diode

Discussion in 'Homework Help' started by omerysmi, Oct 16, 2014.

  1. omerysmi

    Thread Starter Member

    Oct 10, 2014
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    I need to find the lowest zener voltage that will allow current in the following circuit, the current must be little than 3mA

    [​IMG]

    My answer is: Vzener = 4.01V

    Am I right?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That is correct in theory. In practice you may not be able to find a zener with that exact voltage.
     
  3. omerysmi

    Thread Starter Member

    Oct 10, 2014
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    Yes I know, but I only need a solution for the question...so it is a good accuracy?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    I think you missed a word when you said, "the current must be little than 3ma"? Did you mean a little more? A little less? Judging from your answer, I'm guessing you meant a little less.

    Technically, the question is rather poor since it is asking for the lowest zener voltage but then giving an indefinite spec. If you say 4.01 V, then what is to prevent someone from saying that is wrong because a 4.001V would also meet the criteria yet have a lower voltage? And then someone else would claim that that is wrong because you could have a 4.0001V zener, and so on forever.
     
  5. omerysmi

    Thread Starter Member

    Oct 10, 2014
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    Yes but I cannot write a number with so much zeros so i assume that 4.01v is a good accuracy to be an answer for this question, no?
     
  6. WBahn

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    Mar 31, 2012
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    Probably. I would probably be a smart ass with such a question and write something like (4+eplison)V where epsilon is vanishingly close to zero. But then I tend to react rather harshly to poorly constructed questions (even if I'm the one that wrote them).
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Since this is a homework section, the answer to the number of digits you carry out the answer is up to your instructor. Historically, I've rarely seen scientific notation to three significant digits.

    Like it's been stated before, you won't find that zener as it is not a standard value.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    One thing that you might consider doing is providing two answers to the question. The first would be as given -- and you can cover the poor question by saying something like: Vz < 4V -- and then providing an answer that is practical giving the value of the closest standard zener diode that meets spec. This is not only good practice for the "real world", but it gets you in the habit of considering problems from a practical engineering perspective and demonstrates that to the grader and the instructor, who will often then become much more forgiving of minor errors on your part because they will often be viewed as uncharacteristic goofs rather than reflective of a lack of knowledge and skill.
     
    JoeJester likes this.
  9. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello,

    Yes, it is wise to simply state that the zener voltage must be GREATER than 4 volts "Vz>4v" if they are really asking for a current LESS THAN 3ma, but then again they did not specify that it had to be close to 3ma.
    If they asked for a current equal to 3ma then you could simply state "4 volts", but they did not ask for that.
    If they asked for a current that is slightly GREATER than 3ma then you would state "the zener voltage must be less than 4 volts".

    If you state 4.01v then yes that causes a current less than 3ma, but as others have mentioned anything over 4v does too so it's arguable what it should be unless you simply state that is must be over 4v. For example, we could state Vz=5v and still be correct, or Vz=6v, or even Vz=9.999v because with Vz slightly less than 10v the current is definitely less than 3ma and there is nothing that states that it must be close to 3ma :)

    So for this problem the best answer is probably: 4v<Vz<10v, not Vz<4v and the answer 4.01 is not sufficient because a host of other voltages up to but not including 10v also work.

    If they stated that they want the current as close to 3ma as possible, that would be a different story.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Actually, they did: "find the lowest zener voltage...." But you are correct that I messed up and should have said "Vz>4V". My bad -- thanks for catching it.

    I'm reading into the post a bit, because I think the OP missed a word when typing it, but it would appear that they are specifying that "current flowing" means a current of a little less than 3mA. But it's also possible that the last part was supposed to be "the current must be LESS than 3mA", which actually makes a lot more sense.

    It appears to be a poorly crafted question, when all is said and done.
     
  11. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,


    Actually i thought you meant that Vz>4v anyway :)

    Yeah, i agree the question was not worded accurately so we had to interpret the meaning, and this is always going to lead to different opinions about what it really was supposed to be asking.

    The original statement was:
    "I need to find the lowest zener voltage that will allow current in the following circuit, the current must be little than 3mA"

    I took this (based on experience with other languages and their conversion to English when English is a second language) to mean:
    "I need to find the lowest zener voltage that will allow current in the following circuit, the current must be less than 3mA"

    while you took it to mean:
    "I need to find the lowest zener voltage that will allow current in the following circuit, the current must be a little less than 3mA"

    So we see that 'i' took it to be as brief as possible, while 'you' took it to be more practical to what we would really do in real life, and i have to say i like your interpretation better than mine :)
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I took it to be "littler than 3 mA" which would translate to less than 3 mA. I took that to be between 2 mA and 3 mA, because if they wanted to be "littler than 2 mA", they would have stated so.

    Either a 4.3 V and 5.1 V zeners would satisfy the a 2 mA to 3 mA requirement. I suspect the smallest zener they might have in stock is the 5.1 V zener
     
    Last edited: Oct 18, 2014
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