Can I parallel a relay in this situation?

Discussion in 'The Projects Forum' started by likes_shiny_things, Jul 10, 2013.

  1. likes_shiny_things

    Thread Starter New Member

    Sep 11, 2011
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    The attached schematic is for a stand-by generator I have at my weekend home.

    I want to add a relay in the automatic transfer switch breaker panel. The purpose of that relay is to trigger input circuits on a phone dialer to alert me when the generator has been called into service due to utility loss and again when it has gone into stand-by due to utility having been restored.

    As you can see by the diagram, TR1 is energized by 23 going to ground when the control board senses utility loss.

    My question: If I add a 12VDC relay with the coil powered by 15B and 23 (essentially paralleled to TR1), what are the chances that it could damage the transfer control board?

    (I realize you may not be able to give me a definite answer based on the limited information provided, but what I am looking for is an opinion of the likelihood that adding a relay in this manner will or will not damage associated circuitry. I understand that any action I take based on opinions in this forum are at my own risk.)

    TR1 is 12VDC and the specs say the power consumption at rated voltage is approx. 1.2W

    Thank you for your time - I do appreciate it.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I take it you don't have a schematic of the control board? That would certainly help.

    Without that, I am only guessing, but I doubt that adding another 12V relay in parallel with TR1 will do any damage so long as the current drawn by the relay coil is low. I would put a counter EMF diode across the coil of the new relay.
     
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  3. likes_shiny_things

    Thread Starter New Member

    Sep 11, 2011
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    Thank you for your help, tracecom.

    No, I don't.

    Which diode is typically used for what you suggested?
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A 1N4001, or any 1N400x would do.
     
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  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    with the cathode of the diode towards positive
     
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  6. likes_shiny_things

    Thread Starter New Member

    Sep 11, 2011
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    In the original post, I wrote:

    I cut the utility to cause the generator to start and measured #23 to ground and it read about 1.2 K ohms.

    I expected it to read much closer to zero.

    Does this mean I cannot parallel the second relay as I had originally asked?
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Is this a contactor or just a DPDT relay, if the former, you can often get auxiliary contacts to add.
    If it is a DPDT relay, could you not replace it with a 3PDT relay?
    Max.
     
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  8. likes_shiny_things

    Thread Starter New Member

    Sep 11, 2011
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    That's probably what I'll do - I hesitated to do that originally due to physical constraints (it's difficult to get to).

    Given the reading of 1.2K, should I rule out the idea of a second relay?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You could use a small solid-state relay such as this, which requires only a minimum of 2mA to turn on.

    You just need to add a resistor in series with the control input to limit the control current to about 4mA (≈2.7kΩ for 12V control voltage).

    Be sure to observe proper polarity for the control voltage as noted on the relay schematic.
     
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  10. cornishlad

    Member

    Jul 31, 2013
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