"Greg Mayman" bravely wrote to "Mike Ross" (19 Mar 04  08:50:00)
 --- on the heady topic of "{at}%^{at}#$%^ VEROBOARD"

 -=> Mike Ross said to Jasen Betts
 -=> about "{at}%^{at}#$%^ VEROBOARD" on 03-16-04  09:00.....

 MR> Yes a "holding current" rating. It's due to a mechanical
inertia that
 MR> the relay motor works against to both turn on and then to turn off.

 GM> That explanation may be one point of view but it is greatly
 GM> missleading. There isn't any motor as such to operate the relay,
 GM> and there definitely isn't any motor to release it - unless you
 GM> class a spring as a motor.

Not really the same type of spinning motor we commonly think of. What
I meant was more fundamentally a device which converts electric energy
into a mechanical force. For example, the moving parts in a relay form
a linear motor, just like the solenoid in a pinball machine flipper.


 GM> The way it works is this, if you'll pardon me for being a bit
 GM> wordy.

One can make things simpler but they shouldn't be too simplified or we
lose some meaning.


 GM>  As the armature gets
 GM> closer to the pole of the solenoid, the airgap in the magnetic
 GM> circuit gets shorter and the magnetic attraction gets stronger,
 GM> so the relay snaps in for the last part of its travel.

But the question was if this was a positive feedback? An analogy of
this action might be for example an electric arc where a certain
threshold voltage must be attained before the gas ionizes. Then once
it conducts the voltage required to maintain the arc is quite low.
There is no positive feedback to be found in this either. The idea of
a positive feedback requires energy be added to the system. In my
opinion we don't find it in the relay action. Where is energy added?


 GM> The reason why the current level above which the relay will
 GM> operate (turn on) is so much higher than the current below which
 GM> it will release (turn off) is that the pullin current has to
 GM> generate enough magnetic attraction to pull in the armature from
 GM> over a wide air gap. The holding current just has to generate
 GM> enough magnetic attraction to hold the armature in over a much
 GM> narrower air gap.

Your explanation is excellent. The same applies to the magnetizing
threshold in a audio tape oxide particle but there is no positive
feedback in that either.

 Mike
 ****

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