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question on electromechanical relay specifications

Discussion in 'Electronic Design' started by Gus, Nov 21, 2005.

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  1. Gus

    Gus Guest

    I'm looking at some miniature electromechanical relays that use a DC voltage
    to energize the coil. The DC ratings for the contacts are given for
    resistive, inductive, lamp, and low level loads and are wildly different.
    For example with a resistive load its 1A, but only 100mA with a lamp load.
    Why the difference ?

    I've tried to ask a vendor's application engineer but they haven't answered
    yet, and I'd appreciate any insight from this group.
     
  2. Tim Wescott

    Tim Wescott Guest

    * A resistive load will start flowing current according to I = E/R, and
    will do so indefinitely.

    * An inductive load will want to keep flowing current when you break
    it's circuit because V = L dI/dT. When the relay opens and I wants go
    to zero instantly (dI/dT = -infinity) the inductor will generate enough
    voltage to keep the current going -- even if this means sparking across
    the relay contacts (snubbers reduce this, sometimes dramatically).

    * A lamp is rated for its current once it is hot. When it is cold its
    resistance is significantly less than when it is hot; the relay needs to
    supply that inrush current without welding the contacts together.

    * I dunno about the low-level stuff.
     
  3. Tim Shoppa

    Tim Shoppa Guest

    For example with a resistive load its 1A, but only
    Most lamps are rated for their steady-state current. When the filament
    is "cold" they can draw many more times more current than their
    steady-state current. For the relay contacts it's this switched current
    that matters more than steady-state.

    A factor of 10 is about right for many small vacuum-filled (how do you
    fill a container with nothing?) lamps.

    There are tricks to reduce the switch-on surge including "prewarming"
    the filament through a resistor. Prewarming also greatly increases
    total number of lamp on/off cycles before the filament burns out.

    Tim.
     
  4. John  Larkin

    John Larkin Guest


    In theory, the nice gold or silver plating on the contacts is good for
    low-level switching, but once you arc the contacts with bigger loads,
    it's trashed, so may not be suitable for very low-level use after
    that.

    That's what I've heard, anyhow.

    We're using tiny sealed relays for microvolt-level switching, latching
    relays to keep the thermal EMFs down.

    John
     
  5. It is flowing during the 'contact-bounce' time, before the mechanical
    bits stop moving. This can cause welding of the contacts or a
    micro-welding and tearing apart sequence, which rapidly erodes the
    contact surfaces.
     
  6. John  Larkin

    John Larkin Guest

    But most of them are filled with nitrogen.
    I've read that cycling doesn't actually reduce filament life; life is
    dominated by evaporation.

    John
     
  7. w_tom

    w_tom Guest

    "Prewarming" a filament has no effective on power cycling. But
    again, even the industry's own specifications do not define power
    cycling as destructive. Light bulb life expectancy is a function of
    voltage (filament temperature) and hours of operation. This even
    provided by industry standard formulas.

    When a light bulb is cold, it will demand a very large current. If
    the bulb is kept slightly warm, then the intial power on current is
    lower. This means smaller value transistors and less transients on the
    ground plane that would otherwise caught state sensitive devices to
    change logic states.

    It is mythical that power cycling causes incandescant bulb failure.
    But that initial power on current when the bulb is cold can be
    problematic to the control electronics.
     
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