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Acceptable Zener regulator tolerances

Discussion in 'Power Electronics' started by LearningSomethingToday, Mar 12, 2018.

  1. LearningSomethingToday

    LearningSomethingToday

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    Mar 4, 2018
    If I am working on a differential amp circuit that uses a 27volt Zener diode and resistor to form a voltage reference what would be the acceptable deviation from 27 volts? The circuit I am looking at is consistently putting out 25.42 volts. I am not sure if a simple Zener regulator like this has some acceptable error or tolerance. When Zeners are failing do they typically go open,short or sway away from their intended values?

    Thank you!
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009

    there should be that data in the component datasheet for that particular diode
     
  3. duke37

    duke37

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    Jan 9, 2011
    If you want good regulation, why not use a band gap reference e.g. LM317T. Set it to the voltage you want.
     
  4. WHONOES

    WHONOES

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    May 20, 2017
    The voltage present on your zener diode depends on its published tolerance and the amount of current passing through it. ±5% is typical for many types, 5% of 27 is 1.35V so would give a voltage of 25.65V at the extreme end of its negative tolerance.
     
  5. LearningSomethingToday

    LearningSomethingToday

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    Mar 4, 2018
    @davenn : I feel kind of dumb that I did not look a datasheet to start with. I don't know who manufactured this particular diode but I did get a spec sheet for that diode type. It looks like I am a bit out of spec.

    @WHONOES: I am measuring 25.42v. There is a possibility my Fluke meter could be reading off slightly but my gut feeling is it is reading the voltage properly. From looking at one or two datasheets it appears I may be out of spec. Littediode says they have an upper/lower regulation of 26.33v/27.66v. Semtech says they have upper/lower of 26.29v/27.64v

    I have read that resistors can change value as they age but may not fail completely. I have not personally witnessed this except on resistors that look like they have been stressed.
    Can I assume a 30 year old Zener can do something similar as it gets older without failing completely?

    Sorry for questions that seem 'newby'. I have people in my direct circle that are good with electric systems but not necessarily with electronics. I am trying to go a little further

    Thanks
     
  6. LearningSomethingToday

    LearningSomethingToday

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    Mar 4, 2018
    If I were designing a new circuit this could be a good approach. I am working on a 30+ year old design and I am trying to live within the space of the original design, trying to stay as OEM as possible. Thanks for the input.
     
  7. WHONOES

    WHONOES

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    May 20, 2017
    What is the zener type you are using?
    It is possible to get ±2% tolerance parts
     
  8. Alec_t

    Alec_t

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    Jul 7, 2015
    Depending on the circuit the zener is in, its value may not be critical. Can you post a schematic?
     
  9. LearningSomethingToday

    LearningSomethingToday

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    Mar 4, 2018
    There was nothing printed on the Zener other than voltage. It is a glass type. The numbers are barely visible and cannot be read without the aid of a microscope.

    Luckily I needed to order a peculiar value capacitor for a large monitor at my job so I just threw a few 24v and 27v Vishay Zeners on the order to save on shipping. I did order 2% Zeners. They really were not expensive. I think a handful is less than $2
     
  10. LearningSomethingToday

    LearningSomethingToday

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    Mar 4, 2018
    sae.jpg
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    Hmm, the zener voltage in that circuit doesn't seem critical. If it were, say, only 25V then you'd be dropping an extra 2V across the feed resistor, i.e about 4% of the 50V(?) main supply. That would cause slight additional heating of the resistor, which may or may not be rated to cope.
     
  12. LearningSomethingToday

    LearningSomethingToday

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    Mar 4, 2018
    In my quest to find out why I could not successfully get bias dialed in on this unit, and since all other components, looked, tested and smelled ok I started thinking that small changes in reference voltages could impact the biasing circuit negatively. Since bias movements are so small in magnitude I was thinking that a larger change in the reference voltages could throw that whole thing off. Anyway that was just my exploratory theory. It may all be nonsense but for a few cents worth of Zeners, I figured I could do no harm. This unit is an ongoing project and does not belong to a client. It is a learning platform for me so I am not in a rush to get it going.
     
  13. Audioguru

    Audioguru

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    Sep 24, 2016
    The amplifier circuit has lots and lots of negative feedback so that a change of the zener diode voltage will barely change the output voltage.
     
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