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dpdt relay question

Discussion in 'Electronic Basics' started by Ryan Kremser, Oct 1, 2003.

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  1. Ryan Kremser

    Ryan Kremser Guest

    Ok the ratings on a relay that i have are 1A @ 24 DC & 1A @ 120v AC..
    are these values per side of the relay (it being a dpdt has two -I thought)
    or are these totals as in .5A per side?
     
  2. Ryan Kremser

    Ryan Kremser Guest

    ahh forgot to mention it is a hrs2h-s-dc5v from HUIGANG
     
  3. Rein Wiehler

    Rein Wiehler Guest

    those numbers are the contact ratings in that relay.
    You can switch 1A at 24VDC (24W DC load) or 1A at 120VAC (120W AC load).
    Looks like you need 5VDC to operate the relay.
    rw
     
  4. Ryan Kremser

    Ryan Kremser Guest

    ok but there are 2 sets of contacts it being a dpdt relay, are the values
    per set or cumulative
     
  5. Per pole. That is, per "side" - pole is the 'p' in "dpdt".
     
  6. Rein Wiehler

    Rein Wiehler Guest

    thats the rating for each set
    rw
     
  7. David Harmon

    David Harmon Guest

    If you wire the contacts in parallel, is it OK to
    double the current rating?
     
  8. No, because you have no way of ensuring the current is equally split between
    the contacts.
     
  9. Ryan Kremser

    Ryan Kremser Guest

    ok good, so correct me if i'm wrong but reading your posts its my
    understanding that i can run 1 amp through each pole allowing me to switch a
    2 amp device if I place both poles in parallel?
     
  10. bemw

    bemw Guest

    No. At the moment of closing or opening, one pole will react quicker/slower
    than the other, resulting in the single pole briefly carrying the total
    load. Arcing will occur and the contact surfaces may be damaged or welded
    together.
     
  11. Guest

    No, for reasons others have stated. But in addition,
    you should under rate the circuit so that it does not
    use the full current rating of the relay contacts.
    That means that if you are controlling 1 amp of
    current, a relay rated at 1 amp is a poor choice.
    Use a relay with higher rated contacts.

    On top of that, you should add elements to the circuit
    as needed to prevent or minimize arcing of the contacts.
     
  12. JeffM

    JeffM Guest

    Walter has already told you NO.
     
  13. Ryan Kremser

    Ryan Kremser Guest

    Thanks, sorry that was so hard, I missed the first set of posts which had
    answered the question
     
  14. John Fields

    John Fields Guest

    ---
    I think the answers you got were bogus.

    In the first place, even if the resistances of the two contacts were
    different the difference would be in milliohms (or, more likely
    microohms) so the "extra" heating the lower resistance one would suffer
    would be negligible. Just for comparison, think of a couple of
    resistors in parallel with a difference in resistance of 1%. For any
    current flowing through the pair, the smaller resistor will only carry
    about 1% more current than if they were equal. In the second place, the
    temperature coefficient of resistance of metals is generally positive,
    so even if one set of contacts did conduct substantially more current
    than the other one, and got hotter, its resitance would then go up and
    the current causing it to heat would be shunted to the other set of
    contacts until a thermal balance was achieved.

    There _are_ other considerations which have to do with the type of load
    you want to switch, whether you want to do cold or hot switching, and
    whether you're switching AC or DC, but a blanket condemnation of
    paralling contacts to increase the current switched isn't warranted.
     
  15. I wouldn't say I gave him a blanket condemnation (though others may have); I
    just said that in this instance, 0.5 + 0.5 doesn't equal 1.0. I'll stand by
    that math :)

    But John, I'm not sure you're attending to the right aspects. You're
    talking about overheating during normal operation. I don't think that's the
    main thing relays are current-rated for, is it? I mean, a piece of 22-gauge
    wire will carry 3A comfortably, and I've seen relays with contacts and
    internal wiring that are physically larger than that rated for less.

    I am *NOT* an expert on relays, so read this with considerable skepticism,
    but:

    I think the current rating of a relay has more to do with the amount of
    current that it can safely interrupt, without damage to the terminals from
    arcing. That is, the rating is concerned with the situation where the relay
    is closed, current is flowing, and then the relay opens. The two contacts
    of a DPDT relay must open at slightly different times (perhaps 0.1msec
    apart); so, if they're paralleled, one contact is always getting opened with
    the full current on it. It is precisely that brief period of opening that
    matters, because if there's too much current to interrupt, the contacts will
    bond and fail to open.

    More realistically, since we're only talking about a factor of two overload,
    the contacts will still successfully open; but over time, the effect will be
    exactly the same as if the relay were being run at twice its rating, because
    it will always be the case that _at the most critical moment_, all the
    current is flowing through one contact. Little micro-welds will form and be
    physically pulled apart by the relay spring, but they'll be twice as big as
    they're supposed to be.
     
  16. Guest

    Exactly. It's interesting to put a set of relay contacts
    through repetitive make/break cycles, and scope the current
    through the contacts. You'll see plenty of bouncing with
    typical relays. If those contacts are paralleled for increased
    current, the transfer and the bounces would have to be precisely
    in sync to avoid the scenario you mentioned.
     
  17. John Fields

    John Fields Guest

    ---
    The current rating of relay contacts is based on the voltage drop across
    the contacts with a certain current through them. The voltage drop is
    due to the contact resistance, and putting more that the rated current
    through the contact resistance will result in the contacts heating more
    than with the rated current flowing through them, increasing the contact
    resistance, increasing the power dissipation of the contacts, raising
    the temperature of the contacts...
    ---

    ---
    There are two specifications to consider here: One with the current a
    relay is specified to carry with the contacts closed and another with
    the relay "hot-switching" the current.

    Here's a decent link: http://www.aromat.com/pcsd/rti.pdf
    ---
    Relay contacts can't weld on break, unless they bounce, since the plasma
    will only occur after the contacts have separated. They can (and do)
    weld on make, however, since bounce almost always accompanies make.
    ---
    ---
    Again, this only occurs when the contacts close, and if they bounce when
    they open. What _does_ happen when the contacts open (and when they
    bounce) is that the arc/plasma which is created when the contacts open
    causes metal to migrate from one contact to the other. It's this
    phenomenon which causes the contacts to weld closed, since when the
    contacts finally come to rest they can be sitting in a pool of molten
    metal. Especially switching DC, since when the bouncing decays a time
    will come when the arc won't extinguish between bounces because the
    contact separation won't be enough to extinguish the arc.

    Also to be considered is that when one set of contacts closes and
    bounces, the other set will not be far behind and will quench the arc
    created by the first set as soon as the second set closes. Of course if
    the second set bounces before the first set comes to rest another arc
    will be generated by the second set bouncing, but then when the first
    set makes again the arc will be extinguish. Since this arcing will be
    shared by both sets of contacts and will exist for a shorter time than
    if it occurred with only one set of contacts, it may well be possible
    that (on make) the life of the contacts in parallel will be longer than
    twice the life of either, making 0.5 + 0.5 = 1.1 :)
     
  18. That is indeed a great link! Very informative; thanks. Color me eddicated
    :)
     
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