Voltage Regulation using Zener DiodesBryan WitherspoonNovember 8, 2008AbstractIn digital or mixed-signal applications, there is a need to precisely regulate the voltage entering or exiting acircuit. The Zener diode has a unique property that offers a simple, low-cost solution to this problem. Thisapplication note discusses the implementation of this voltage regulator and its advantages and drawbacks.Keywords: Zener diode, Circuit, Voltage regulator, Digital, Mixed SignalIntroductionLow voltage, low current applications like digital or mixed-signal circuitry require precise voltages for certaintasks like powering a microcontroller, or providing a reference voltage for measurements.In the laboratory, voltage regulation may be viewed as unnecessary or redundant when fine control overcircuit inputs exists. In most real-world applications, however, the environments in which our circuits existare much more harsh and unforgiving. It then becomes necessary to properly protect circuitry, especially inthose which may be sensitive to spikes in voltage or current.There are many integrated circuit voltage regulators on the market today. These tend to be expensive,unnecessarily complex, may be subject to temperature effects, and typically require a much higher voltage inorder to maintain its listed voltage.In applications where current draw is sufficently small, a more attractive solution exists which only uses aresistor and a unique semiconductor called a Zener diode (http://en.wikipedia.org/wiki/Zener_diode)Zener Diode PropertiesThe Zener diode is a unique device which exploits a propertyof diodes that is generally avoided in standard diodeapplications. Typical diodes have a very high reversebreakdown voltage, the voltage at which the diode begins toconduct in the reverse direction. This voltage is generallyavoided in standard diodes, but Zener diodes are designedand manufactured to have a very specific reverse breakdownVoltage Regulation using Zener Diodes http://www.egr.msu.edu/classes/ece480/goodman/fall08/group0...1 of 3 11/10/08 11:20 AMFigure 1: V-I characteristics of a typical Zener diode.Source: http://www.necel.com/en/faq/f_diode.htmlFigure 2: Shunt voltage regulator.Source: http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htmvoltage. On top of that, the transition into reversebreakdown is very sharp. Figure 1 shows the V-Icharacteristics of a typical Zener diode.Exploiting this characteristic of Zener diodes allows forsimple, inexpensive voltage regulators. There are severaldrawbacks, however, that will be discussed shortly.Building a Voltage RegulatorWith an ideal Zener diode, construction of a voltageregulator is easy, simply connect the diode between theunregulated voltage and ground. Since the exploitedproperty of Zener diodes is the reverse breakdown voltage,connect the anode of the diode to ground, not the cathode.Unfortunately, this is only an ideal circuit. Like most passiveelements, if enough current is allowed to pass through thediode, it will be destroyed. A resistor will be added to ourcircuit to limit the current through the diode.First, select the Zener diode based on the voltagerequirements of the circuit.Next, determine the maximum current draw of your circuit. For a robust solution, the diode must be able tohandle the entirety of this current. Zener diodes have power ratings, so to determine the maximum powerthe diode will take, multiply the maximum current draw by the Zener voltage.The resistor will shoulder the rest of the voltage drop not handled by the Zener diode, which is the differenceof the source voltage and the Zener voltage. Select the resistor value using Ohm's Law, where the resistanceis the voltage drop across the resistor divided by the maximum current draw. The Zener diode itself requiressome current to stay in reverse breakdown, typically in the order of 5mA, so the resistor value may belowered slightly to accomodate.Drawbacks and RecommendationsAs with most voltage regulators, the source voltage must be slightly higher than the Zener voltage in order tokeep the Zener diode in reverse breakdown. Otherwise, the output voltage will simply follow the inputVoltage Regulation using Zener Diodes http://www.egr.msu.edu/classes/ece480/goodman/fall08/group0...2 of 3 11/10/08 11:20 AMvoltage.As touched on earlier, both the diode and resistor must have power ratings high enough to handle all thecurrent should the circuit suddenly stop drawing current. This means that this solution may be impracticalfor circuits that require high voltage or high current. An 8V regulator with a 12V source that requires 1A ofcurrent requires a 250Ω resistor rated at 4W and an 8V Zener diode rated at 8W. Typically, Zener diodes areonly available at up to 5W, so in even a medium voltage/current application like this example, it may bemore practical to use an integrated circuit regulator.ConclusionThe Zener diode, with its accurate and specific reverse breakdown voltage, allows for a simple, inexpensivevoltage regulator. Combined with the right resistor, fine control over both the voltage and the supply currentcan be attained.However, the low power ratings of standard Zener diodes and resistors make this solution impractical forhigh power devices.Copyright ©2008 Bryan Witherspoon (mailto:[email protected])Voltage Regulation using Zener Diodes http://www.egr.msu.edu/classes/ece480/goodman/fall08/group0...3 of 3 11/10/08 11:20
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