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Homework Help: Zener diode as voltage regulator.

  1. Feb 26, 2013 #1
    The following link is a picture of my zener diode set up with variable load resistor. The Source is 15VDC and the first resistor is a constant 330 ohm. When I use 470ohm up to 15Kohm for the Load resistor RL, I get a 6V voltage drop across the diode. However, when I go down to 330 ohm for RL, I get Vo to be 0.

    I have come up with an explanation for this, but I'm not too sure about it. Please read over and correct me if I am wrong.

    When the load resistor falls out of range(too small) it takes all of the current away from the zener diode therefore the zener diode is no longer working in its breakdown region. If the diode is within range (390 ohm and above), there is enough current flowing through the diode to kickstart the diode and bring it to conducting state(breakdown).

    Thank you for your help. Below is a picture of the way I have the circuit set up. On the right side is where the load resistor RL would be.

  2. jcsd
  3. Feb 26, 2013 #2


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    Staff: Mentor

    Did you directly measure the output of the voltage source when the output of your circuit dropped to zero (to confirm that it was still producing 15V)? It may have curtailed its output if has short-circuit protection or current limiting.

    The important characteristic of the zener diode is its threshold voltage. If it "sees" less than its threshold voltage, it looks like an open circuit. One would then expect the output of your circuit to follow the potential determined by the voltage divider comprised of the series resistor R and the load resistance.
  4. Feb 27, 2013 #3
    ok I interpreted my data wrong. Forget what I originally posted. Here's what I got for my data. For resistor ranging from 300 to 15K ohms, the voltage ranged from 6.07-6.18V. It seems that the zener diode is working in breakdown voltage and holds almost constant voltage for a wide range of resistance of RL. Am I correct in saying that the Zener is regulating the Voltage drop across RL?
  5. Feb 27, 2013 #4


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    I would write that it is regulating the output voltage Vout.
  6. Feb 27, 2013 #5

    rude man

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    No. See previous poster.
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