add delay to relay

Discussion in 'The Projects Forum' started by jls667, Aug 11, 2013.

  1. jls667

    Thread Starter Member

    Jun 5, 2013
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    How would I add about a 1 sec delay to a relay? The relay coil is 12vdc and the relay is switching a 120vac 7amp motor. The coil will not be switched with anything more fancy than a manual switch.
     
  2. jls667

    Thread Starter Member

    Jun 5, 2013
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    I forgot to mention that I need an on delay, not an off delay for this project.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Since you already have 12VDC available, you have several options. One is to put a resistor and capacitor in series - an RC tank - and use the voltage on the capacitor to trigger a transistor. The transistor needs ~0.7V to turn on, and with the right choice of R & C, you could prevent that until the capacitor has reached that threshold.

    A more elegant solution uses the 555 timer IC. This would give more precise control of the delay time.

    Since the witch is manual, can we assume switching frequency is low?
     
  4. jls667

    Thread Starter Member

    Jun 5, 2013
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    The timing does not need precision. I just want another relay to come on first.
     
  5. Mussawar

    Member

    Oct 17, 2011
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    Take a look at attachment. If you want to switch b/w delay ON and Delay OFF, just interchange the places of R and C. Try with 4k7 and 100uF. Good Luck :)
     
  6. #12

    Expert

    Nov 30, 2010
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    We have done things like this before. This drawing shows that not using a transistor results in having to buy a huge capacitor. Go with what wayneh said in post#3.
     
  7. jls667

    Thread Starter Member

    Jun 5, 2013
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    I just bought a 555 timer chip. Is what I described called a Monostable mode of a 555 chip? I am a bit confused by the term one-shot pulse.
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Yes, one-shot is the same as monostable.

    If you have a spare pair of contacts in the first relay, you could have it turn on the second: no timing required.
     
  9. jls667

    Thread Starter Member

    Jun 5, 2013
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    #12

    I liked your idea for a delay that did not need a transistor. What size capacitor and resistor would I need for a small delay of 1/2 a second or so?
     
  10. #12

    Expert

    Nov 30, 2010
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    The math is on the drawing. Just plug in "1/2" instead of "2".
     
  11. jls667

    Thread Starter Member

    Jun 5, 2013
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    I do not understand where some of the numbers came from in the calculation. Why is R 80 when the resister seems to be 160 ohms. What is v0? My DC source is 12 vdc. Should I stick to a resister of 160 ohms and just use a large capacitor? I may have too many questions.
     
  12. #12

    Expert

    Nov 30, 2010
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    The two resistances in the circuit form a functional impedance of 80 ohms at the top at the capacitor. That circuit won't work for you because you don't have 24 volts to work with. (Yet another good reason to use a transistor as a switch.)
     
  13. LDC3

    Active Member

    Apr 27, 2013
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    160
    Yes, R should have been 160Ω, #12 made an error.
    Vo is the voltage out. In this case it is 8.4V, which will give enough current to energize the relay.
    If your source is 12V, and not 24V, then the number in the equation needs to be corrected.
    Let's see what are numbers work out to be:
    C = -t/R ln (Vo/dV)
    C = -(1/2)/R ln (8.4V/12V)
    C = -(0.5)/R * (-0.3567)
    C = 0.1783 / R
    If you use an 80Ω resistor, then C should be about 2.2mF.
     
  14. #12

    Expert

    Nov 30, 2010
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    Go ahead and try it. I'm just not on my "A" game tonight.
    I should have gone to bed about 2 hours ago.
     
    Last edited: Aug 13, 2013
  15. ScottWang

    Moderator

    Aug 23, 2012
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    An easy way to get 1 sec delay time, I tried it with 5V but not 12V, you can try it and modify the R values to meet for your needs.

    R1 - for charge
    R2 - for discharge

    [​IMG]
     
  16. jls667

    Thread Starter Member

    Jun 5, 2013
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    That circuit looks easy. Thanks.
     
  17. jls667

    Thread Starter Member

    Jun 5, 2013
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    That easy circuit did not seem to work reliably for me. I also tried the 555 timer. For some reason there was no delay. I will have to check my circuit.
     
  18. #12

    Expert

    Nov 30, 2010
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    There is something very wrong with the circuit in post#13. It has a 5 volt supply and a 12 volt relay coil. It simply won't work unless you reverse the connections. Then adjustment of the resistor values could make it work much like the circuit in post#6. Again, you start out with more voltage than the relay needs and waste some of it to get the delay to work.

    The 555 circuit is very much more adjustable with way smaller capacitors. Post your schematic and we'll see if you misplaced something.
     
  19. ScottWang

    Moderator

    Aug 23, 2012
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    Thanks for you pointed out.

    That was a labeled error, because I copied that 12V relay from my another circuit, it may makes some new ee players confusing, but if anyone who had experience about the relay, it is not a problem and knew that kind of the mistake won't harm anything, if you using 12V relay then it just won't work, and I also mentioned that I tested it with 5V not 12V.

    Yes, the circuit is similar as you posted at #6, I think the basic RC circuit can producing a delay time, that is a common sense, I tried it about two years ago, because almost of time that I always used bjt to driving the relay, so I were triing to testing a easy way, will it possible to work when a relay coil through a resistor, will it work properly, the answer is positive, but the resistor can't be too large, otherwise the relay can't get the enough current.

    [​IMG]
     
    #12 likes this.
  20. wayneh

    Expert

    Sep 9, 2010
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    I guess I don't see the attraction of using a 50¢ capacitor and reducing the power to the relay just to avoid using a 10¢ transistor. One less part? Whoopee.
     
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