Flashing output to drive a relay, help please!

Discussion in 'General Electronics Chat' started by jay4708, Jul 28, 2014.

  1. jay4708

    Thread Starter New Member

    Jul 28, 2014
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    Hi

    My electronics are a little rusty, I need a bit of help with the following if you don't mind, thanks all.

    I have an output that once triggered pulses low with respect to +24vdc as shown below:

    (The lamp is just a load for this example drawing)
    [​IMG]

    What I need is for that flashing output to drive a relay for as long as the output is active, I don't want the relay to turn on and off with the output I need the relay to be on whilst the output continues to flash.

    Your input is much appreciated thanks.

    John
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    Can the pulse sink enough current to drive relay directly?
     
  3. jay4708

    Thread Starter New Member

    Jul 28, 2014
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    Yes but I need the relay to stay on and not go on off on off ....
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
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    If source is stiff enough you just need a 1000uf or so across a small relay coil to keep it on for a 1/2sec.

    If not, and the high charging current could damage the source, you will need a transistor or ic driver and access to the +&-24
    vdc.
     
  5. jay4708

    Thread Starter New Member

    Jul 28, 2014
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    Do you mean like this:

    [​IMG]

    I would be interested in seeing a circuit of how I could get round this with a transistor just in case the output is not up to driving the relay and yes I do have access to the +24vdc and ov, there is no -v.

    John
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    When converting a pulse train to a steady state signal, a common issue is turn-off delay. Whatever circuit you end up with, it is functioning as a missing-pulse detector - the relay turns off when the pulses go missing. The problem is how long to wait to make sure the pulses really have stopped. With a high speec digital counting circuit you still have to wait longer than 1/2 cycle to be sure. Analog timer circuits like putting a cap across the coil work very well, but the cap probably has a 20& tolerance so you have to oversize it to make sure it never is too small. So, what kind of turn-off accuracy do you need?

    ak
     
  7. jay4708

    Thread Starter New Member

    Jul 28, 2014
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    The turn off accuracy doesn't need to be great, I don't mind the relay staying on for a second or two.
     
  8. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    Put a diode in series with the relay
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
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    What is producing the pulses and what voltage are they?
     
  10. jay4708

    Thread Starter New Member

    Jul 28, 2014
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    The pulsed 0v is an output from a nurse call system control board, when an emergency call is activated the output is pulsed low to 0v and lamps are energised with respect to +24vdc.

    Thanks for any help.

    John
     
  11. inwo

    Well-Known Member

    Nov 7, 2013
    2,435
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    If pulse drives lamps directly, then it should supply enough average current to hold in a small relay.

    If a second or so extra hold time isn't issue, then try the cap.
     
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