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NE555D PROBLEMS

Richard Brune
Prodigy 20 points
Other Parts Discussed in Thread: NE555

I designed a circuit a few years ago that uses an NE555D for a debouncer and I've had some intermittent failures of the timer. Replacement of the timer gets the circuit working temporarily, but the 555 always eventually fails to work, and exibits the following:

Pin 6&7 remain at close to VCC

Pin 2 remains low, and seems to be shorted to ground providing at low resistance (1k) path from pin 2 to VCC does not pull up voltage at Pin 2, enough to reset.

Pin 3 remains high (Near VCC)

Disconnecting power temporarily does not correct the problem, and the above listed conditions are exibited with open connections to pin 2 & 3, through a 1 meg scope or a 10 meg meter.

I'd like to know what causes this so I can provide protection for the 555

Thanks, Richard

  • 0
    TI__Guru**** 157436 points

    Hello Richard Brune,

     

    If the trigger is held low, the output will stay high unless the reset pin is low. The highest priority input is Reset then Trigger and finally Threshold.

    Damage to the trigger pin is either EOS or ESD. Do you have low impedance sources connected to the trigger? Can static voltage sources (people) contact the trigger or get within a couple inches of it?

     

  • 0 in reply to Ron Michallick
    Prodigy 20 points

    Thanks for your reply, Ron.

    I'm considering ESD now, as I've done extensive measurements of the currents through both the input and the output during all the known conditions. There is no opportunity for a person to get within a few inches of the trigger while in normal operation, but there are necessarily long leads going to a set of contactors that initiate the timing cycle. Is there a simple component that can be placed near the trigger that will protect it from static ?

  • 0 in reply to Richard Brune
    TI__Guru**** 157436 points

    There are many ways to pick up transient signals on long cables especially if the cable is not shielded or twisted.

    Having a resistor between the cable and the NE555 trigger combined with either a ceramic capacitor or TVS diode to ground should provide good protection.