Zener Diode Voltage Regulator
A device which maintains the output voltage of an ordinary power supply constant irrespective of load variations or changes in input AC voltage is known as a voltage regulator. There are basic two types of voltage regulators:
2. Shunt voltage regulator.
The series regulator is placed in series with the load. On the other hand, the shunt regulator is placed in parallel with the load. Each type of Voltage regulator provides an output voltage that remains constant even if the input voltage varies or the load current changes.
Zener Diode voltage Regulator
As the zener diode is operated in the breakdown or zener region, the voltage across it is substantially constant for a large change of current through it. This characteristic permits it to be used as a voltage regulator. As long as input voltage Vln is greater than zener voltage V2, the zener operates in the breakdown region and maintains constant voltage across the load. The series limiting resistance Rs limits the input current. The zener will maintain constant voltage across the load in spite of changes in the load current or input voltage. As the load current increases, the zener current decreases so that current through resistance Rs is constant. As output voltage = Vln - IRS, and I is constant, therefore, output voltage remains unchanged.
As the input voltage Vin increases, more current will flow through the zener, the voltage drop across Rs will increase but load voltage would remain constant. The reverse would be true that is if the input voltage Vin decreases the load voltage remains constant.
Limitations of Zener Diode Voltage Regulator
A zener diode regulator has the following limitations. Zener Diode Voltage Regulator has low efficiency for heavy load currents. It is because if the load current is large there will be considerable power loss in the series limiting resistance. The output voltage slightly changes due to zener impedance as Vout = V2 + lz Zz. Changes in load current produce changes in zener current. Consequently the output voltage also changes. Therefore the use of this circuit is limited to only such applications where variations in load current and input voltage are small.
Another form of Zener diode circuit is an overvoltage protection circuit. While power supplies are normally reliable, the effects of the series pass transistor or FET can be catastrophic if it fails by forming a short circuit. In this case the full unregulated voltage would be placed onto the circuits using the regulated power. This could destroy all the chips being powered.
One solution is to use a crowbar circuit. When this form of circuit detects an overvoltage situation it fires an SCR. This quickly holds down the output voltage and in the instance shown, it blows a fuse that disconnects the input source power.