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Getting a 2.5v reference voltage.

Discussion in 'Electronic Basics' started by Simon, Apr 22, 2004.

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  1. Simon

    Simon Guest

    I have a circuit that specified a LM431 coupled with an a resisitor and
    potentiometer to give an reference voltage of 2.5v to input in a comparator.
    Im unable to get the LM431 from my usual supplier and so have done a google
    search which returned a similar circuit using the LM338 for a 2.5v
    reference - however once again my usually supplier does not have this part.

    I can however get a LM317 - from what I can see it does the same job as both
    the LM431 and LM338 - Can anyone explain what the differerence is, with the
    aim of getting a 2.5 reference voltage.

    Whu is a regulator needed in any case? Could 2.5v not be acheived by a
    simple potential divider?
  2. References generally produce lower noise, have tighter tolerances
    (initial and drift with time and temperature) and operate on less
    power than regulators. But they share many similarities. A divider
    produces an accurate voltage only when unloaded (or loaded with a
    perfectly fixed load current) and does not compensate for any changes
    in supply voltage. Otherwise, they are quite useful.
  3. Fred Bloggs

    Fred Bloggs Guest

    A more confident answer could depend upon the specifics of the
    comparator circuit, but generally a potentiometric divide will do unless
    there is some other loading on the 2.5V that pulls the voltage down due
    to potentiometer resistance.
  4. Just a note- if you can't get an LM431 or exact equivalent from your
    usually supplier you need a new supplier for your usually business.

    They are made by National, Philips, Fairchild, Zetex, Korea
    Electronics, TI, etc. A real jellybean part. Last I looked, just about
    every PC power supply used one.

    Best regards,
    Spehro Pefhany
  5. Paul_Morphy

    Paul_Morphy Guest


    You don't say where you are, but if you're in the U.S., B.G. Micro
    ( has an IPS338AV which substitutes nicely for the LM338. I
    just bought one from them for use in a 1.25-16 V regulated supply. It's a
    fat regulator, though, capable of sourcing at least 5 A. Seems overmuch for
    a reference-V source. B.G. Micro also has the LM317T for 69 cents. Do a
    Google search for "LM317 data" for application info. For small orders, if
    you ask in advance, B.G. Micro will just send the parts in a padded
    envelope, saving you from having to pay their $6 min shipping charge.
    Regulators are on page 10 of their online (PDF) catalog.

  6. Tim Wescott

    Tim Wescott Guest

    Use a voltage reference when you need good absolute accuracy.
    Conveniently enough if you have a ratiometric sensor and can supply it
    from your chip supply, the best reference to use for your ADC is also
    your chip supply.
  7. Or in the UK, for the TL431, 4 different manufacturers, all in stock:-

    Best regards,
    Spehro Pefhany
  8. R.Lewis

    R.Lewis Guest

    The '431' is such a common part that I find it difficult to believe that any
    supplier would be unable to source it.
    It is made by a host of companies.
    Maybe they know it better as a TL431, or TL431A.
  9. Joerg

    Joerg Guest


    As the others have suggested, widen your search parameters for the 431. It is as
    widespread as sand on the beaches. A distributor not having any of these sounds
    to me like a grocery that doesn't have salt. There is also the LMV431 version
    which might work for you.

    Regards, Joerg.
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