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Thursday, 12 July 2012

Zener Diode As Shunt Regulator

Voltage regulator provides regulated output whether load current or input voltage is changed or not . Zener Diode completes this task very well. As these diodes work in breakdown region that is why they are used as voltage regulators. A simple Zener Voltage regulators is shown in the diagram given below.

Here ‘Vin’ in input DC voltage in which the occurred changes are to regulate. Zener diode is installed across ‘Vin’ in reverse direction. And load resistance (RL) is applied across Zener diode . Constant Voltage are required across ( RL) when the value of supply voltage ‘Vin’ increases from zener voltages (Vz) then the diode conducts and a large Current flows through series resistance ( Rs) , that is equal to the sum of diode current ( Iz) and load current ( IL) .
I = IL + Iz
“At Every State “     Vout = Vz
And                                       Vin = IRs + Vout
Putting the value of V out
                                            Vin = IRs + Vz
1) 1st Condition:
      Suppose that (RL) is constant and supply voltage (Vin) is increased. is this condition the reverse current (Iz) flow in very low value until the breakdown voltage. But when the (Vin) increases then breakdown voltage then Internal resistance of zener diode decreases and (Iz) increases rapidly due to this , the total current I is also increases thus voltage drop across series resistance Rs also increase
Hence output voltage remains constant , because
V out =  V in – Irs
Opposite to this, if the value of (V in) is decreased then a low current flows through diode. In this way, the total current ‘I’ also decreases and as a result low voltages are dropped across ‘Rs’ so output voltage still remains constant .
2nd Condition:
Suppose that (Vin) is constant but diode current (Iz) changes when load current (IL) increases then (Iz) decreases but current I doesn’t change an output voltage still remain constant in this condition because
V out = vin – Irs
=Vin – (Iz + IL) Rs since (I = Iz+ IL)
Similarly when IL decreases then (Iz) is increases but current I doesn’t change and output voltage still remains constant in this condition.



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