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Back EMF protection

Discussion in 'General Electronics Discussion' started by BodhiSci, Mar 16, 2015.

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  1. BodhiSci

    BodhiSci

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    Jul 4, 2014
    KrisBlue helped me with a project a ways back (thank you btw, I didn't get to reply to last message). Anyways, we used a diode to prevent flyback from the solenoid. (The zener makes the solenoid response quicker, it isn't included in the second picture, but I do have it in some versions of it. Just forget the zener for now though) This electronics guys that works with my school's lab changed it from the first picture's setup to the second pictures setup. It seems to me that the the bottom end of the diode should be connected after the solenoid, not before it like he has it now. I need to make the second version of this and I want to make sure it is correct. Thank you.


    Clip from Kris solenoid pic.png

    TO THIS:

    Solenoid Question.png
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, the second version (the one with the black background) is incorrect.

    As the mosfet switches off, the voltage across the solenoid will rise until the mosfet breaks down (avalanche failure). Some mosfets can handle this at defined power, but it is unlikely that a mosfet in an optocoupler would be avalanche rated. It would probably fail quite rapidly.

    The diode needs to be across the solenoid, NOT the mosfet and the solenoid.
     
  3. BodhiSci

    BodhiSci

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    Jul 4, 2014
    Ok Thanks Steve. Before we settled on the 1N4004. I don't know the solenoid specs off hand. Hopefully that's good. I will change it back, It was probably a misunderstanding.
     
  4. BodhiSci

    BodhiSci

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    Jul 4, 2014
    Would that have worked at all for flyback protection? It isnt across the solenoid so I don't see how.
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    no it wouldnt
     
  6. Kiwi

    Kiwi

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    Jan 28, 2013
    "The zener makes the solenoid response quicker..."
    Could someone please explain this?
    I just use a 1N4007 or 1N5408 diode across solenoids etc.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The current will continue to flow until all the energy is dissipated.

    If you used a perfect diode and your solenoid had a zero resistance the current would continue to flow for ever...

    With a normal diode and a normal solenoid energy is lost due to the resistance in the solenoid and the voltage drop across the diode.

    If you use something with a higher voltage drop the energy will be dissipated faster. A zener diode has the advantage of a constant voltage drop so the maximum voltage is limited but the voltage remains high even as the current drops.
     
  8. BobK

    BobK

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    Jan 5, 2010
    Steve.

    Are you sure it is just energy dissipation through the internal resistance?

    I always thought it had to do with the equation for the voltage across an inductor:

    dI/dt = V / L

    A larger voltage across the inductor means a faster decrease in the current. Though, it makes me wonder where the energy goes. Perhaps faster changing current means more radiated energy?

    Edit: Okay, never mind, it is the voltage drop across the diodes that dissipates the energy, not the internal resistance.

    Bob
     
    Last edited: Mar 18, 2015
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Thanks for posting the actual math involved Bob.

    As it illustrates, the rate of change in current is proportional to the voltage.

    A normal diode has a low voltage drop causing the current to fall slowly and thus the solenoid to drop out slowly.

    A zener has a higher voltage drop and thus the rate at which the current falls is faster leading to the solenoid dropping out faster.

    Without any current path being provided the solenoid will drop out extremely fast BUT consequently the voltage is extremely high. This spike of voltage will be enough to "find" a path for current. This can cause arcing on switch contacts or breakdown of semiconductor devices. The diode or zener or other means we use is there to limit the voltage by providing a safe alternative path.
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Sometimes we include a low-valued resistor in series with the diode to dissipate the energy stored in the magnetic field of the solenoid. This will also help speed up the solenoid current decay.
     
  11. Kiwi

    Kiwi

    322
    74
    Jan 28, 2013
    Thanks for the replies guys.

    From doing a bit of Googling I see that adding a reversed biased diode will protect the switching contact from high voltage spikes, but can adversely the response time of the solenoid.
    I found this article on the Crydom website. They did a test of the drop-out time and voltage spike for a relay with various suppression methods. The single diode had the lowest voltage spike, but it took six times longer for the relay to drop-out. Might start to run the risk of contact arcing?

    Looks like there is more to it than just chucking a diode across inductive loads.
    Might have to get out the scope and have a play with some relays and solenoids.
     

    Attached Files:

  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You will note that in that document, the use of a diode and a zener give the drop out time as close as possible to that of an un-suppressed load.

    It's a bit of a shame they didn't also try higher voltage zeners.

    The problem with contact arcing happens when the suppressed load is a relay, and that relay is being used to control another unsuppressed load. One cure here is to suppress that load as well.
     
  13. duke37

    duke37

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    Jan 9, 2011
    A capacitor can also be used, it will need to be sized to accept the inductive energy without giving too high a voltage.

    This is the method used in old car ignition systems where the points needed to open without a spark.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    For AC loads a resistor in series with a capacitor is often used. Whilst this does result in ringing, the resistor ensures that it is well damped without being so high a value that it results in a significant voltage peak.
     
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