Zener Diode Voltage Regulator: Principles and Operation
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Zener Diode as a Voltage Regulator
The function of a regulator is to provide a constant output voltage to a load connected in parallel with it, despite ripples in the supply voltage or variations in the load current. The Zener diode continues to regulate the voltage until the diode's current falls below the minimum IZ(min) value in the reverse breakdown region.
It permits current to flow in the forward direction as normal, but also allows it to flow in the reverse direction when the voltage exceeds the breakdown voltage, known as the Zener voltage. The Zener diode is specifically manufactured to have a reverse voltage breakdown at a precise level. In breakdown, the voltage across the Zener diode remains nearly constant over a wide range of currents, making it highly effective as a shunt voltage regulator.
Principles of Shunt Regulation
The purpose of a voltage regulator is to maintain a constant voltage across a load regardless of variations in the applied input voltage and load current. A typical Zener diode shunt regulator is shown in the figure below.
The series resistor is selected so that when the input voltage is at VIN(min) and the load current is at IL(max), the current through the Zener diode is at least IZ(min). For all other combinations of input voltage and load current, the Zener diode conducts the excess current, maintaining a constant voltage across the load. The Zener conducts the least current when the load current is highest and the most current when the load current is lowest.
Circuit Advantages and Operation
- No Load Conditions: Shunt regulators can be used to dissipate total power through the series resistance and the Zener diode.
- Fault Protection: Shunt regulators have an inherent current-limiting advantage under load fault conditions because the series resistor limits excess current.
A Zener diode with a breakdown voltage VZ is reverse-connected to an input voltage source Vi across a load resistance RL and a series resistor RS. The voltage across the Zener remains steady at its breakdown voltage VZ for all values of Zener current IZ, provided the current remains in the breakdown region. Consequently, a regulated DC output voltage V0 = VZ is obtained across RL, provided the input voltage remains within the specified minimum and maximum range.