Please critique my fault tester circuit

Discussion in 'The Projects Forum' started by strantor, Apr 30, 2012.

  1. wayneh

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    Sep 9, 2010
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    Adding a little hysteresis to your comparator might help with the chatter. Something like a 1 to 5MΩ resistor from the output back to the input.
     
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  2. strantor

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    Oct 3, 2010
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    From the output back to the + input or the - input?
     
  3. strantor

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    I tried 2Mohm to + input, and I tried 1Mohm each to - inputs, and did not notice a change in relay chatter.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    The hysteresis resistor goes back to the non-inverting, + input. The idea is it adds a bit of the output - either state - to the input, but at high impedance. That requires the input signal to overcome the feedback before it can trigger a flip to the opposite state. Try a 100k and see if you can tell a change.

    Another source of chatter that I've fought with was due to wimpy ground wires for the load, like maybe a shared ground wire for the load and for the PCB. So every time the load would start to kick on, "ground" would shift up a few mV. Doesn't sound like much, but adding a simple wire fixed it. Always good to isolate the brain from the load as much as possible.
     
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  5. strantor

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    Will do.
    That's probably it. The whole thing is powered with 30-something AWG wires. Will upgrade tomorrow

    Thanks for all the help BTW.
     
  6. strantor

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    Oct 3, 2010
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    I put my scope between the GND of my relay switching transistor and the GND of my power supply, and noticed a 1.2V noise while the oscillations were occuring. I replaced the ground wire with a better one, and got it down to 1V. I replaced the V+ wire and they went down to .5V, but still oscillating. Tried the 100Kohm hysterisis resistor, no difference. I put .1μF caps between my opto output and +V and oscillations went away. Funny, now I remember having the same problem with my arduino model; when I was watching the input from the optos, it would swing wildly (maybe 100 points out of 255) before I put the caps in; afterwards I got a nice steady output. Not sure why I decided to omit those caps in the new design. So it's working good now, just need to draw up & (try to) make a PCB for it.

    Thanks again!:)
     
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