Typically, Zener diodes are intended to offer constant output voltage to a given load connected parallel to it irrespective of the variations in voltage supply or load current fluctuations. The diode regulates the voltage continuously for as long as the current lies below minimum value. The diode allows current to flow forward direction although when a Zener voltage is reached, the current is allowed to flow in the reverse direction.
Basically, a voltage regulator is tasked with maintenance of a constant voltage through a load irrespective of variations in applied input voltage as well as variations in load current. For a conventional Zener diode shunt regulator, the resistor is chosen in such manner that when input voltage is at minimum (VIN(min)) and load current is at maximum (IL(max)), then the current flowing through Zener diode is at minimum (Iz(min)).
Then for all other arrangements of input voltage as well as load current, the Zener diode conducts excess current therefore keeping a constant voltage via the load. The Zener transmits the minimum current when loading current is maximum and conducts maximum current when load current is the minimal.
This is a type of voltage regulator used for positive voltage which might be set using selected two resistors that are connected between output and ground. Its capability is often dependent on style made use of. It has many different casing ranging from SOIC-8 to TO-3. Rather than having an Input, an Output and a Ground, it has an Input, an Output and a Reference.
Alternatively, when the LM317 is meant to regulate output at 1.25 Volts above Reference pin, the value of the resistor sets current across the resistors. The current obtained by reference pin is meager and may be ignored for as long as resistor current lies between 1 mA and 10 mA.
Giving R1 a standard value, such as 120 or even 240 Ohms, the current is set through R2 also. All one has to do