would zener work as pull-down?

Discussion in 'General Electronics Chat' started by palkapalka, Mar 8, 2016.

  1. palkapalka

    Thread Starter New Member

    Aug 15, 2015
    11
    0
    Hi,

    Would zener work as pull-down for ADC0 node?
    I want to have zero on ADC channel with nothing connected to P1.
    Zener is SMAZ5V1
    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    No, that's a job for a resistor. If accuracy is a specification, you might have to account for the voltage change caused by installing a voltage divider. It depends on the rest of the circuit.
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    When I think of a pull-down, I think of something that is going to pull/force the voltage down to 0 V. A resistor provides this function.

    The zener diode will not pull the voltage down to 0 V, but could stop the voltage from going above 5.1 V. Other voltages also possible.
     
  4. palkapalka

    Thread Starter New Member

    Aug 15, 2015
    11
    0
    Thank you for prompt responses, guys.
    Yes - this is a schematic for overvoltage protection.
    Now I have it working without R and D - and ADC reads 4.9V if nothing is connected to P1. If something (battery) is connected the voltage is correct.
    Now I want to upgrade it to have overvoltage protection with zener and I also want to avoid the situation with non-zero reading with floating pin1 on P1.
    My though was that since zener conducts slightly (reverse current) it would make pin1 on P1 to be at ground level with nothing connected to P1.
     
  5. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    You do over voltage protection with a resistor in series with the input and two normal diodes connecting from the signal line to the +5V power supply line and the ground line.
    Then you add a resistor from the input to ground, just enough to drain the leakage current of the ADC input pin. Might be over 100K. Read your datasheet.
     
    palkapalka likes this.
  6. palkapalka

    Thread Starter New Member

    Aug 15, 2015
    11
    0
    That is another way of doing this.
    This is a very good explanation - thanks. ADC data sheet says 10uA - so 100K works. I will put it parallel to zener:
    [​IMG]
     
  7. #12

    Expert

    Nov 30, 2010
    16,257
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    Watch that input quality if the circuit is going to run hot. Leakage currents increase with temperature.
     
  8. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    The zener does conduct slightly, reverse current. However, when it does, as you see in your results, the voltage across it is 5V, not 0V as when you have a very small current through a resistor. Your zener diode does not follow Ohm's law like a simple resistor.
     
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