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Zener Diodes
23 questions By Tony R. Kuphaldt
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Question 16 of 23
Zener diodes are simple and useful devices for building voltage regulator circuits, but are times when you may have to improvise in the absence of the proper zener diode. Explain how normal diodes might be used as crude substitutes for a zener diode in the following circuit:
Reveal answer
Notes:I have actually done this before in home-made circuitry. Voltage regulation isn’t that good (especially the temperature dependence), but it is better than no regulation at all!
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Question 17 of 23
Suppose you needed to build a simple voltage regulator circuit with a regulation point of 4.5 volts, but had no zener diodes to work with. Can you think of a way normal diodes could be used for the purpose instead?
Reveal answer
(The number of series-connected diodes required depends on the actual forward voltage drop of each diode under loaded conditions.)
Notes:Some students may suggest to use normal diodes backwards, exploiting the reverse-breakdown phenomenon common to all PN junctions. Whether or not this suggestion is made, ask your students why it would not be a practical solution in this case.
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Question 18 of 23
At what load resistance value will this voltage regulator circuit begin to lose its ability to regulate voltage? Also, determine whether the voltage regulation is lost for load resistance values greater than this threshold value, or less than this threshold value.
Reveal answerThere will be no load voltage regulation for any load resistance values less than 15 kΩ.
Follow-up question: calculate the power dissipated by all components in this circuit, if Rload = 30 kΩ.
Challenge question: write an equation solving for the minimum load resistance required to maintain voltage regulation.
Notes:For those students struggling with the “greater than”/“less than” issue, suggest to them that they imagine the load resistance assuming extreme values: first 0 ohms, and then infinite ohms. After they do this, ask them to determine under which of these extreme conditions is the load voltage regulation still maintained.
Performing “thought experiments” with extreme component values is a highly effective problem-solving technique for many applications, and is one you should stress to your students often.
It should be noted that the calculated answer shown here will not precisely match a real zener diode circuit, due to the fact that zener diodes tend to gradually taper off in current as the applied voltage nears the zener voltage rating rather than current sharply dropping to zero as a simpler model would predict.
