Need help with a project please

Discussion in 'The Projects Forum' started by chris f, Aug 2, 2013.

  1. chris f

    Thread Starter New Member

    Aug 2, 2013
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    Im working on a timer circuit but the problem i have sometimes is when load is applied to the coil on the relay it wont stay activated long enough to break the circuit and it just resets the time all over. i was wondering if i should ad a capacitor inline ? any help would be appreciated thanks .
     
  2. #12

    Expert

    Nov 30, 2010
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    ICM timers that I know about work with AC. You haven't said if this is AC or DC. What are you trying to accomplish? How is the timer supposed to work? Delay the start-up after power is applied, or something else?
     
  3. chris f

    Thread Starter New Member

    Aug 2, 2013
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    im sorry its 120 volts ac . the contactor block controls a three phase motor. the way its suppose to work is when the operator hits green start switch. that sends current to coil on contactor block wich in turn pulls in contactor on side on block. thats where it holds and also send current to the 120 volt ac input on the timer. when desired countdown is acheived it sends 120 volts load to coil on relay which interrupts whole circuit. But the problem i have is it seems like some times it wont hold the relay on long enough to break the connection.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    It seems to me that you have set up a race condition.

    When the time expires, it sends voltage to the relay which opens it's contacts. So now both the contactor and the timer lose power. But if the timer loses power first, then it releases the relay, which sends power back to the contactor's side contacts and, if the contactor hasn't released them yet, the contactor and timer are powered back up.

    Since these are AC signals, I don't think a capacitor is going to be of much help. But maybe I'm wrong.

    To eliminate the race condition, you might use an approach that I developed back when I had to do asynchronous event-driven logic in CMOS ICs that had no clock (or that had to squelch the clock during noise-sensitive processing). Basically, I said that if Event A needed to make Event B happen, then I would use logic that once Event A was detected would latch a signal that would continue telling Event B to happen until the detection that Event B had actually happened told the latch that it could shut up. I called this an "event handshaking protcol" (and I'm certain I'm not the first person to come up with it).

    So if you can get a contactor that has a NO DPST set of contacts, then you might wire it like the following:

    [​IMG]

    The idea is that as long as the contactor is closed it will continue to provide power to the timer. When the timer expires it tells the relay to interupt power to the contactor, but it does NOT interrupt power to the timer. That only happens once the contactor responds to the relay's action and actually opens. Only then does the timer stop powering the relay and it again sends power back to the one set of contacts on the side of the contactor, but at that point the contactor has already opened.
     
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  5. #12

    Expert

    Nov 30, 2010
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    There are other ways to wire this if you don't have enough N.O. contacts on the contactor, but WBahn has spotted the problem (and saved me the work of drawing it for you). Add another relay in parallel with the contactor coil if you need more N.O. contacts.
     
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  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    If you can't get any AUX contacts for the contactor and you don't want to spend any more money, you could make the CR latch in the off condition with the modification shown in the red box (I'm assuming that your relay has a NO contact and you just didn't draw it). This should achieve your needs, just with the added caveat that now once your operation has timed out and you want to restart it, you need to press the stop button first.

    [​IMG]





    EDIT: I guess this also depends on whether or not your relay makes a full switch. I guess it's possible that the contact might not make it 100% of the distance from the NC contact to the NO contact before the power is removed again.
     
    Last edited: Aug 2, 2013
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  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I am wondering why Ground is used as a conductor, this should never be done in a mains fed circuit?
    Unless this is a wrong expression of neutral?
    Max.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    It is almost certainly the neutral.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Probably, But not at all evident from the drawing? ;)
    In fact the reverse.
    Max.
     
  10. chris f

    Thread Starter New Member

    Aug 2, 2013
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    ok so after messing around and trying the different methods that you guys helped with i still had similar results. but after i powered the timer from a seperate source it works perfectly. so im wondering if i am pulling to much current away from the relay coil from powering the coil on the contactor since it was on the same line. i noticed when i tested voltage on the load on the timer it read 108 volts (not enough to fully power the relay?) any thoughts?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You should be getting full 120v across the timer?
    Check the other side of the L3 contactor coil, where is the top end of L3 fed from?
    IMO you should not be getting any appreciable volt drop in that circuit?
    Max.
     
    Last edited: Aug 7, 2013
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Any update? :confused:
    Max.
     
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