Section Index Owatonna Steele County Amateur Radio 24-Jun-2016

 Zener Diode Regulator Circuit The Zener diode is similar to a general-purpose diode. When a reverse voltage exceeds the diode breakdown voltage, current starts to flow through the diode. For a Zener diode, the current increases dramatically to the maximum circuit value (which is usually limited by a series resistor). The reverse saturation current remains fairly constant over a wide range of applied voltages. The "Zener voltage" is when the voltage across the Zener diode becomes stable. Because of this charecteristic, the Zener diode is often used as a voltage regulator in low power circuits. Designing a voltage regulator using a Zener diode requires the following information: ES(min) The minimum unregulated voltage supplied to the circuit. ES(max) The maximum unregulated voltage supplied to the circuit. EZ The rated voltage of the Zener Diode. IL(max) The maximum current drawn by the load. IL(min) The minimum current drawn by the load (should normally be 0 VDC). Knowing the above pieces of information, the calculations are fairly straight forward: The resistor (RS) limits the power that needs to be absorbed by the Zener diode: RS = [ ES(min) - EZ ] / [ 1.1 * IL(max) ] The power rating of RS is calculated by the equation: PR = [ ES(max) - EZ ] * IL(max) The power rating of the Zener diode is based on no current being delivered to a load: PZ = EZ * [ { ( ES(max) - EZ ) / RS } - IL(min) ] As an example, calculate the required values for the following circuit: ES varies between 11 VDC and 13 VDC. EZ is 5VDC. IL(max) is 100 mA. IL(min) is 4 mA, but assume it will be 0 mA. The calculations are: RS = [ ES(min) - EZ ] / [ 1.1 * IL(max) ] RS = [ 11 - 5 ] / [ 1.1 * 0.100 ] = [ 6 / 0.110 ] = 54.5 ohms PR = [ ES(max) - EZ ] * IL(max) PR = [ 13 - 5 ] * [ 0.100 ] = [ 7 * 0.100 ] = 0.7 watts PZ = EZ * [ { ( ES(max) - EZ ) / RS } - IL(min) ] PZ = 5 * [ { ( 13 - 5 ) / 54.5 } - 0 = 5 * [ { 7 / 54.5 } - 0 ] = 5 * [ 0.128 - 0 ] = 5 * 0.128 = 0.642 watts

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