Simple Zener Regulator
The image shows a simple Zener voltage regulator. It is a shunt regulator and operates by way of the Zener diode's action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the Zener breakdown region. The resistor R1 supplies the Zener current IZ as well as the load current IR2 (R2 is the load). R1 can be calculated as -
where, VZ is the Zener voltage, and IR2 is the required load current.
This regulator is used for very simple low power applications where the currents involved are very small and the load is permanently connected across the Zener diode (such as voltage reference or voltage source circuits). Once R1 has been calculated, removing R2 will cause the full load current (plus the Zener current) to flow through the diode and may exceed the diode's maximum current rating thereby damaging it. The regulation of this circuit is also not very good because the Zener current (and hence the Zener voltage) will vary depending on VS and inversely depending on the load current.
Read more about this topic: Linear Regulator
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