Convert 0 5V PWM signal to +2.5V -2.5V signal

Discussion in 'Embedded Systems and Microcontrollers' started by hengis, Sep 9, 2011.

  1. hengis

    Thread Starter New Member

    Sep 9, 2011
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    I wish to take a 1kHz PWM signal and create a symmetrical 2.5V signal from it. I do not want to have polarisation effects altering my measurements. The frequency if that is the correct term is about 1kHz Any help would be appreciated
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Do you have +2.5v and -2.5v supplies available?

    If not, do you have at least a negative supply available that is <= -2.5v?
     
  3. hengis

    Thread Starter New Member

    Sep 9, 2011
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    Thanks for your quick reply. No to both questions. I would have to add a circuit to create such a voltage.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    A virtual ground might be a solution.

    Why do you need a +2.5V and a -2.5V signal?
     
  5. hengis

    Thread Starter New Member

    Sep 9, 2011
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    I want to measure water content (wetness) in plant pots. A dc signal causes polarisation about the electrodes. Using an alternating +2.5V pulse followed by a -2,5V pulse gets over this problem. Using a MCU is more practical in a greenhouse particularly as I am reluctant to "play" with mains voltages in a greenhouse with water and dampness all around.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Just insert a series capacitor. That keeps out the DC but for the small leakage current of the cap itself.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    But if it's PWM you'll have more charge transfer in one direction than the other, unless the duty cycle is exactly 50%, i.e. a square wave.
     
  8. ErnieM

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    Apr 24, 2011
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    That is not true.
     
  9. John P

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    I don't see how it can not be true. Suppose you have +2.5V feeding through a resistance to Gnd for 9 seconds, followed by -2.5V through the same resistance to Gnd for 1 second, i.e. a very slow PWM signal. Won't the ratio of the amount of charge transfered be the same as the ratio of time intervals, 9:1? Note that I'm talking about "charge" rather than "current" because while it's flowing, the current is constant. And I assume it's integrated charge that affects the electrodes chemically.
     
  10. ErnieM

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    Apr 24, 2011
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    As a capacitor is incapable of passing a DC current then the " integrated charge that affects the electrodes chemically" is zero irregardless of the driving signal.

    Aside: While it is possible to get a constant current to flow thru a capacitor, you need to drive it in a very special way to accomplist this, or in other words, the current in a cap is only a constant in very special circumstances. A linearly changing voltage can accomplish this.
     
  11. John P

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    Oct 14, 2008
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    Oh, you're assuming that a capacitor is in the circuit. I was assuming it wasn't. Mystery solved.
     
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