Open collector to relay...giving me a 12v shock all day long

Discussion in 'The Projects Forum' started by MMMatt, Jun 25, 2014.

  1. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    I have a open collector to a relay and am using a 12v power supply. The first time the open collector opened the relay fired and I got a signal from my contact closure. Now the relay wont disengage and I am getting a 12V shock on either side of my contact closure wires. What have I done? I wired the coil on the relay to + on the power supply and + on the open collector side, then tied the - on the power supply to the - on the open collector. Did I do that correctly?
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
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    If you did not have a diode (1N400x or similar) across the coil of the relay, its likely you fried the transistor when it turned off due to back EMF. Put a diode across the coil, banded end towards the +12V supply.

    Welcome to AAC BTW.

    EDIT: Re-reading your post:
    The circuit should be
    +12V to one end of the coil.
    Collector of the NPN transistor to the other end of the coil
    Emitter to ground.
    Diode across the coil.

    Current into the base of the transistor turns on the relay.
     
  3. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    Ok, lets see if my brain is following you. I did wire it correctly but I need a diode and may have fried the transistor.
     
  4. MrCarlos

    Active Member

    Jan 2, 2010
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    Take a look at the attached image
     
  5. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    Wait so I sent current the other way? Why would this fry the transistor? Shouldn't it allow current both ways as long as the current doesn't exceed its rating?
     
  6. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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  7. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    I really wish I knew what the heck am I doing. I must have gotten some bad advice.
     
  8. JohnInTX

    Moderator

    Jun 26, 2012
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    It does. But what do you do with the energy stored in the coil's magnetic field when it collapses? It generates voltage in the opposite direction - and lots of it. Its not unusual to see several hundred volts across a coil when it releases.

    I'm assuming you are using a circuit like the sketch attached.
    The other attachment explains what happens in a coil when its released and what to do about it.

    Mr. Carlos's circuit is essentially the same as mine but he thinks you might have welded contacts. Could be, but it sounds to me like you fried the driver. Check both.

    It wouldn't hurt to post a sketch of your circuit.
     
  9. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    Ok here is my wiring..
     
  10. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    With any luck I just fried the relay which was a very low voltage/amperage relay to start with.
     
  11. JohnInTX

    Moderator

    Jun 26, 2012
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    Not exactly sure where the diagonal wire connects in your diagram but either my or MrCarlos' driver diagram will work if you add a diode across the relay coil.
    Transistors conduct (safely) only in one direction. The collapsing magnetic field in the coil will cause very high voltages (likely exceeding its rating) which will break the transistor. That's why you need the diode.
     
    Last edited: Jun 25, 2014
  12. MrCarlos

    Active Member

    Jan 2, 2010
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    Sorry but I do NOT understand what you mean by "OPEN Collector"
     
  13. Pencil

    Active Member

    Dec 8, 2009
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    Could you be specific as to what component, IC or transistor
    that you are connecting to? Also what is the pin number
    on the component you are connecting to?
     
  14. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    How would I test the diode to see if its still working?
     
  15. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    Pencil I am not sure how to answer that. The open collector has a + and a - output. As far as the circuit goes from the outputs back into the board IIRC the mfg said there was a opto-isolator. When you are asking for pin number are you looking at my diagram?
     
  16. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    Ok I get voltage going through the collector when its activated so I didnt fry the transistor. Fwew!
     
  17. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    If you have the minus side of the power supply to the collector than what is the emitter connected to? :confused: The power supply minus side should go to the emitter not the collector. And add the diode across the relay coil as suggested.
     
  18. MrCarlos

    Active Member

    Jan 2, 2010
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    134
    Hello MMMatt

    Please, Take a picture with your web cam of what you call "The open collector has a + and a - output" and upload it here.
    Let us see what it is.
     
  19. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    Please forgive my ignorance. I do not know what a emitter is other than the logic of it and where in franksville it is located. Would these simply be going to a ground on the board?
     
  20. MMMatt

    Thread Starter New Member

    Jun 25, 2014
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    Here is the cable and the schematic wiring it into their board. All I am trying to do is get a contact closure without frying anything. This should explain the +/- open collector info
     
    Last edited: Jun 25, 2014
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