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Zener diode problem

Discussion in 'General Electronics Chat' started by kpatz, Aug 4, 2010.

  1. kpatz

    kpatz New Member

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    I'm working on a project where I need a 3.3V reference for a PIC A/D. I tried using a BZX55B3V3 which is a 3.3V Zener using a circuit like below:

    [​IMG]

    Initially I tried it with R=10K and Uin = 4.5-5V. In theory, Uout should be 3.3V under no/minimal load, right? I got closer to 2.2V. I tried reducing R to 1K and that got me somewhere in the 2.8-3 volt range. When I used a 9V battery for Uin and R=470Ω I got around 3.5V, with about 10 mA draw.

    So what gives? Why would a 3.3V zener conduct at 2.2V at low current levels? The only thing connected at the output was my DVM, so it isn't an output impedance issue loading the circuit. Since this is a battery-operated circuit, Uin can vary depending on battery strength, and I don't want my reference to be drawing several mA of current to get the correct voltage, but I need my reference voltage to be reasonably stable. I couldn't find a precision shunt reference @ 3.3V that was in stock, which is why I went with the zener.
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Don't use a zener, use a precision voltage reference - check my PIC analogue tutorial for an example (you can add a preset to make it adjustable if you really need 3,3V? - or just add an attenuator to drop the input voltage).
     
  3. ronv

    ronv Well-Known Member Most Helpful Member

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    Zeners, especially the lower voltage ones don't have as sharp a knee as the pictures show. Check the "test current" in the data sheet. My bet is it is 5ma. -- So 240 ohms for your 4.5 volts would do it.
     
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    As Ron said, to get anything like the rated Vz, the current through the Zener must be 5 to 10mA.

    While not as good as a REF01, an LM/TL431 will do quite well. You can set its output voltage to 2.495V on up...
     
  6. kpatz

    kpatz New Member

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    Maybe I'll use this in my next revision. It's hard to breadboard SOT23 parts though. Too bad they stopped making the TO-92 version.

    I have precision 3V references in TO-92 already, I'll just use one of those for now. It's close enough to 3.3V, I can compensate in the firmware.
     
    Last edited: Aug 4, 2010
  7. nickelflippr

    nickelflippr Member

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    Some Pics have an internal Fixed Voltage Reference (FVR) that can be assigned to the a-d module, like the 18f14k50 for instance. Looks like the new enhanced midrange (16F1xxx) devices have this too
     

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