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Homework Help: Zener diode current loads

  1. Jan 31, 2010 #1
    I have to answer this question in front of the class tomorrow. Been struggling all weekend with no joy.

    Determine the minimum and maximum loads currents for which the zener diode on the figure below will maintain regulation. What is the minimum RL that can be used.

    Vz=12v, Izk=3mA and Izm=90mA

    Please see attached image file.

    If I figure the circuit without the load, apparently this will tell me how much the circuit will draw from the 24V power supply. It will still be regulated.
    I don't understand this. I'm beginning to feel stupid. When I figure out a circuit with a supply/resistor/zener diode, am I treating this as though it is an ordinary diode in forward bias? And what exactly am I hoping to end up with?

    Please see diagram attached.

    Attached Files:

  2. jcsd
  3. Jan 31, 2010 #2
    This is not a difficult problem if you keep in mind that the Zener diode has an almost constant reverse voltage over a range of reverse current. That is what this problem is about, nothing to do with forward conduction. The key is to assume the output is at 12V.

    You also know the feed voltage is 24V, and that the series resistor is 470 ohms.

    You should be therefore be able to see what the voltage across the 470 ohms is.

    Then find the current through the 470 ohms resistor (I=V/R).

    You already have the maximum and minimum zener currents, so the rest should be easy.

    Finally, wouldn't this be better posted in the "homework section?
  4. Jan 31, 2010 #3
    The voltage across the limiting resistor is 12v. Therefore the current through it is 12v/470ohms=25.5mA

    You already have the maximum and minimum zener currents, so the rest should be easy.

    Please don't assume this, as I am very confused by this.

    Yes. I made a mistake. This should be in homework section.
  5. Jan 31, 2010 #4
    ok. so 25.5mA is within the range of the zener diode so the current will flow in reverse bias. This is considering the circuit without a load.
    If we attach the load, it will now draw away the current from the zener. The zener cannot get below 3mA or it will not function correctly. So the load (attached in parallel to the zener) must not allow a current below 3mA to flow through the zener diode.
    How am I doing. If correct. Now im stuck
  6. Jan 31, 2010 #5
    We have 25.5mA flowing into the node joining resistor, zener, and load.

    The zener needs to draw at least Izk = 3mA out of the node

    The sum of all currents into / out of any node is zero.

    How much current must therefore flow out into the load???
    Last edited: Jan 31, 2010
  7. Feb 2, 2010 #6
    25.5mA - 3mA = 25.2mA
  8. Feb 2, 2010 #7
    Erm, I would think that 25.5mA-3mA = 22.5mA (But note I got that wrong the first time - slippery things,these plastic keyboards!)

    Assuming that last was just a slip of the finger, just for interest, what do you think made the problem so difficult in the first place?
  9. Feb 2, 2010 #8
    Plastic was it :-
    Hay sorry Adjuster. Not very good with cryptic stuff. huh?
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