Snubber Diode

Discussion in 'General Electronics Chat' started by qitara, Jun 30, 2013.

  1. qitara

    qitara Thread Starter Member

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    Hi Guys

    I got a solenoid that is getting the relay that is driving it going bad due to the contacts getting welded, so now its time to install a diode across the solenoid, only thing is that I am not a pro at sizing the correct diode, So i would be thank full if some of you guys could help :)


    the Solenoid is a 900W DC solenoid running on 110VDC.



    P.S

    If I change the Electromechanically relay with a SSR that has a load voltage of 0-400VDC and load current of 40 amp, will I still need to install the diode ?
    Last edited: Jun 30, 2013
  2. GopherT

    GopherT Well-Known Member

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    No, but check the manufacturer's DATASHEET and application notes to be sure of all requirements. I prefer the SSR option.
  3. qitara

    qitara Thread Starter Member

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    Well I like the SSR option as well, but what I don't know is if it will run fine with the coil pumping current when its begin disconnected

    if the solenoid causes the contacts on the electromechanical relay to get welded, what would it do to the SSR, will the SSR work fine without a diode across the solenoid ?
  4. GopherT

    GopherT Well-Known Member

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    Check the manufacturer's DATASHEET. They will have recommendations (there may be a built-in diode or they may recommend adding one. Check it out.
  5. qitara

    qitara Thread Starter Member

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    checked the datasheet, but nothing was found there only the diagram showing a diode on the load side (terminal), I cant see how the diode will work when its not across the load ?
  6. GopherT

    GopherT Well-Known Member

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    SSRs can handle AC or DC loads.

    DC loads
    This type of load should be considered inductive, and a diode should be placed across the load to absorb any surges during turn-off.
  7. qitara

    qitara Thread Starter Member

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    Any idea what type of Diode I should use for a 900W Solenoid running on 120 VDC ?
  8. strantor

    strantor Well-Known Member

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    The same current that flows through the coil will flow through the diode when the circuit is opened. So 900W/120V=7.5A. Your diode should be rated AT LEAST 120V/7.5A. Getting something rated a little higher than that wouldn't hurt.
  9. THE_RB

    THE_RB AAC Fanatic!

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    A coil diode won't stop the contacts being welded together.

    You need an RC snubber across the coil contacts, to reduce arcing when the contacts open. Or a relay rated for enough DC contact current with your type of load.
  10. crutschow

    crutschow AAC Fanatic!

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    Au contraire. Don't understand why you say that. :confused: A reverse-biased diode across the coil will most definitely minimize the problem of the contacts being welded. The diode will carry the coil current when the contacts open, keeping the voltage transient voltage no higher than the forward voltage drop of the diode and essentially eliminating any contact arcing.

    The transient voltage will be less than typically generated by an RC snubber. Of course the effect of using a diode is that it slows the release time of the solenoid as compared to an RC or zener diode snubber. So, if speed is important, you will want to consider using one of those instead.
    Bernard and strantor like this.
  11. strantor

    strantor Well-Known Member

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    I also had beef with RB's statement, but I know who the bigger expert is. So assumed he was right and I was wrong. I wanted to do the digging and pull some references but I didn't have the time to find anything that spelled it out. So I'm glad you spoke up, let me know that my understanding isn't fundamentally wrong.
  12. creakndale

    creakndale Active Member

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    1500W Transient Voltage Suppression (TVS) Diode. Like this one from Littelfuse:
    Part number: 1.5KE150A, Unidirectional, 1500W, Vr=128v

    creakndale
  13. strantor

    strantor Well-Known Member

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    That one seems a little ill equipped, with a peak surge current of 1mA.

    Here's a list of other TVSs that have voltage ratings >130V and amperage ratings >8A.

    Note: I've never used a TVS diode for this purpose; I usually use a plain ol' rectifier diode.
  14. ian field

    ian field Active Member

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  15. strantor

    strantor Well-Known Member

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    Again, I believe this incorrect. If adding a diode actually aggravates the arcing problem, then why is it such a common practice to add the diode? Why can I buy relays and solenoids that have the diode installed from the factory?

    The reason coils cause arcing across the contacts because they are inductors, and inductors oppose changes in the flow of current. When you suddenly open the circuit, the inductor will keep the current flowing at whatever voltage is necessary (up to infinity, theoretically) until the field collapses. So with no diode, the voltage just spikes until whatever voltage is necessary to arc across and complete the circuit. When you install the diode, that current now has another option of where to go, and it does choose to take the path of the diode, not the open contacts. With the diode installed and the new current path option in place, the coil does not need to increase it's voltage to bridge any gaps and complete the circuit, so no spike occurs. Upon opening of the contacts (@T0), the voltage across the contacts should be 0V, and rapidly approach source voltage (120V). So if upon opening the contacts the voltage across them is 0, then why are you saying that this will make the arcing worse? Can you provide a published reference that supports your claims? Because I have not heard this before you & (to an extent) RB said it. The way I have just explained it is the way that I understand it, and if I am wrong I welcome the correction.
  16. ian field

    ian field Active Member

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    I was referring to the relay contacts opening more slowly - this happens because the catch diode shunts the current back through the coil.

    Maybe I just read more application notes than you do.
  17. ian field

    ian field Active Member

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    ***************************************************

    http://educypedia.karadimov.info/library/13c3264.pdf
  18. creakndale

    creakndale Active Member

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    Perhaps it is better to check the actual datasheet instead of relying on Mouser's typographical error?

    creakndale
  19. strantor

    strantor Well-Known Member

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    Ah I see the confusion, you're talking about suppression on a relay coil, and everybody else is talking about suppression on a solenoid (which is what OP asked about).
  20. THE_RB

    THE_RB AAC Fanatic!

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    Whoops! You are correct, I missed in post #1 that he was using a relay to drive a relay, and thought there was only one relay with both coil and contacts being very separate! :eek: :)

    Please ignore my previous post.
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