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Relay contact rating

Discussion in 'Electronic Design' started by [email protected], Mar 1, 2013.

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  1. Guest

    I have a relay whse contact rating is specified as follows

    Power : 460VA / 48Watts
    Current : 2A @ 24V DC or 230V AC

    If I am not wrong, 460VA is from 230V AC * 2A. If my applications is required to switch 16V AC max with 5A max current (i.e 80VA), Can I use this relay? or max current is 2A only irrespective of voltage and AC or DC.

    Load is inductive (4 ohm loud speaker) if this matters.

  2. Syd Rumpo

    Syd Rumpo Guest

    5A is too much for 2A contacts.

    But. If this is for audio as you imply, then would you ever really see
    this current for any significant length of time?

    Phil Allison will know.

  3. Guest is for Audio and peak current will only be for short bursts.
  4. Syd Rumpo

    Syd Rumpo Guest

    In my opinion, it'll be fine as long as there's no safety hazard in the
    event of the relay contacts not opening, but I wouldn't do it
    commercially. It's outside the specification of the relay so there's no
    comeback if someone dies in a raging inferno.

  5. Phil Allison

    Phil Allison Guest

    ** For domestic hi-fi use, it should be fine.

    Switching will very rarely be simultaneous with the full 16 volts and 5
    mps - plus with audio signals, arcing upon opening is much less than with
    50/60Hz power or DC. Reason being that a low zero voltage will occur in less
    than 10mS and stop any arc.

    The average current passed by the contacts with audio program is gonna be
    well less than 2 amps.

    Have you ever seen the puny pushbutton switches used in most hi-fi amps for
    speaker switching?

    ..... Phil
  6. Phil Allison

    Phil Allison Guest

    "Syd Rumpo"
    ** You a fan ?

    .... Phil :)
  7. Uwe Hercksen

    Uwe Hercksen Guest


    if you switch DC, the maximum current is 2 A and the maximum voltage is
    24 V, you may switch 2 A and 16 V also, but not 3 A and 16 V.

  8. Phil Allison

    Phil Allison Guest

    "John Fields"
    ** Horse shit.

    The 2A figure is the relay's rated switching capacity under at max
    conditions with a stated number of cycles.

    When closed, the contacts can handle far more.

    ** The arc has to quench itself as the gap between contacts increases
    so 2A and 24VDC are the max conditions where this happens reliably
    over a large number of operations.

    Relays and small toggle switches are lousy at switching DC of any size.

    The use of speaker relays in audio amps is a non trivial subject with which
    I am very familiar.

    And you are not.

    ..... Phil
  9. Phil Allison

    Phil Allison Guest

    "John Fields"
    "Phil Allison"

    ** Total fuckwittery.

    ** Look harder, it sometimes called the holding current.

    and, since ohmic contacts generally have a
    ** More horse shit.

    The resistance of a such contacts varies inversely with current.

    That 2A rating is the SWITCHING CAPACITY !!!

    You bullshitting ass.

    ** Means SFA.
    ** Completely asinine.

    Got any idea why so many audio amps have speaker relays at all ?

    ..... Phil
  10. Phil Allison

    Phil Allison Guest

    "John Fields"

    ** You really are a TOTAL FUCKWIT !!

    ** LOL - that is the relay's "holding voltage".

    SCRs and triacs have "holding currents".

    The contact current rating is rarely given for non-switching as it is an
    unusual app - but it *IS* the one we have here.

    ** Common contact metals form oxide and sulphide coatings that have a
    negative resistance characteristic.

    It's all in the literature.

    ** See above.

    Most people buy relays to SWITCH a load.

    ** Of course it does - you pedantic, autistic twat.

    ** Is that some autistic, smartarse way of alluding to switch on/off muting

    FOAD you vile pig.

    ..... Phil
  11. Usually called "carry current" or "carrying current" ratings.

    Relay makers don't seem to be very bold in such ratings- sometimes
    they are just the same as the switching ratings.

    I had an old Motorola app note that claimed some "well designed" 5A
    relays could carry 50A.

    These days, they'd cr*p on the engineers for wasting so much valuable
    material rather than call the product well designed.

