Digital Circuit fails when put on batteries

Discussion in 'The Projects Forum' started by Don8, Jul 22, 2014.

  1. kubeek

    Expert

    Sep 20, 2005
    5,585
    1,094
    The problem you are experiencing migth be common mode noise present on the batteries. The optocouplers have a bit of capacitance from input to output, which will couple the noise from the switcher through the circuit. I would do what sensacell suggests, especially the schmitt trigger and lower value pullup. Also some HF filtering on the inputs might help. And I would change all the ICs before you try again.
     
  2. Don8

    Thread Starter Member

    Jul 22, 2014
    31
    1
    Yes
     
  3. Don8

    Thread Starter Member

    Jul 22, 2014
    31
    1
    I think this is my problem. I believe the primary and secondary are capacitively coupled. Perhaps the biasing of the battery stack is creating voltages across the optocopler.
     
  4. Don8

    Thread Starter Member

    Jul 22, 2014
    31
    1
    I tried a lower pull up it worked well. The transition time was quicker which meant it spent less time in an undefined region.
     
  5. Don8

    Thread Starter Member

    Jul 22, 2014
    31
    1
    I looked for that. I probed directly to every Vdd and Vss pin. I saw no difference with or without a capacitor over the same Vss and Vdd.
     
  6. Don8

    Thread Starter Member

    Jul 22, 2014
    31
    1
    The ICs stopped failing after I decreased the current driving the optos LEDs and I changed the resistor network to discrete resistors. I believe I was overtaxing the CMOS chips by making them drive 15ma into a LED and the resistor network insulation was breaking down. My last problem was clock jitter. I tried caps to stop it, but it was ineffective. I really wanted the caps to work too since the only other solution was increasing the current through the optocouplers, which I don't believe the CMOS inverter can handle.
     
  7. kubeek

    Expert

    Sep 20, 2005
    5,585
    1,094
    What caps did you try to fix the clock? Slowing the edges rarely ever works, usually just makes things worse. Try adding as schmitt trigger on the clock right after the optocoupler.
     
  8. Sensacell

    Moderator

    Jun 19, 2012
    2,428
    803
    The kind of nasty glitches that the bypass capacitors fix are almost impossible to catch on a scope, that is why I always put them in- needed or not...
     
Loading...