RS232 Relay

Discussion in 'The Projects Forum' started by booeboy, May 29, 2010.

  1. booeboy

    Thread Starter Member

    May 4, 2010
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    0
    I have a 12 volt relay. It draws 15ma. Is it safe to drive this via rs232 dtr line? Are protection diodes needed? Do I also need a diode on the coil side to keep it from going to negative voltage? Thanks.
    Lyle
     
  2. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    I would not drive a relay directly off of the dtr line, you will most likely fry the usart ic....

    I would drive a transistor with the dtr line to trigger the relay, have the relay powered from a separate 12 volt source...

    B. Morse
     
  3. booeboy

    Thread Starter Member

    May 4, 2010
    31
    0
    What if I used the parallel port and a 5 volt relay? I could power it directly from here? I would drive it from 2 lines. Would this be safe? Are protection diodes needed?
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    :eek:!
    Either way you do it, you should still use some kind of buffer between the port and the relay, never connect a relay directly to a parallel port pin or to a serial port pin, use some buffer IC's such as the 74367 TTL Hex buffer/Inverter IC or similar Newer IC to buffer the port pins....

    Unless you have money to replace the motherboard after you toast its ports....:rolleyes:

    B. Morse
     
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