    Best regards,
    Spehro Pefhany
  12. Phil Allison

    Phil Allison Guest

    "Spehro Pefhany"
    "Phil Allison"

    ** OK - that is the term I was searching for.

    ** Must have big contacts with lotsa pressure.

    .... Phil
  13. SoothSayer

    SoothSayer Guest


    Such materials have those characteristics, but nobody uses such
    materials these days, and therefore sees no such resistive media

    Common MODERN contact media is typically Platinum. You probably read
    your spec sheet 50 years ago. That was how long ago it applied. Hell,
    the PINBALL industry cleared all that hash up. Where the **** were you?
    Contacts on reed type switching assemblies, which nearly all are some
    form of, have been low or NO oxide materials for decades. Even an
    enclosed switch has little short 'reeds' with contact heads on the ends.

    Platinum exhibits exactly ZERO oxidation, even after 100 YEARS of
    exposure. The only way to coat it with anything is to have super moist,
    particle laden air.

    Both it AND Gold are the best metals at NOT EVER getting ANY 'tarnish'
    on them. Especially Platinum. Look at the valence shell!

    Imbalances can cause contacts with momentary HV pulses involved to
    migrate surface molecules from one contact and meld them onto the other,
    thereby causing degradation.
  14. Phil Allison

    Phil Allison Guest

    "Phil Allison"
    ** ROTFL !!

    On what planet ??


    The topic is ** POWER** relays !!!!!

    Silver and silver alloys are the dominant metals used.

    Pure Silver, Silver Nickel, Silver Cadmium Oxide, Silver Tin Oxide et alia.

    Silver forms a sulphide that makes it look dark and is an insulator.

    Platinum is used in specialty telecoms relays with low amp ratings - as it
    has high resistance.

    Gold ( plating) is still ideal for small signal relays, but is too soft for
    high current switching.

    Try reading a fucking catalogue some day, you half wit pinball freak.

    .... Phil
  15. P E Schoen

    P E Schoen Guest

    "Phil Allison" wrote in message
    It's also sometimes called thermal contact rating, which may also be
    expressed as a duty cycle. There is also the I^2t rating which is used for
    fuse selection.
    Ratings such as this are usually shown for power contactors and motor
    starter relays. They are rated for a certain HP and locked rotor current. We
    use contactors for switching transformer taps and setting series and
    parallel connection, under "dry" conditions with essentially zero current
    and voltage (other than leakage through an SCR snubber).

    Here is a rather extensive glossary and explanation of relay terms:

    Don't see platinum in any of the contact materials. Gold (mostly flashed and
    plated for low level signals), of course, and silver alloys with palladium
    and ruthenium.

    Here is an extensive study of relay contacts, and platinum is only mentioned
    once regarding plating them with gold.

    More information: (4.3 MB)

  16. P E Schoen

    P E Schoen Guest

    "SoothSayer" wrote in message
    Platinum contacts were used some 90 years ago and some problems were found: (13

  17. josephkk

    josephkk Guest

    Hello, the current rating is 2 A. Use at 5A is beyond ratings, very poor
    life at best.

  18. Phil Allison

    Phil Allison Guest

    "joseph kook"

    Hello, the current rating is 2 A. Use at 5A is beyond ratings, very poor
    life at best.

    ** Got any idea what the peak to RMS ratio of music programme is ??

    Think 5:1 or more times, in terms of voltage or current.

    So the average RMS current is not likely to exceed 1 amp.

    .... Phil
  19. whit3rd

    whit3rd Guest

    [and used to switch a low-impedance audio signal to a speaker]
    But, the relay rating is for DC or 50/60 Hz AC power supplies, and the audio
    signal is broadband AC. That means many harmonics might add
    to make transient peaks far beyond any reasonable ratings.

    A 5x 'crest factor' is commonly applied to audio AC, and that means
    a 1A average current is handled by >5A transistors in an amplifier.
    The transistors can fail in a millisecond or so with overstress.

    The relay is not going to be repeatably switching peak currents,
    it's going to have a few transient peaks while completely open or
    completely closed. So, 1A of heat-producing current is associated
    with 5A peaks, AND THE RELAY ONLY CARES ABOUT THE 1A number.
    The relay will take seconds, not milliseconds, to fail with overcurrent.
  20. Phil Allison

    Phil Allison Guest

    "John Fields = TROLL"

    ** A continuous signal has a crest factor.

    Compressed rock/pop music has similar characteristics to ( audio band
    limited) pink noise, which has a crest factor of about 13dB.

    It represents a typical, worst case scenario for a domestic amplifier.

    **** head.

    ..... Phil
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