Zener Diodes
Discrete Semiconductor Devices and Circuits
Don’t just sit there! Build something!! 
Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.
You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the änswers” instead of a book or another person. For successful circuitbuilding exercises, follow these steps:
 1.
 Carefully measure and record all component values prior to circuit construction, choosing resistor values high enough to make damage to any active components unlikely.
 2.
 Draw the schematic diagram for the circuit to be analyzed.
 3.
 Carefully build this circuit on a breadboard or other convenient medium.
 4.
 Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements onebyone on the diagram.
 5.
 Mathematically analyze the circuit, solving for all voltage and current values.
 6.
 Carefully measure all voltages and currents, to verify the accuracy of your analysis.
 7.
 If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully recalculate the values and remeasure.
When students are first learning about semiconductor devices, and are most likely to damage them by making improper connections in their circuits, I recommend they experiment with large, highwattage components (1N4001 rectifying diodes, TO220 or TO3 case power transistors, etc.), and using drycell battery power sources rather than a benchtop power supply. This decreases the likelihood of component damage.
As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter “loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ.
One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another timesaving technique is to reuse the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.
Not all “zener” diodes break down in the exact same manner. Some operate on the principle of zener breakdown, while others operate on the principle of avalanche breakdown. How do the temperature coefficients of these two zener diode types compare, and how are you able to discern whether a zener diode uses one principle or the other just from its breakdown voltage rating?
Correspondingly, is there a way we could determine the type of breakdown action from experimental measurements on a zener diode? Explain how such an experiment might be set up.
Suppose you had the boring job of manually maintaining the output voltage of a DC generator constant. Your one and only control over voltage is the setting of a rheostat:

What would you have to do to maintain the load voltage constant if the load resistance changed so as to draw more current? Being that your only control over load voltage is the adjustment of a variable resistance in series with the generator, what does this imply about the generator’s output voltage (directly across the generator terminals), compared to the target load voltage?
Suppose you had the boring job of manually maintaining the output voltage of a DC generator constant. Your one and only control over voltage is the setting of a rheostat:

What would you have to do to maintain the load voltage constant if the load resistance changed so as to draw more current? Being that your only control over load voltage is the adjustment of a variable resistance in parallel with the load, what does this imply about the generator’s output voltage (directly across the generator terminals), compared to the target load voltage?
Calculate the current through the zener diode for the given values of load resistance in this circuit:

 •
 R_{load} = 1 kΩ ; I_{zener} =
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 R_{load} = 910 Ω ; I_{zener} =
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 R_{load} = 680 Ω ; I_{zener} =
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 R_{load} = 470 Ω ; I_{zener} =
 •
 R_{load} = 330 Ω ; I_{zener} =
Do you see any relationship between load current and zener diode current? If so, explain what that relationship is.
Calculate the power dissipated by the 5volt zener diode for the following values of motor current (assume the battery voltage remains constant at 12 volts):

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 I_{motor} = 20 mA ; P_{zener} =
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 I_{motor} = 50 mA ; P_{zener} =
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 I_{motor} = 90 mA ; P_{zener} =
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 I_{motor} = 120 mA ; P_{zener} =
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 I_{motor} = 150 mA ; P_{zener} =
Calculate the current through the zener diode for the given values of input (source) voltage in this circuit:

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 V_{source} = 25 V ; I_{zener} =
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 V_{source} = 20 V ; I_{zener} =
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 V_{source} = 15 V ; I_{zener} =
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 V_{source} = 10 V ; I_{zener} =
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 V_{source} = 5 V ; I_{zener} =
Do you see any relationship between source voltage and zener diode current? If so, explain what that relationship is.
Qualitatively determine what will happen to the series resistor current and the zener diode current in this voltage regulator circuit if the load current suddenly decreases. Assume that the zener diode’s behavior is ideal; i.e. its voltage drop holds absolutely constant throughout its operating range.

I_{Rseries} = (increase, decrease, or unchanged?)
I_{zener} = (increase, decrease, or unchanged?)
Qualitatively determine what will happen to the load current and the zener diode current in this voltage regulator circuit if the source voltage suddenly increases. Assume that the zener diode’s behavior is ideal; i.e. its voltage drop holds absolutely constant throughout its operating range.

I_{load} = (increase, decrease, or unchanged?)
I_{zener} = (increase, decrease, or unchanged?)
Precision voltage reference regulators are often constructed of two zener diodes connected in series like this:

Explain why two zener diodes provide greater stability than a single zener diode would, and also draw a circuit showing a voltage source, so this component functions as a complete voltage reference.
Find one or two real zener diodes and bring them with you to class for discussion. Identify as much information as you can about your diodes prior to discussion:
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 Polarity (which terminal is cathode and which is anode)
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 Forward voltage drop
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 Continuous current rating
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 Surge current rating
 •
 Continuous power rating
 •
 Zener voltage rating
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