LMC555 timer questions

Discussion in 'General Electronics Chat' started by Fred_S, Nov 7, 2006.

  1. Fred_S

    Thread Starter New Member

    Nov 7, 2006
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    I have a timer that keeps failing with local transients and have a few questions.

    First: What if any is the normal failure mode ? i.e. If the device gets knocked
    out will the output stay high or low.

    Second: Does the control pin (5) need to be tied to gnd or bypassed as I see in the datasheet test circuit (.001uF cap)

    In our application the 555 is driving a current control power fet and it's possible causing the fet to blow (violently I might add :eek: )

    Any help would be great,

    thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Hi,

    Any chance you can post up a schematic? We really can't give an answer with one.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    Just as beenthere indicated, We are going to need a schematic if we are to have any chance at all of assisting you in getting to the bottom of your problem.

    hgmjr
     
  4. Fred_S

    Thread Starter New Member

    Nov 7, 2006
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    Thanks for the replay, I can't release the entire drawing without a bunch of hassle but here are the assosiated parts of the 555. Notice the discharge and modulation pins are floating, think this is a problem ?

    I'm just troublshooting this device so I'm not exactly sure what the original engineers intent was but if the 555 output stays HIGH for an extended length of time this would probably cause the power FET to blow as it's driving a pretty low R coil.

    Thanks again for the help, hope this image link works.

    Fred

    [​IMG]
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Do the voltage swings of the 60Hz or the LM358 output take the inputs of the 555 outside its supply rails?
     
  6. Fred_S

    Thread Starter New Member

    Nov 7, 2006
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    Pebe - The trigger gets it's signal from a comparator with the same VCC so I don't think so aside from possible transients.


     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
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    And what about the 50Hz?
     
  8. Fred_S

    Thread Starter New Member

    Nov 7, 2006
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    I'm sorry pebe I don't understand your last post. You asked if the voltage swings take the 555 trigger out of the supply rails, I answered no exept for possible switching trantients from the comparator. The trigger is fed from the LM358 cinfigured as a comparator. Both are powere from a 12V Vcc so the input never sees the 60HZ line, only a 6V squarewave.

    What about 50Hz ?

    Again my concern is the floating discharge (pin 7) and control voltage inputs (pin 5) of the 555 and the possibility that a failure of the LMC555 or latchup could cause the output to stay high.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    Greetings Fred_S,

    I think pebe was also asking if the 60Hz signal source feeding the reset pin is exceeding either of the rails of the 555 timer.

    As for the control pin 5, you should NOT tie it directly to ground. Most datasheets I have encountered on the 555 timer recommend a bypass cap from pin 5 to ground. I think it would be a good idea to take this precaution in you circuit.

    As for the discharge pin, you can leave that pin unconnected since it is an uncommited or open-collector NPN transistor. It is not likely to cause your circuit to misbehave.

    hgmjr
     
  10. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Hi Fred,
    Yes, sorry. As hgmjr said, I was referring to the 60Hz input tor the reset pin. Being in the UK using 50Hz mains, that was a slip of the pen (or rather, finger). If that 60Hz source is centred on 0v then it may damage the 555.
     
  11. Fred_S

    Thread Starter New Member

    Nov 7, 2006
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    0
    Thanks for the replies guys, yes I did see that (.001uF cap) in the datasheet as well, will try that but it seems my problem may not be transients as we originaly suspected. To pass on some interesting results of a test I hit several parts of the device with as ESD tester including several direct hits on the floating and used pins of the LMC555 and not one failure. This speaks well of the 555 and I'm suprised considering it's the CMOS version. I did blow the LM358 though but 10-15KVDC is a pretty rough test.

    Thanks again for the input, I'll keep trying to make this thing fail..

    Fredrick
     
  12. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    Presumably the 60Hz square wave is mains-derived. I would not be happy with it being fed directly into the reset pin unless I was sure that there could be no transients on it. It might be wise to feed it via a 100K resistor to give the protection diodes in the 555 a chance - it will make no difference to the circuit operation.
     
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