piclist 2001\01\29\231958a >
Thread: 2.5KW Load
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face BY : Roman Black email (remove spam text)



Mike Harrison wrote:
>
> On Mon, 29 Jan 2001 23:57:25 +1100, you wrote:
>
> >I do it in battery powered devices with a large capacitor
> >and resistor. The cap is BEFORE the relay's switching semi,
> >and connected to V+ by the resistor. On first turn on the
> >full voltage comes from the cap to pull-in the relay, and
> >as soon as the cap discharges the resistor provides enough
> >holding current. I normally aim for about 35% holding
> >current. Never had a problem with it. (also decouples
> >the supply from the relay back emf quite well) :o)
> >-Roman
> One thing to be aware of with doing this - if the product may be
> subject to mechanical shock (e.g. a handheld unit that can be dropped
> onto the bench) , this can cause the relay to drop out, and the low
> holding current will not be enough to pull it back in.


Yep, that's definitely a good point. Generally I've found
that the pull in current is *much* higher than the hold in
current, because when holding, the armature is closed and
there is no magnetic air gap. Most hold in currents can
be reliably less than 10% of the pull-in current, but
I use about 35% as a general rule. At 35% I think it
would be practically impossible for a drop shock to
pull it open, based on how hard it is to pry open
with a screwdriver etc. But I would advise caution if
someone was to push that limit by using very low
holding currents. :o)
-Roman

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