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Relay contact ratings.

Discussion in 'Hobby Electronics' started by Sylvia Else, Dec 8, 2011.

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  1. Sylvia Else

    Sylvia Else Guest

    The defrost timer in my fridge is failing, and given the cost of a
    'genuine' replacement part, I was pondering the option of making
    electronic timer driving an electromechanical relay.

    It probably won't happen, but when I was looking at relay specs, I found
    that their ratings are usually specified as a highish reactive power,
    and a much lower real power.

    is typical, specifying in this case 300 W / 2500 VA.

    I find this difficult to fathom. Imagine a purely inductive 2500VA load.
    So no real power being switched, but it would arc like crazy. What am I
    missing here?

  2. Phil Allison

    Phil Allison Guest

    "Stupider than Anyone Else"

    ** The 300W figure refers to DC switching - ie 10 amps at 30 volts.

    The 2500VA figure refers to resistive loads and 250VAC power.

    There is nothing about inductive load switching.

    See the data sheet.


    .... Phil
  3. Sylvia Else

    Sylvia Else Guest

    Care to point, in the data sheet, to where that distinction is drawn.

  4. **Transformers and motors are notoriously nasty for relays to deal with.
    TRIACs are a much better choice for such loads. Easy enough to design a
    simple circuit, or, if you want a really easy way out, just buy a
    suitably rated Solid State Relay (SSR). SSRs and VERY easy and safe to use.
  5. Sylvia Else

    Sylvia Else Guest

    It's complicated by the fact that the defrost timer switch is a SPDT -
    it switches between the compressor (via the thermostat, I presume) and
    the defrost heater. In my relatively cursory search, I haven't found a
    SPDT SSR. Could use two, I suppose, but a failure mode that leaves both
    heater and compressor running seems more likely than with an
    electromechanical relay.

    The existing switch has contacts just as a relay would; they just don't
    have an electromagnetic actuator.

    The switch only cycles four times a day, so it's not so demanding in
    terms of contact life.

  6. mike

    mike Guest

    You can often get more life out of a mechanical defrost timer by opening
    up the
    motor and oiling the bearings.
  7. F Murtz

    F Murtz Guest

    I hope you are not being sarcastic.
  8. Sylvia Else

    Sylvia Else Guest

    It could be a reference to the absence of bearings - except to the
    extent that the metal axle rotates in a hole in the plastic housing. At
    least, that's the situation in the one I have.

    Perhaps I shouldn't be concerned about having a "genuine" part, because
    the original looks about as cheaply made as it could get while still
    working at all.

  9. Sylvia Else

    Sylvia Else Guest

    That makes more sense. RS Components don't help by reversing the order
    of the power ratings relative to the voltage and current ratings.

    Quoting power ratings seems redundant.

  10. Something like the Omron G7L is properly rated for motor loads. You
    could use a second relay for the heater.

    Best regards,
    Spehro Pefhany
  11. ehsjr

    ehsjr Guest

    The price of the relay at the url you posted is ridiculous.
    Surely you can get something much cheaper and equally robust.
    There are relays made for appliances like air conditioners
    available from Digikey for < $5.00 so it's likely you can
    get something where you are for much less than the ~ $82.00 at
    the RS Australia site. You might be able to use an automotive
    relay. Generally their contacts are rated at 30 or 40 amps at
    12 VDC, and they'll handle 240 AC nicely.

  12. mike

    mike Guest

    That could be a problem. Mine has a standard clock motor with metal
    case and bronze-looking bearings.

    A clock motor has about zero torque. Takes very little friction in
    the first few gear reduction stages to stop it. hardened grease can
    easily do it. Just cleaning out the dried up grease can make it work.

    Mine ran for another decade after I cleaned/oiled it.
    Replacement was about $13, but it failed after a week. Second replacement
    timer lasted years before I retired the fridge.
  13. Phil Allison

    Phil Allison Guest



    Even though SSR's and Triacs are quite rugged, something like a surge
    from a lightning strike or similar event could cause one or both of
    these triacs/SSR's to short out,

    ** Nonsense.

    If the max voltage rating of a triac is exceeded by a spike on the AC
    upply - it simply turns on for the rest of that half cycle.

    .... Phil
  14. Phil Allison

    Phil Allison Guest

    "John Fields" .
    ** Problem being, there is simply not enough gap available to break the arc
    that forms if the DC ratings are exceeded.

    For the type of relay in question, a permanent arc will form with DC if the
    current flow is say 10 amps and the voltage across the contacts is over 40
    volts. This means there is over 400 watts of heat, mostly being dissipated
    into the contacts and destroying them in a few seconds.

    In the world of high powered audio amplifiers it is still common to see such
    relays used to protect speakers from DC high fault currents and turn on/off
    transients. In the case of former, the relay is not capable of doing the

    .... Phil
  15. **Unlikely. In fact, IME, TRIACs (appropriately rated ones) are vastly
    more reliable than relays when driving highly inductive loads. In fact,
    I've been using several for around 30 years, without issue. That is not
    to say that TRIACs cannot fail. They can and do and usually shorted.
    Which, in Sylvia's case, may prove inconvenient.

    and since fridges are turned on for
    **IME, the vast majority of lightning problems occur via TV antennas.
    Power line issues are massively over-stated. Again, the only time I can
    pin point a power line "surge" as the direct cause of a problem was a
    very long time ago, when a 5kV bearer fell across the 240VAC main
    overhead lines. The damage was considerable and affected several blocks.
    **I agree. However, in terms of longevity, TRIACs win hands down.
  16. **Tapco CP500 anyone? Crap design, with useless relay protection.
  17. Phil Allison

    Phil Allison Guest

    "Trevor Wilson"

    ** Plus the telephone line - modems and TAMs drop like flies when there is
    a thunderstorm.

    ** It's an issue in many rural areas - the solution to which is fitting
    varistors in the power box.

    ** Funny how microwave ovens all seem to have relays turning on the big

    .... Phil
  18. I would not use a 12V automotive relay to switch mains voltage,
    particularly inductive loads.. coil-to-contact dielectric strength of
    those POS relays is << 1kV. I like to see 4kV or so. Are the housings
    required to be flame retardant?
  19. swanny

    swanny Guest

    Seen that happen. Contacts weld, amp and speakers go boom.
  20. Phil Allison

    Phil Allison Guest

    "Phil Allison"
    ** I have made a bit of a study of this in an attempt to find a solution.

    Conclusions are:

    1. There is no readily available relay capable of breaking the voltages and
    currents involved if a 1000 wpc amp goes DC.

    2. If you wire a changeover relay so it shorts the speaker and disconnects
    the amp at the same time - chances are good the speakers will be OK, but
    the relay contacts will be burnt by the resulting arc to ground. The amp
    must have DC rail fuses if this is to work.

    3. A large ( ie octal base ) relay with 4mm clearance contacts PLUS a
    strong magnet next to the contacts CAN
    work with amps up to about 400wpc. The magnet pulls at the arc and helps
    break it.

    4. A triac "crowbar" will also work but gives no on/off transient
    suppression. The amp must be able to take a dead short without internal
    failure and not have response down to subsonics or DC.

    .... Phil
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