Re: The mechanism behind bouncing...


"KILOWATT" <kilowatt"nospam"@softhome.net> wrote in message
news:45c3aa6d$0$31564$c3e8da3@xxxxxxxxxxxxxxxxxxxx
Hi... thanks for your attention.

I just wish to know the precise reason why for example, a digital counter
may count many pulses on it's clock input when the clock is feed via a non
noise-free source like a mechanical switch. It is because when the
contacts
makes/breaks, arcing (i've read somewhere that there can be a possibility
of
arcing even at low voltage) occurs, or if it's because of the very rough
surface (microscopically-speaking) of the switch contacts, were the metal
molecules grinds (and possibly flexes) together, during switch activation?
TIA for your reply.



The atoms of the two materials are not configured in such a way that there
is complete contact. If they were then the materials would be fused. Since
there are not fused and they slide there is friction involved and this
friction causes the contacts to move farther a part and then closer
together. So the average distance between the constants is changing
significantly campared to when is not moving and they are making good
contact. So now the electric field is changing because of the distances
changing between the contacts. As the contacts move farther away the field
becomes weaker but now we have a capacitive effect. This effect creates a
force between the contacts that attract them. One now has a kinematic force
pulling the contacts away(so it can slide), one of friction that wants to
stop the slide, and one of capacitance that is attractive(I'm sure there are
more too).

Anyways, So there are all these forces that are interacting and the end
result is this oscillation of the contacts moving toward and away from each
other. One always gets "arcing" but thats kinda relative turn. (In some
sense all electronic flow is "arcing".) Generally this term is used when
there is significant macroscopic arcing and this requires much higher
voltages than the microscopic arcing that always happens. (Its just a matter
of degree). This oscillation that happen is called bouncing and the net
effect is to increase and decrease the resistance but with a trend towards
increasing it(which happens very sharply when the contacts finally break
away).

When a switch bounces it does turn on and off instantaneously a few times
then finally settles on off. Nothing is every instanenous in the real
world. The net resistance is dependent on many factors and one of them is
the distance between the contacts. Theres a point where the distance becomes
so great that microscopic forces do not have any significant effect. The
bound that you see on an oscilloscope occurs right at this point where D
becomes more significant and the other forces do not. This point has to do
with how much of the two contacts are actually in contact although it
happens over the whole surface since some points on the contacts are more in
contact than other points. In this case we have an average that approximates
the whole surfeces very well unless the contacts are very distorted.


I guess ultimately the effect I'm talking about is sorta middle ground
between the atomic and the macroscopic. Whats really important here is what
happens when the two contacts are about to completely leave each other and
not what happens before(although it could be significant it is just a
smaller version of the main one). Once you understand this then its not
hard to see that it happens many times before but just on a smaller and
smaller scale. (One could say that bouncing is always happening so its just
a matter of degree).

Anyways, I'm kinda rambling there but maybe it will shed a little light on
your problem.

Jon


.

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