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Searching for engineer experienced in 24V vehicle electrical design.

Discussion in 'Electronic Design' started by [email protected], Mar 4, 2005.

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  1. Guest

    Hello,

    My company decided to port our 12V automotive product design to 24V for
    sale in the European truck market. I am experiencing a problem with
    the transistion and was wondering if you, or someone you may know,
    would be willing to help. I am attempting to either prevent, or quickly
    extinguish, the arcing effect across a set of relay contacts.

    Reply to this post if you think you can help and I'll supply more
    details.

    Gerb
     
  2. Guest

  3. Guest

    Because this is an area that I'm interested in, I googled this a bit
    further and came up with instances of people using a MOV for preventing
    this sort of thing. In general, the aim is to prevent the current surge
    associated with switching inductive and to a certain extent
    capacitative loads. The MOV, diode idea and transient voltage
    suppressors are all candidates. Google any of the above, or "relay arc"
    for more info.

    I'd be curious to know what you come up with.

    Chris
     
  4. Guest

    I am switching a wire wound resistor using a 24V high current relay.
    The relay acts as a high side driver between the battery and grounded
    load. Wire wound resistors have some inductance but it is very low (I
    believe below 1uH). I do not think this is a contributing factor to my
    problem. I may be wrong.

    SAE says 24V automotive systems typically sit at 28V while charging.
    They can drift up to 32V under normal operating conditions. I am
    driving a 1 ohm load. My product seems to have no problem passing 28A
    at 28V. A pretty good spark is generated when the relay contacts
    separate, but this is manageable. My problem occurs when system
    voltage rises to 32A at 32V. When the relay contacts separate a
    sustained arc of approx 2-3 seconds is created. This arc is severely
    damaging to the relay contacts. It melts the contact acting as anode.
    One or two of these arcs kills the relay. The anode contact melts away
    and the relay can no longer conduct.

    I have tried all recommended approaches. RC network across relay
    contacts, RC network across load, diode across load, MOV. No
    suppression network seems to work. The only thing I found that works
    is to increase the gap between relay contacts. I have not had a chance
    to do extensive testing on modified relays yet to see if it is the
    cure.

    My stock relays have fine grain silver contacts and are gapped 0.4mm
    apart. The relays I modified are gapped to approx 0.85mm. They seem
    to start arcing at about 36V which is outside the normal operation
    range of the vehicle and should be good enough for me.

    I'm looking for an expert opinion from someone who has been down this
    road before. Am I missing something? What does it take to extinguish
    an arc? Am I on the right path, etc....

    Thanks for your input Chris. I welcome you, and others, to join in for
    more trouble shooting.

    Gerb
     
  5. Rich Grise

    Rich Grise Guest

    Use a car starter solenoid for the relay. Better yet, truck. They handle
    hundreds of amps, albeit intermittently. You could use both - the starter
    solenoid closes a few tens or hundreds of millisecs before the main one,
    and stays closed for awhile when the main one opens; but you might not
    need this.

    Obviously, you are running this thing right on the hairy edge of its
    spec - you just need a beefier part.

    Good Luck!
    Rich
     
  6. Completely off the subject, but I suddenly wondered whether wire wound
    resistors introduce significant inductance. I've always thought of them
    as just resistors that were manufactured a different way.
     
  7. Tim Shoppa

    Tim Shoppa Guest

    My stock relays have fine grain silver contacts and are
    What are your "stock relays" rated at? 0.4mm seems like a really small
    gap for even a 12V power relay.
    Way too close for me! Especially in an environment with extremely
    broad temperature and humidity requirements...

    Tim.
     
  8. On Wed, 9 Mar 2005 01:28:38 +0000 (UTC), the renowned
    Yes, some most certainly do. It can cause problems with current sense
    resistors, for example.

    Of course in this case, you've got the inductance of the resistor plus
    the inductance related to the loop area of the wiring. And
    high-current DC circuits just love to arc away anyhow.

    One way around it would be to use a hybrid switch.


    Best regards,
    Spehro Pefhany
     
  9. Rich Grise

    Rich Grise Guest

    Maybe he should be looking for a "Contactor".

