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'[OT]:Current Sensor - current breaking (long-winde'
2001\04\05@200428 by Barry Gershenfeld

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>>         In the motor cicuit, once you open the switch, the current has got
>> to go somewhere. It cannot instantaneously dissappear (because that would
>> require the voltage to be infinite). Where does it go? back to ground,
>> through your resistor. It decays exponentially, not linearly. The faster
>> your PWM frequency, the less the motor current decays in the off period.
>
>And the circuit is probably completed by the nice bright arc on your switch
>contacts.
>
>Bob Ammerman

Now we're getting somewhere!  I've puzzled about this long and hard.
That energy HAS to go somewhere.  It starts by jumping the switch,
as Bob pointed out.  But you can open a switch faster than the
energy goes away.  Or you can use an electronic thingie.  That
voltage goes WAY UP, trying to get out somehow.  It will try to
bust its way through the semiconductor's junctions, (which is why
we sometimes hang diodes and stuff across them).  But if we don't
let it out, where does it go?

All you guys with callsigns should be raising your hand.  It flows
up the wire until it gets to the open switch part.  All the electrons
cram in up there.  The wire and associated parts act as a stray
CAPACITOR, storing the charge there.  Immediately it all reverses
and heads back down the wire towards ground.  Through the motor
and whatnot.  But it doesn't disappear because with all those
electrons hurrying for the exits, they leave behind a deficiency
(holes, positive charge, take your pick), so then all the electrons
come charging up the wire (pun) back towards the switch end again.

Each time through they will lose energy in the resistance of the
motor and whatever else they flow through, and also, some of energy
will radiate off the wiring into space as a radio transmission.  It
will die out after a time (milliseconds).  And
thus the answer is that it oscillates at a frequency determined by
the wiring lengths (inductance) and closeness to ground (capacitance).
You 'scope will show this.  And your radio goes "bzzt" or "click"
at the same time.

Barry
WA2QMI

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2001\04\06@044042 by Roman Black

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Barry Gershenfeld wrote:

> >And the circuit is probably completed by the nice bright arc on your switch
> >contacts.
> >
> >Bob Ammerman
>
> Now we're getting somewhere!  I've puzzled about this long and hard.
> That energy HAS to go somewhere.  It starts by jumping the switch,
> as Bob pointed out.  But you can open a switch faster than the
> energy goes away.  Or you can use an electronic thingie.

Yep, it's called a diode across the motor. Or the
relay coil, or across anything inductive you are
switching on and off. Unless you are deliberately
building an ignition coil and spark plug! :o)
-Roman

PS. If you put your diode across the motor and
current sense resistor (them in series) the diode
will still allow PWM etc and the resistor will
still allow current sensing AVERAGE of the motor.
-Roman

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