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zener diode question

Discussion in 'Electronic Design' started by R.Lewis, Jun 29, 2004.

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  1. R.Lewis

    R.Lewis Guest

    Is there any practical difference between the various small (sub 1W) zeners
    such as the BZX55,79, & 85's (plus others such as BZY's that I cannot fully
    bring to mind at this moment)
    All I need is 3v6 and 4v7, 5%, 100mW max, through hole, but neither passing
    more than 1mA at 2.0 volts Vf.
    ..
     
  2. Robert Baer

    Robert Baer Guest

    Firstly, ordinary zeners below 6V are crappy and get worse as the
    voltage gets lower, and seems to get almost as bad as using an ordinary
    diode in the forward region.
    I would suggest that you use an LM285/LM385 adjustable voltage
    reference.
     
  3. John Fields

    John Fields Guest

    On Tue, 29 Jun 2004 23:17:31 +0100, "R.Lewis" <h.lewis-not this
     
  4. Joerg

    Joerg Guest

    The LM385 is a good suggestion. In case it has to be rock bottom in cost and you can spare an opamp somewhere, the LMV431 (fixed) would be an alternative.

    I can only corroborate what the other posters said about zeners and I replaced a lot of them with this reference after complaints about tolerances.

    Regards, Joerg
     
  5. Joerg

    Joerg Guest

    Correction: The LMV431 is also adjustable.

    Regards, Joerg
     
  6. colin

    colin Guest

    as people have said i think, low voltage ones have poor zener efect and even
    poorer with low current. ie large slope resistance and variuation with
    temperature and wide variation of its knee (ie point at wich zener curent
    dominates over leakage curent)

    but if its just a simple level shifter where u have a wide margin and dont
    care too much about voltage then it can be simplest solution, but u need a
    minimum curent to flow through it before you see the rated zener voltage.

    i wouldnt count on geting close to 5% of its specified voltage with only
    2volts or 1ma tho, u might be too close to its 'knee' to predict what
    volt/curent you get.

    Colin =^.^=
     
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