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Common ground for different voltages in relay circuit

Discussion in 'General Electronics Discussion' started by towijo, Feb 17, 2013.

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  1. towijo

    towijo

    5
    0
    Feb 16, 2013
    Hello,

    My goal is to control a 12 VDC device (on/off) from a microcontroller using a relay card. The relay needs 12 VDC operating power (I use the same 12 VDC source for both relay and "target device") and then i should use a small current to turn the switch on/off. This small current share the same common (ground) as the larger (12 VDC) current.

    Maby this is pretty straight forward but is it safe to share the common (ground) with my microcontroller and the larger voltage? The microcontroller seems kindof fragile.

    I have attached schematics on how i think it should be. Correct me if im wrong.

    Regards
    Towijo
     

    Attached Files:

  2. pwdixon

    pwdixon

    52
    0
    Oct 14, 2012
    There's no real reason for there to be any problems with a common ground, in fact there must be a common ground for this circuit to work at all.
     
  3. towijo

    towijo

    5
    0
    Feb 16, 2013
    ok, sounds good. Thanks for the input! so i guess the schematics looked ok?
     
  4. pwdixon

    pwdixon

    52
    0
    Oct 14, 2012
    I didn't say the circuit was good just that the common ground shouldn't be a problem.

    You circuit may well be ok but I'd need to do much more research on your parts before I could endorse your schematic I'm afraid.
     
  5. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    At bare minimum you need flyback protection on the motor...

    You also should have smoothing/decoupling capacitors on the supplies especially with chatter the relay will be making...

    The 5VDC supply can also be pulled off the 12VDC supply using a voltage regulator instead of another power source...
     
  6. pwdixon

    pwdixon

    52
    0
    Oct 14, 2012
    Looking at the datasheet for the relay board I did notice that the design doesn't include a flyback diode to protect the relay drive transistor so that might be a cause for concern.

    And in the time it took me to look up the relay datasheet and post someone has already pointed this out......
     
  7. towijo

    towijo

    5
    0
    Feb 16, 2013
    OK, thanks a lot guys! im going to read up on flyback protection and smoothing capasitors. and get the curcuit right!
     
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