"George White" bravely wrote to "MIKE ROSS" (16 Mar 04  23:47:30)
 --- on the heady topic of "{at}%^{at}#$%^ veroboard"

 GW> On 16-Mar-04, MIKE ROSS wrote to Jasen Betts:
 JB>> relays have inherent in themselves a measure of mechanical
 JB>> positive feedback.

 MR> Yes a "holding current" rating.

 GW> No, it's actually a genuine, mechanical, posetive feedback. (Yes, I've
 GW> spent time designing relays in the past, when there was much more
 GW> electro-mechanical stuff about). When a relay is open there is a
 GW> significant gap in the magnetic circuit. When it is energised the
 GW> movement of the armature towards the pole reduces the air gap, and
 GW> provides the positive feedback. In fact all relays have some form of
 GW> air gap built in to stop them holding closed on the residual magnetism
 GW> of the pole, a peice of non macnetic material of some kind somewhere
 GW> in the magnetic circuit. On old relays it was often a copper rivet, on
 GW> modern ones usually part of the plastic mouldings used in the
 GW> manufacture.

This magnetic path break is sometimes hard to find but the armature is
often mounted using a brass spring hinge. One thing for sure is it
takes a whole lot less current to hold a relay so that indicates some
kind of reinforcement mechanism.

However, a positive feedback entails returning energy to a system
during part of an oscillation cycle. In the case of a relay, it really
exhibits an electromechanical hysterisis.  Many things exhibit
hysterisis without ever requiring a positive feedback. If it was
really positive feedback then wouldn't a relay oscillate? (bounce or
chatter is something else)

In my own understanding one description is that it exhibits a positive
reluctance (i.e. increase in magnetic conductance) but not a feedback.
Say, if one were to add capacitance to this system could it oscillate?
I would appreciate if you would explain this better to me?

 Mike
 ****

... Inductor designers do it in the gap.
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