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EXPERIMENT
6.1: Title: To design a voltage regulator using Zener diode. Discuss the behavior of the regulator for various loads 6.2: Objectives: To construct a Zener diode voltage regulator To measure its line and load regulation. 6.3: Apparatus: Zener diode, Resistor Variable DC power supply Milliammeter. Voltmeter. Rheostat. Wire. 6.4: Theory: The Zener diode is like a general-purpose signal diode. When biased in the forward direction it behaves just like a normal signal diode, but when a reverse voltage is applied to it, the voltage remains constant for a wide range of currents. Avalanche Breakdown: There is a limit for the reverse voltage. Reverse voltage can increase until the diode breakdown voltage reaches. This point is called Avalanche Breakdown region. At this stage, maximum current will flow through the Zener diode. This breakdown point is referred as “Zener voltage”. 6.5: Characteristics: The Zener Diode is used in its "reverse bias". From the I-V Characteristics curve we can study that the Zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the Zener diodes current remains between the breakdown current Iz(min) and the maximum current rating Iz(max).
This ability to control itself can be used to great effect to regulate or stabilize a voltage source against supply or load variations. The fact that the voltage across the diode in the breakdown region is almost constant turns out to be an important application of the Zener diode as a voltage regulator. The forward bias region of a Zener diode is identical to that of a regular diode. The typical forward voltage at room temperature with a current of around 1 mA is around 0.6 volts. In the reverse bias condition, the Zener diode is an open circuit and only a small leakage current is flowing as shown on the exaggerated plot. As the breakdown voltage is approached, the current will begin to avalanche. The initial transition from leakage to breakdown is soft but then the current rapidly increases as shown on the plot. The voltage across the Zener diode in the breakdown region is very nearly constant with only a small increase in voltage with increasing current. At some high current level, the power dissipation of the diode becomes excessive and the part is destroyed. There is a minimum Zener current, Iz(min), that places the operating point in the desired breakdown. There is a maximum Zener current, Iz(max), at which the power dissipation drives the junction temperature to the maximum allowed. Beyond that current, the diode can be damaged. Zener diodes are available from about 2.4 to 200 volts typically using the same sequence of values as used for the 5% resistor series –2.4, 2.7, 3.0 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, etc. All Zener diodes have a power rating, Pz. From Watt’s law the
There are two type of regulations such as: a) Line Regulation In this type of regulation, series resistance and load resistance are fixed only input voltage is changing. Output voltage remains the same as long as the input voltage is maintained above a minimum value. Percentage of line regulation can be calculated by, Where Vo is the output voltage and Vin is the input voltage and ΔVo is the change in output voltage for a Vo is the change in output voltage for a particular change in input voltage ΔVo is the change in output voltage for a Vin. b) Load Regulation: In this type of regulation, input voltage is fixed and the load resistance is varying. Output volt remains same, as long as the load resistance is maintained above a minimum value. Percentage of load regulation, Where VNL is the null load resistor voltage (i.e. remove the load resistance and measure the voltage across the Zener Diode) and VFL is the full load resistor voltage. 6.7: Designing a Voltage Regulator: When selecting the Zener diode, be sure that its maximum power rating is not exceeded. Imax= Maximum current for Zener diode. Vin= Input voltage (it is known). Vs= Voltage across series resistance. Vl= Voltage across the load resistance. Is= Current passing through the series resistance. Iz= Current passing through the Zener diode.
Il: Current passing through the load resistance.