    Cheers!
    Rich
     
  10. Or hire a sub-contactor.



    Best regards,
    Spehro Pefhany
     
  11. Tim Shoppa

    Tim Shoppa Guest

    Maybe he should be looking for a "Contactor".

    For a 1A resistive load? This is well within the capabilities of any
    automotive relay and they're made by the billions. They also have this
    tendency to fail shorted, especially if they're a horn relay :)

    Tim.
     
  12. John Fields

    John Fields Guest

     
  13. John Fields

    John Fields Guest

    ---
    I think the most important things to do when trying to quench an arc
    are to get the contacts as far apart as possible as quickly as
    possible, and if you know that you're going to be using them in that
    kind of service to get them made with the proper contact material.

    Opening up the gap between the contacts on relays you already have
    might help, but it may be that lowering the contact pressure by doing
    that will hurt you in the long run by allowing bounce to last for a
    longer time and changing the thermal characteristics of the contacts.
    With less pressure their resistance will increase slightly, causing
    them to heat up slightly, increasing their resistance a little more...
    Also, because of the reduced pressure, the contacts won't separate as
    quickly as they did before, which will give the arc a chance to live
    longer. It's probably not _that_ serious, but they were designed the
    way they were for a reason and when you start fooling around with the
    mechanicals it doesn't usually end up serendipetously.

    I think you're going to wind up in trouble unless you get the right
    relays for the job. That is, _manufactured_ with the proper contact
    spacing, current rating and contact materials. What does your
    European competition use?
     
  14. Don Murray

    Don Murray Guest

    Hi,
    your problem is an old one when switching high currents and the
    practical solution to this is a relay with blowout magnets at the
    contacts.These extinguish the arc quickly by forming a magnetic field
    thst
    repels the plasma field .hope this helps
     
  15. [...]

    One solution, that I've used sometimes, is to simply put a (FET)
    transistor across the contacts. Turn the transistor on shortly before
    the contacts open, and turn it off when they are fully open. It's a
    simple solution -- and often cheaper than the alternatives.
     
  16. Fred Bloggs

    Fred Bloggs Guest

    Right- at these current levels, any passive type suppression network
    will be huge. The problem is that contact voltage vs current curve
    exceeds the sustained arc threshold for your relay. You can use a
    smaller auxiliary relay to keep this in check with a voltage divider. At
    32V the main relay opens on a 22V open circuit voltage, and the smaller
    aux relay then drops out after delay and breaks the circuit completely
    at a much reduced current 11 amps. K2 would be a 12V relay coil.
    View in a fixed-width font such as Courier.
     
  17. Fred Bloggs

    Fred Bloggs Guest

    The current rating of the contacts is a strong function of the force
    with which they are held closed. He most certainly has reduced this
    rating significantly by doubling the spacing- maybe halved it. Fiddling
    with the armature travel is a dumb thing to do.
     
  18. Hmm.... my solution was going to involve a timed
    MOSFET, but I don't think I'll bother now.

    Maybe add a diode from the 1-ohm to Ground though.
    This clamps any negative HV spike from the cabling
    inductance.
     
  19. Tim Shoppa

    Tim Shoppa Guest

    LOL, it's a one _ohm_ load...

    My god, the mind boggles. 28V * 28A = 784Watts. Into a resistor. On
    a truck. What the heck is this guy doing? Icemelting? Space heating?

    Tim.
     
  20. Tony Williams wrote...
    Nah. Just plop an Infineon PROFET Smart SIPMOS switch in there.
    Get raw performance, short-circuit and thermal protection, and
    an error indication, all at a low cost. For example, use a
    BTS443, BTS432, BTS442, etc.

    At first they called these a "Sense Highside Switch," and later,
    protected-POWER "Smart High Side Switches," to help your searching.
    http://www.infineon.com/cgi/ecrm.dll/ecrm/scripts/prod_cat.jsp?oid=-8171
    Poke around a bit... BTW, DigiKey has a nice in-stock selection.
    http://www.infineon.com/cmc_upload/migrated_files/document_files/Application_Notes/36341.pdf
     
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