Make a latch with a relay?

Discussion in 'The Projects Forum' started by Man_in_UK, Apr 24, 2012.

  1. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0
    Please can you suggest a solution?

    A long time ago we used to make so much out of relays but my brain is not working as it should. I am sure a simple circuit with a relay or two will fix this problem but I am stuck. I would like to keep it simple with some sort of relay latch for now as whatever I do this week, will no doubt change next week as I discover another problem to solve.

    (the easiest way to describe the function and use of this is to translate the problem into a bulb and fan situation)

    I have a light bulb that comes on every hour and stays on for 10 minutes. On for 10 mins, off for 50 mins. The supply to this bulb also powers a fan. The fan will always be on at the same time as the bulb but I want to be able to cut the fan power using a single push button if I should be standing near it.

    I need a single push button to stop the fan - but 50 minutes later when the bulb comes on again, the fan must run as normal.

    I cannot use a toggle switch as there is a danger of me leaving it turned off accidently.


    Sounds so easy. I know it must be easy to make a single drop out latch using a relay but my head will not get to where it needs to be.

    Please help.
     
  2. cmwilson7

    New Member

    Apr 24, 2012
    9
    1
    Think this is what you are looking for?
     
    Man_in_UK likes this.
  3. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0
    Thank you for posting back to my problem.

    Its funny how the brain works. I had been thinking all day as to how to make a relay drop out, and stay out. Until I went to bed last night and it hit me in the face .... don't drop the relay out, pull it in.

    Now I am frustrated that I came here and asked such a stupid question.

    Thanks again
     
  4. williamj

    Active Member

    Sep 3, 2009
    180
    32
    Man in UK,

    I've been thinking about this thread for several daqys now and I finally figured out what it was that was bothering me. In cmwilson7's diagram the relay is wired normally closed (NC) and the switch appears to be a maintained type. If it is maintained the the circuit works but as pointed out in you original post a maintained switch was undesireable becuase you my forget to reset the switch. If on the other hand the switch in cmwilson7's diagram is momentary then once the switch is released the relay would re-energized and the fan would re-start.

    It is my opinion that two relays (1 spdt, 1 dpdt) would be required to obtain you desired results. The spdt relay would be to seperate fan control from light control thus making it possible to turn the fan off without affecting the light.

    I've attached some diagram to illustrate what I thinking. Hope this works for you.

    williamj
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    I agree

    I disagree.

    [​IMG]

    (one SPDT, one N.O. momentary pushbutton)
     
  6. williamj

    Active Member

    Sep 3, 2009
    180
    32
    strantor,

    Excelent! I knew there was an easier way to do this but for some reason I just couldn't wrap my head around it. Thank you for setting things straight!

    williamj
     
  7. cmwilson7

    New Member

    Apr 24, 2012
    9
    1
    With the circuit I drew up, the relay is latched after the momentary switch is pressed. The normally open contact of the relay feeds the coil after the switch is depressed, energizing the coil initially. Once the source drops out (at the end of the 10 min cycle), the relay is reset. The circuit I drew accomplishes what the ladder logic shown above does. The relay won't "re-energize" (should say de-energize) because it is latched from the N.O. contact after the coil is initially energized from the momentary switch.
     
    Last edited: Apr 29, 2012
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    So it does. Sorry. I guess we both missed that.
     
  9. williamj

    Active Member

    Sep 3, 2009
    180
    32
    cmwilson7,

    Yiu are correct. My apologies.

    williamj
     
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