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The very simplest of voltage regulator circuits can be created using a simple series resistor and a zener diode. This is a shunt regulator circuit, and it consists of a series resistor with a zener diode connected down to ground, across the load.
Zener voltage regulator circuits are normally used for low power applications where only a reasonable level of stabilisation is required. A zener diode circuit will be able to provide a reasonable level of stabilisation, but for more exacting requirements a more sophisticated circuit is required.
The zener diode operates under reverse bias. As the voltage increases across the diode, it does not conduct at first. However when the voltage reaches a certain level, the diode starts to conduct, and it will try to maintain the same voltage across the diode for almost all levels of current. In this way, if a load is placed across the zener diode, in the simple regulator circuit the voltage will be maintained despite and variations in the current requirements for the load. The zener diode will take up the variations in current required to keep the voltage across the diode the same.
Obviously the zener diode cannot accommodate an infinite variation of current. There is a minimum level of current it must carry to maintain the voltage, and if too much current is taken, then it will overheat, and could be destroyed. When designing a simple regulator circuit, it is necessary to design the circuit to accommodate the likely variations.