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Easy zener diode circuit questions

  1. Nov 5, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-11-5_17-27-43.png


    3. The attempt at a solution

    So basically, as the question says, you need to calculate the current in the zener and load when the supply voltage is 10V and then again when it's 15V.

    FOR 10V SUPPLY

    Voltage across resistor = 10-5 = 5v

    Supply current = 5/1000 = 5mA

    Voltage across load = 5V as Zener and load are in parallel

    Current through load = 5/1000 = 5mA

    Current through Zener = Supply current – Load current = 5 – 5 = 0



    FOR 15V SUPPLY

    Voltage across resistor = 15-5 = 10v

    Supply current = 10/1000 = 10mA

    Voltage across load = 5V as Zener and load are in parallel

    Current through load = 5/1000 = 5mA

    Current through Zener = Supply current – Load current = 10 – 5 = 5 mA


    This doesn't look correct. I've never did a question with zener diodes before and I'm confused. What is it I'm doing wrong?
     
  2. jcsd
  3. Nov 5, 2016 #2

    cnh1995

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    Looks correct to me.
     
  4. Nov 5, 2016 #3

    gneill

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    Your calculations are fine as far as they go, however...

    The designation "5V1" on the zener diode is a fairly common way to specify the zener voltage where the "V" is placed to represent the decimal point. I don't know if this syntax was explained to you. So the zener in question has a 5.1 V threshold.

    This will affect your calculations. In particular, if there's not enough voltage "left" after the drop across R1 to let the zener operate then the zener can't conduct any current; it's as if the zener is removed from the circuit. I suggest that the first thing to check is whether or not a zener in that location will be able to conduct: remove the zener and check the potential at that node. If it's less than the zener voltage then the zener will have no effect on the circuit. If it's higher than the zener voltage, then the zener can conduct and "pull" the voltage down to its threshold voltage of 5.1 V.
     
  5. Nov 5, 2016 #4
    So the current through the zener for the 10v supply is 0A?

    My lecturer was saying that instead of using 5V for the zener, I should be using 5.1V because that's its breakdown voltage or something.
     
  6. Nov 5, 2016 #5

    cnh1995

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    Right. I didn't know 5V1 means 5.1V. Post #2 clarifies that.
     
  7. Nov 5, 2016 #6
    Should I use 5.1 in the calculation or just 5?
     
  8. Nov 5, 2016 #7

    gneill

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    The zener voltage is 5.1 V. Use 5.1 V. And do the check I suggested for each of your power supply voltage cases.
     
  9. Nov 5, 2016 #8
    FOR 10V SUPPLY

    Voltage across resistor = 10-5.1 = 4.9v

    Supply current = 4.9/1000 = 4.9mA

    Voltage across load = 5.1V as Zener and load are in parallel

    Current through load = 5.1/1000 = 5.1mA

    Current through Zener = Supply current – Load current = 4.9 – 5.1 = -0.2 mA



    FOR 15V SUPPLY

    Voltage across resistor = 15-5.1 = 9.9v

    Supply current = 9.9/1000 = 9.9mA

    Voltage across load = 5.1V as Zener and load are in parallel

    Current through load = 5.1/1000 = 5.1mA

    Current through Zener = Supply current – Load current = 9.9 – 5.1 = 4.8 mA


    So for the current through the zener for the 10V supply, it's -0.2mA? I don't understand. Is that correct?
     
  10. Nov 5, 2016 #9

    cnh1995

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    This means the zener is not working and has no effect on the circuit. You should analyse the circuit by removing the zener. It is not conducting any current.
     
  11. Nov 5, 2016 #10

    gneill

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    Re-read the second paragraph of my reply in post #3.
     
  12. Nov 5, 2016 #11
    Okay, so that calculation is correct but the zener just doesn't do anything in the circuit?
     
  13. Nov 5, 2016 #12

    gneill

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    The calculation is fine mathematically, but it doesn't apply to the circuit you're analyzing. You're making an assumption about the potential at the top of the zener which is not true for this case.
     
  14. Nov 5, 2016 #13
    Okay, so lets say I get that question in my test and I do the calculations that I have just shown using 5.1V as the zener voltage. Will I get the marks for that?
     
  15. Nov 5, 2016 #14

    gneill

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    No. At best you'll get partial marks. You failed to recognize that the zener was not going to conduct at all because it was being "asked" to operate outside of its range of operation for zener mode.

    Always check whether or not the zener is going to be able to conduct in a given situation. If it cannot conduct then it won't behave as a zener; it'll just behave as an open circuit.
     
  16. Nov 5, 2016 #15
    So the 15V supply one is correct?

    For the 10V supply one, should the calculations shown not prove that it doesn't conduct? After I prove it doesn't conduct, should I then not just then write a statement saying "the zener will not conduct..."

    If I don't show they calculations for the 10V supply, how do I know by just looking at it that it won't conduct?
     
  17. Nov 5, 2016 #16

    gneill

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    Yes.
    You could say that. But you still need to state the zener current and load current explicitly in order to answer the question.
    You first do the check that I explained in post #3. Just do the check. It's that simple.

    Once you have gained experience you'll be able to spot these "iffy" or "borderline" cases by looking at the part values in the circuit. I see that R1 and R2 have the same value and would form a voltage divider if the zener were not there. As a voltage divider they would want to produce a potential of half the supply voltage at their junction. Half of 10 volts is 5 volts, which is less than the zener voltage of 5.1 V, so the zener won't be able to turn on.
     
  18. Nov 5, 2016 #17
    I'm sorry for going on but what do you mean I need to state explicitly the load and zener current? Didn't I do that?

    Zener currrent = - 0.2mA
    Load current 5.1mA

    I calculated the supply current to be 4.9mA so how can the load current be 5.1?
     
  19. Nov 5, 2016 #18

    gneill

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    You stated two incorrect values. No points for that.

    You made an incorrect assumption, that the zener would hold its voltage at 5.1 V. With the given circuit and a 10 V supply the zener could not do that (which is why you want to do the check first). So the load current and the source current, both calculated assuming that the zener would be at 5.1 V, ended up with incorrect values.
     
  20. Nov 5, 2016 #19
    Okay, so before doing any calculations I check if it will work and if not, I state that it won't conduct. What about the values that "I need to state explicitly?" What are the values I need to state to get all the marks?
     
  21. Nov 5, 2016 #20

    gneill

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    You need to state that the zener current is zero and the actual load current (by a new calculation).
     
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