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Low Zener Voltage?

Discussion in 'Electronic Basics' started by eromlignod, Jul 29, 2007.

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  1. eromlignod

    eromlignod Guest

    Hi Guys:

    This is a simple one. I've got a 0 to 5V square wave that I want to
    convert to a 0 to 3.3V square wave. I tried putting a series resistor
    and a 3.3V zener diode to ground.

    The wave stays nice and square, but it comes out as 0 to 2.0 volts. I
    have tried varying the value of the resistor, but it doesn't help.
    What am I doing wrong? Is there a better way to do what I'm trying to
    do?

    Thanks for any replies.

    Don
     
  2. A better answer depends on several things. What is the
    frequency of the square wave? How fast must the rise and
    fall times be?

    What is the source of the 5 volt wave? How much load can it
    tolerate before sagging too much?

    What load will be places on the 3.3 volt output?

    Some possibilities:

    2 resistor divider.

    Resistor and green LED divider.

    3.3 volt linear regulator, e.g.:
    http://www.zetex.com/3.0/pdf/ZMR330.pdf

    Resistor and 3.3 volt shunt regulator.
     
  3. Jamie

    Jamie Guest

    Bad Zener?, not enough Resistance and there for you're loading the
    output source down to much? or maybe you have a capacitance problem?

    You should first verify that the zener is operating correctly.
    connect it to a supply via a resistor that places the zener in it's
    normal operating range of current. Then perform a Voltage test across
    it.
     
  4. Phil Allison

    Phil Allison Guest

    "eromlignod"

    ** You did not READ the spec for your 3.3 volt zener.

    Eg:

    The 1N746A is a 400mW, *nominal* 3.3 volt zener, with rated test current of
    20mA !!

    The 1N4728 is a 1 watt, *nominal* 3.3 volt zener with rated tests current
    of 76mA !!!!!

    At low current of say 1 mA , the conduction voltage of either type may well
    be only 2.0 volts.

    Low voltage zeners are *darn soggy * and vague voltage references.

    Change the zener to one with a higher * nominal* rated zener voltage, till
    you get the 3.3 volts you want.



    ........ Phil
     
  5. Eeyore

    Eeyore Guest

    Zeners are very 'sloppy' at regulating voltages. You're putting too little
    current through the zener.

    An 'active clamp' is perhaps the best answer (or a dedicated buffer for voltage
    translation) but do you need to implement this with very few parts ?

    Graham
     
  6. Eeyore

    Eeyore Guest

    Trust you not to have a clue.

    Graham
     
  7. neon

    neon

    1,325
    0
    Oct 21, 2006
    no there are ot bad zeners they jus do not exsists for your application. there are some ref diodes around 2.5 but they are not zeners as diodes. what you want requires an active device
     
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