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.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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.
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.
"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 cycl> There are tricks to reduce the switch-on surge including "prewarming"
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.
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.
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~ Adrian Tuddenham ~
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