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Basics of using zener diodes

Discussion in 'General Electronics Chat' started by diy didi, Jan 2, 2018.

  1. diy didi

    diy didi Member

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    This is a back to basics question. More specifically how to calculate the series resistor to for the zener diode if one is for example making a series pass regulator with a transistor or a CCS.
    Experimenting with zeners led me to the conclusion that a resistor value calculated for a 15V 1W zener will differ to one calculated for a 15V 400mW zener.
    The 1W needs a higher min Iz to keep regulation. But what is the min Iz needed? I know datasheets sometimes state this, but sometimes one has an unknown brand zener. What then?
    Anyone have a quick fail safe method?
     
  2. Pommie

    Pommie Well-Known Member Most Helpful Member

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    If you are just using a zener to get a lower voltage then the series resistor must drop the excess voltage at the required current.

    For example, if you have a 12V supply and need 5V as 20mA then the series resistor needs to drop 7V at 20mA - R=7/0.02 = 350Ω. On no load the zener will conduct the full 20mA and will dissipate 5*0.02 (0.1) Watts. The resistor will dissipate 7*0.02 = 0.14W.

    HTH

    Mike.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It really depends on the load conditions as well, and if they are constant or varying.
     
  4. dave miyares

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    A 1W Zener does not need 1 watt. That is the maximum wattage. It will work at a much lower level.

    I have always used current (like Ponnie) in post #2. You can get the same answer by using watts.
    -Stay away from the 1W level. 0.5 is safe.
    -If your load always is 0.5w then this is simple. But life is not simple. Many loads vary. For an example: 0.2 to 0.5 watts.
    -Push 0.6W at the Zener and the load will take 0.2 to 0.5 of that leaving 0.4 to 0.1 watts for the Zener. If the load is unplugged the Zener can easy take 0.6W.

    You should also do the math for changes in supply. From post #2. (12V to 5V) But the 12V supplies might be 11 to 13V. So you need to through enough power at the (Zener//load) so it works at 11V and does not burn out the resistor or Zener at 13V.

    I make a table of 4 states. Each state must have some Zener current or wattage. Every state must keep the watts in the resistor below safe levels. The watts in the Zener must not get above rated levels.
    1) Min load, Min supply
    2) Min load, Max supply
    3) Max load, Min supply
    4) Max load, Max supply
     
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  6. Colin

    Colin Active Member

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    "For example, if you have a 12V supply and need 5V as 20mA then the series resistor needs to drop 7V at 20mA - R=7/0.02 = 350Ω. On no load the zener will conduct the full 20mA and will dissipate 5*0.02 (0.1) Watts. The resistor will dissipate 7*0.02 = 0.14W."

    This is the very reasoning to avoid.
    With this reasoning you have allowed the circuit to drop out of regulation.
     
  7. Pommie

    Pommie Well-Known Member Most Helpful Member

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    OK, I should have said max 20 mA. However, unless you want very rough regulation, you would not use a zener shunt regulator.

    Mike.
     
  8. dave miyares

    Dave New Member

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  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    C10=10V Zener. You can see the "regulation" or voltage for a 10V Zener. "Good or rough" regulation is not well defined.
    upload_2018-1-4_17-20-57.png
    It is true that Zeners below 5V are not so good.
    upload_2018-1-4_17-23-36.png
    "BZT52C"
     
  10. Pommie

    Pommie Well-Known Member Most Helpful Member

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    That is a lot better than I remembered it.

    Mike.
     
  11. Colin

    Colin Active Member

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    "That is a lot better than I remembered it. "

    Things have improved in the past 40 years.
     
  12. Pommie

    Pommie Well-Known Member Most Helpful Member

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    However, the basic principal is still the same - see post #2.

    Mike.
     
  13. Colin

    Colin Active Member

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    It is not a basic principal
     
  14. Pommie

    Pommie Well-Known Member Most Helpful Member

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    What is it then Colin?
     
  15. Colin

    Colin Active Member

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    the basic principle is still the same
     
  16. Pommie

    Pommie Well-Known Member Most Helpful Member

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    Would you care to enlighten me on "The basic principal"?
     
  17. Colin

    Colin Active Member

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    I have no idea what "The basic principal" is.
     
  18. Pommie

    Pommie Well-Known Member Most Helpful Member

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    You seem to know what it isn't but not what it is!!

    Or are we differentiating between A basic principle and THE basic principle.
     
  19. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    I read it initially as a lighted hearted attempt at a spelling correction, i.e, principal to principle.
     
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  20. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    To answer diy didi's specific question, to design a basic shunt Zener voltage regulator, then start with case #3 (Max load, Min Supply). To provide the rated regulation, the minimum current through the Zener must be > Iz (test current) = 5mA for most of the "BZT" Zener series as shown on the plots in post #7.
     
  21. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I am going to be the "donkey's behind again" and disagree.
    I think the "test current" is only the current they test at. Not the minimum current.
    From the BZX84 series data sheet. Note the voltage remains very good down well below uA. This type of diode was chose to show what I want to show. There are other diode families where the high voltage Zeners do not work well at 1uA.
    upload_2018-1-5_7-58-43.png
    On the right side, the 7.5V part is good to 2uA. Depending on what good means.
    upload_2018-1-5_8-1-54.png
    As long as I am being contrary; You can now get many types of Zeners at 1% and some at 0.5%.
    Next item; temperature stability. This depends on which series of diode and what voltage with in a series. Example: most (certainly not all) families of diodes are best for temperature near 5V.
     
  22. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    I agree, but designing for Iz as a minimum Zener current makes for a reliable design. Usually, you do not know the max current drawn by the load that precisely.
     

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