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Design Challenge : relay spike protection

Catwell
This is a common problem with relays. The relay coil acts like an inductor when power is removed, and it sends a spike of power back into your circuit. And let's say you are originally putting 5V through the coil. When the power is removed, in some cases, the coil could send a spike of any level, even in the range of 400V, back into your circuit. And the problems cause by that are many, from burning out circuitry to tremendous electromagnetic interference.
 
 
I have seen spikes that oscillate between hundreds of volts positive and negative. Like below:
 
image
The challenge is to eliminate the problem in the cheapest way, with as few components as possible. Let's just say space and money are scarce for this project. And the relay is connected to very sensitive circuitry.
 
 
Use the relay drawing here to draw your ideas.
image
The relay drawing here has NC & NO contacts.
 
Cabe
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  • 0
    Am I missing something here? The usual way is to put a 1N4001 or similar diode reverse biased across the relay coil.
  • 0 in reply to Zad

    Mike,

     

    The problem with just a diode is you only suppress half of the transient wave form.

     

    I have tested relays that were diode protected, and I still get intense interference. In fact, the signals were so strong that it would disconnect my mouse from the computer it was connected to.

     

    Cabe

  • 0 in reply to Catwell

    Hi Cabe,

     

    Could you give us more details as to how you are actuating this relay? Mike's diode recomendation is the textbook way to handle this problem and it's usually not serious you just have to make sure the diode can handle the current of the coil.

     

    Perhaps, there is some detail that we are not aware that would make this solution fail, how are you measuring the waveform? Is the relay AC or DC actuated?

     

    If you let us know we may be able to figure this out.

     

    Best Regards,

     

    Jorge Garcia

  • 0 in reply to Jorge_Garcia

    For the sake of example, I'll use the one I noticed this problem on:

     

    24 VDC control voltage

     

    I watched the transient wave form on an oscilloscope.

     

    Cabe

  • 0 in reply to Catwell

    Hi Cabe,

     

    This is interesting. The textbook solution to this problem is to place a diode reverse parallel just like Mike mentioned and I know it works because I've used it myself. The waveform used has to be taken with a grain of salt for a few reasons. If you place a diode like mike has mentioned you won't get any ringing(maybe a little but nothing like the waveforms your seeing) because the diode will provide a path for the inductor current to dissipate hence the inductor voltage isn't going to sky rocket trying to maintain current flow.

     

    I can't think of a way to test it, but I think that the waveforms your obtaining have something to do with your scope. Nothing inherently broken with your scope but just thinking about the front end of the scope. We generally model an ADC as a parallel resistor capacitor combination, when you connect this in parallel with the inductor you effectively obtain a resonant circuit which I think accounts for the waveforms your seeing.

     

    I've included a PDF which I think should clear things up a bit, however the only way to know is to test the hypothesis and I can't think of any way to do that.

     

    For sure, the solution to your problem is a diode in the arrangement previously mentioned it does work, none of your components will overheat or suffer any un due stress.  We just need to find a way to explain the results your getting.

     

    hth,

     

    Best Regards,

    Jorge Garcia

    Attachments:
    imageResponse for Cabe.pdf
  • 0 in reply to Jorge_Garcia
    Dave Jones has a blog with a good example of ringing generated in the oscilloscope probe: http://www.youtube.com/watch?v=sUG_sjS67K4
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