Mystery grounding issue

Discussion in 'General Electronics Chat' started by jambalaya, Jan 17, 2014.

  1. jambalaya

    Thread Starter New Member

    Jan 17, 2014
    4
    0
    Hi!

    I have a battery operated toy that uses capacitive touch switches. When operated from a 9V switch mode power supply, the buttons behave totally erratically! However, when operated from the battery, it works really well. When operated from a proper big lab power supply, it works really well too.

    Furthermore, if I touch the metal case when using the 9V switch mode power supply, it works really well..

    So, given these conditions above, my question is: what could cause this and how can I fix it so it works with a simple 9V power supply?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    its called common mode noise. smps have a lot of switching transients and hash on both + and _ output lines. the other supply dosnt.
     
  3. jambalaya

    Thread Starter New Member

    Jan 17, 2014
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    0
    But why does everything work fine with the smps, if I am touching the metal case?
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    To start, try connecting a 0.1uf ceramic disc capacitor across the power supply lines.
    Then next, add a 10-100uf electrolytic.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    also, try running both lwires through a ferite noise choke. the problem going away because of touching the metal case is because the case isnt at a real ground. not a wire to ground, more a large surface area ground strap. high frequency pulses and noise are more like rf, and skin effect rules apply. m( skin effect high frequency currents flow through the outer surfaces of a wire, increasing the resistance to high frequency and rf.)
     
  6. jambalaya

    Thread Starter New Member

    Jan 17, 2014
    4
    0
    I tried this but the result is the same (I also think there are caps there for this purpose already).

    The switch mode power supply already has a ferrite choke on the plug.
     
  7. jambalaya

    Thread Starter New Member

    Jan 17, 2014
    4
    0
    Another way to ask the question.. what can I connect to the case instead of my finger, to fix this problem :)
     
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