Time delay circuit

Discussion in 'General Electronics Chat' started by davidw, Jan 8, 2010.

  1. davidw

    Thread Starter Member

    Jan 12, 2009
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    Hello everyone. Looking for some assistance designing a circuit.

    I am trying to set a time delay for turning on my air handler fan for heating to allow the heat exchanger to heat up before the fan turns on. It takes about 6 minutes from when the thermostat calls for heat to when hot air starts coming out of the ducts.

    The G terminal on my thermostat goes to +24VDC when it calls for the air handler fan to turn on. The B terminal goes to +24VDC at the same time to call for heat.

    The timer circuit should do the following:
    1. When B terminal goes +24V, interrupt the G terminal signal and wait around 5 minutes (best if that is configurable between 3 and 7 minutes)
    2. When the time delay completes, put the G terminal to +24V.
    3. G terminal should go back to off when no signal is received on B terminal (or G would work too I think).
    Can anyone help with a hopefully simple circuit to accomplish this? I am ok with creating the circuit but a list of the specific components with a diagram would be great. Thank you for your help!

    David
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    What kind circuits do you want to use. I always do this kind of work with a cheap microcontroller. But not all are comfortable with such devices
     
  3. davidw

    Thread Starter Member

    Jan 12, 2009
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    Yeah... Not sure I am so comfortable with a microcontroller. I would assume I would have to program it? Are there other simple options?
     
  4. VoodooMojo

    Active Member

    Nov 28, 2009
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    there are many easy solutions.

    what are the specifications of the fan motor?
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    Yes you have to program it ;) But you can easily use some timer circuits and logic gates. I would guess a 555 timer in monostable configuration, some simple logic gates, and a few transistors would do the job. But I leave the scene to experts in this field ;)
     
  6. davidw

    Thread Starter Member

    Jan 12, 2009
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    Hi, the fan specs are below. Not sure why you need that though?

    Volts: 200 - 230
    Ph/Hz: 60
    F.L Amps - L.R Amps: 6.8

    Anything else you need on the specs?
     
  7. VoodooMojo

    Active Member

    Nov 28, 2009
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    I was thinking of another route to do this. That is why I asked for motor specs. Lets disregard that for now.


    take a close look at this kit and the assembly and let us know if you are comfortable with assembling it or another like circuit.
    It cost about $15.00

    this is for use with a 15 volt supply but we can work with that.
    Going from 24 to 15 is not much of a project.

    The relay included with the kit can handle 10 amps at 28 volts.
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    If I understand the setup correct, so is the fan controlled by +24 volt signal from the thermostat. I guess the +24 volt activate some sort of relay.
     
  9. VoodooMojo

    Active Member

    Nov 28, 2009
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  10. davidw

    Thread Starter Member

    Jan 12, 2009
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    Yes. The fan is controlled by the +24V signal, so it must have a relay or something since the fan is 240VAC

    That first circuit kit as a whole bunch of solder points! I am not so great at soldering, but I could probably get it done!

    That second kit looks more complete. But I need the signal line to control the fan line. Can I do that with this kit?
     
  11. VoodooMojo

    Active Member

    Nov 28, 2009
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    yes,
    the http://www.smarthomeusa.com/ShopByMa.../Item/ELK-960/ will control the 24 volt output that leaves terminal G and energizes the existing relay or other component that in turn runs the fan motor.

    it is complete, and maybe UL approved!

    especially when you consider what might happen if the fan did not kick on.
     
  12. davidw

    Thread Starter Member

    Jan 12, 2009
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    So I can use this to read a signal from terminal B to control a seperate signal from terminal G? Not sure I see that on this model. Where would I hook the lines up to? Sorry for my lack on knowledge on this! Thank you!
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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  14. VoodooMojo

    Active Member

    Nov 28, 2009
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    if you look in the manual you will see the jumpers.
    JP1 will be set to minutes
    JP2 doesn't matter
    JP3 will be set to END
    JP4 doesnt matter.

    the trigger input is not used.
    minus will be common on your existing unit and the timing board.
    the plus will be fed from your terminal B
    the relay connection will be where the wire removed from G will go. Connect a jumper wire from the plus terminal of the timer board to the common relay terminal.


    I called the service department at the company. 828-397-4200
    very good service team.
    They said that what you are doing is done on a routine basis.

    I can draw this up if you like.
     
  15. t06afre

    AAC Fanatic!

    May 11, 2009
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    Would it not be better to put some heat sensing device on the heat exchanger. As you want the fan not to be started before the heat exchanger is warmed up? So for the fan to start the thermostat must be on AND the heat exchanger warmed up.
     
  16. VoodooMojo

    Active Member

    Nov 28, 2009
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    that is always an option.
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
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    Back in the old days, there was a spiral wound thermal sensor that sat between sections of the heat exchanger. As it warmed, it turned a cam which acted on two limit switches. The first switch it hit turned on the fan and IF it went all the way to the second switch, the burner turned off since this was considered above the highest safe temperature for the furnace.
     
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