PID project more confusion about buffers and zeners

Discussion in 'The Projects Forum' started by justtrying, Aug 15, 2011.

  1. justtrying

    Thread Starter Active Member

    Mar 9, 2011
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    351
    As I am trying to put the project together, more things are falling apart. I guess as they should.

    Anyway, this morning I burned through three zeners and a potentiometer trying to get some numbers for the current output of the buffers as related to analog input setup.

    What I get/do not get is:

    it seems to me that current has to be limited at the set-point where zener diodes are. Current coming into the circuit will define the current coming out of the circuit, and you cannot get more than went in ( I am trying to work out a more specific relationship). If zeners fail due to too much current, how do I select the right one? Do I choose one with high power rating? Right now I am using 1N4734 and haven't been able to find information on that.

    The power supply used for this will have at most 0.5 amps of current.

    p.s. I have figured what the zener voltage for these should be, so that is no longer an issue
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    the diodes your referring to are not associated with circuit power, they are reference voltages only into high impedance opamps. You need to give us your choice of resistor values, R26 thru to R30
     
  3. justtrying

    Thread Starter Active Member

    Mar 9, 2011
    329
    351
    resistor values are part of my general problem as well. For now, I have been using 1k for R27 and R29 with a 20K pot for R28. Why am I using these? Not certain. I have tried different values for R26 and R30 to see how it affects current output. My observations are as per my first post.

    Basically, I guess I am not sure how to look at this properly.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Some thoughts in attachment ....
     
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  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    with TNK's info then, you will want isolate this section by removing all your jumps, double checking your values/orientation and measuring to confirm.
     
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