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strange feedback

Discussion in 'General Electronics' started by bryan, Jul 27, 2004.

  1. bryan

    bryan Guest

    I have a counter that shuts off a relay . Whenever I send 110vac
    through the relay the counter malfunctions. There are 2 transistors
    and 2 schmitt triggers between the counter and the relay. I have a
    diode across the coil of the relay so I don't understand why it does
    this. If there is no 110vac going through the relay the counter works
    perfectly.Why does the 110 affect the counter and how do I fix it?
  2. Robert Baer

    Robert Baer Guest

    Insufficent and contradictory info.
  3. bryan

    bryan Guest

    (bryan) wrote in message
    Why would 110v going through the contacts of a relay affect the
    circuit that drives the coil of the relay?
  4. Robert Baer

    Robert Baer Guest

    As long as the two circuits (the coil circuitry and the contact
    circuitry) are completely different, then there would be no effect as
    you describe.
    Now if the coil is driven by the same 110V supply that is switched,
    there still would be no interaction as you intimate, but one could see
    some correlation.
    For example, the contacts are controlling a capacitive or inductive
    load, so the "make" or the "break" spike would be seen on the supply to
    the coil - because it is the *same* supply to the non-resistive load.
    If the supply has some (relatively) appreciable internal impedance,
    the switching of the load by the contacts then would reduce the
    measurable supply voltage (the loss would be "inside" the supply), which
    is also supplying the coil.
    BUT in either case, the current thru the contacts are actually *not*
    affecting the coil circuitry.
    What you see are the results of the loading of the (coil) supply.
  5. bryan

    bryan Guest

    Thanks , I decided to change my circuit. Rather than use the 74161
    counter I am using a 7474 to make a counter . The problem went away.
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