therefore any change in input voltage results the change in the voltage drop across the resistance without any change in voltage across the Zener diode.
Therefore Zener diode acts as a voltage regulator.
A voltage stabilizer is made up of transformer capacitors and transistors and its main function is to give a constant voltage supply.There are various types of stablizers and each of them has a unique way of operation.
A voltage stablizer is connected to a power source which in turn costs the capacitors. When a device is connected to this stablizer, it attracts its power from the capacitors and the battery not from the source. It is able to maintain a fixed current even when there are power drops or surges. Thus, a voltage stablizer maintains a constant current.
The objective of a step up transformer is to convert a low voltage at high current into a high voltage at low current.