Hi MIKE,

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. It's due to a mechanical inertia
 MR> that the relay motor works against to both turn on and then to
 MR> turn off. That's not exactly a "feedback" but the electronic
 MR> equivalent would indeed require a positive feedback to behave in
 MR> that way.

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

George

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