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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
Attachments:
image
  • 0
    Am I missing something here? The usual way is to put a 1N4001 or similar diode reverse biased across the relay coil.
  • 0

    Yes yes yes!!!

    Mike you are right! Some times there isn't 1N4001 diode and there is mostly 1N4007 diode in trade!

    I forgot to emphasize the reverse polarity!!!

     

    Chris

  • 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
  • 0 in reply to Catwell

    One possible solution would be to alter the drive you are using to power the relay coil.

    I can't see why the 'industry solution' of the reverse diode protection doesn't work for you, but hey ho.

     

    I know you've seen the voltage oscillation on the oscilloscope, and i think that the diode will protect against the reverse current and the forward current will be so small (current, not voltage) as to be a non issue.

     

    However, you could drive the relay using something like a ULN2003 (for more than one relay) or a darlington plus reverse diode for single relays - the darlington will sniff it's nose at any risidual current.

     

    Finally, make sure you are not exceeding the rating of the diode switch contacts, and therefore setting up more current, and therefore a bigger magnetic field, than the relay is designed for.

  • 0
    Besides feedback and EMI, not suppressing transient spikes in relays reduces the life of the contacts significantly. Like welding, metal is deposited from one contact to another. In extreme cases, the contacts completely fuse together. Using a single diode suppressor, text book answer, will still let this effect occur.


    This is an excerpt from a Tyco Electronics technical support document:
    Many engineers use a rectifier diode alone to provide the transient
    suppression for relay coils. While this is cost effective and fully eliminates
    the transient voltage, its impact on relay performance can be devastating.
    Problems of unexplained, random "tack welding" frequently occur in these
    systems. In some applications, this problem is merely a minor nuisance
    or inconvenience and the controller or operator will cycle the relay until
    the proper response is obtained. In many applications; however, the first
    occurrence may cause a complete system failure or even present a
    hazardous situation. It is important that these systems be designed with
    another method of relay suppression.
     
    image
    In the above figure you can see the current ripple after the deenergizing of the coil. This is the result of the magnetic restraining force fighting against the relay's spring opening force, or the slow decay of magnetic flux in the relay contacts. This shows the momentary welding effect.
     
     
    The Tyco document continues:
    The use of a reversed-biased rectifier diode in series with a zener diode
    will provide the best solution when the relay can be polarized. This
    suppression is often recommended by Siemens Electromechanical
    Components (SEC) for use in automotive circuits. The impact on release
    dynamics is minimal and poses no loss of reliability. This is normally a
    low-cost method and the only design precaution is to select a zener with
    an appropriate breakdown voltage and impulse power specifications
    adequate for the relay in its application. In printed circuit board applications
    with transistors used as relay drivers, the zener diode can be placed "across"
    the transistor; that is, for a common emitter circuit, cathode connected to
    collector and anode connected to the emitter (the series rectifier diode is
    not used in this type of circuit).
     
    image
    Adding a zener diode limits the relay coil's switch voltage closer to the switch rating. So one can effectively suppress enormous voltage spikes and avoid contact welding with little money and added complexity. See above


    I am adding the documents I found on the subject explaining in detail all the issues with relays. I recommend reading all of them and make all the practices, there in, standard in your designs.


    Cabe
    Attachments:
    imageslac-tn-65-052.pdf
    image125.pdf
    image13c3311.pdf
    image13c3264.pdf
  • 0 in reply to KennyMillar

    Kenny,

     

    When you disengage the relay, no matter what you drive it with, the coil deenergizes causing a massive spike larger than anything you planned for it. See the reply post I did on the subject. The documents attached explain everything.

     

    Cabe