Ron Tebo wrote:
> snip
> Tom:
>
> If we had one of those engineer guys available....(sigh).... they could
> probably tell you where to install a condenser (and what value) to stop the
> arcing without affecting performance!
>
> Ron Tebo
Ok, getting back to some fundamentals here:
When a switch feeds current to a coil (ignition coil, windings on a motor) you
will have
the tendancy for arcing when the switch opens. As the switch opens, the current
theoretically trys to stop instaneously. The magnetic lines around the coil
collapse and
generate a voltage (back emf). It is pretty easy for even modest current
flowing in a
small coil to generate several hundred volts in back emf. This will arc over
the gap
created as the contact are opened. A capacitor placed across the contacts
comes into
play as the contacts are opened - current flows for a brief time, exponentially
going to
zero current. It doesn't take much capacitance to really knock down the
voltage. As I
recall (and I may be remembering incorrectly) the voltage is a function of
di/dt -
instaneously interrupting the current generates infinite voltage, allowing the
current
to taper to zero even for a microsecond, reduces the back emf tremendously.
What value of capacitance ? I don't know, why don't you experiment? Start
with a 0.1uf
cap and see if that reduces the visual arc. It would be a better test if you
could hook
the sitch up to an oscilliscope and measure the difference, but lacking that
you could
just eyeball it. Wire the cap to bridge the switch contacts, if you have a
rotary
switch, wire from the common to each switched contact with a separate cap.
BTW, this may or may not make a significant difference to the life of the
switch.
Arcing of contacts is not news to a switch designer and I would bet that they
took this
into account when the switch was designed. As the contacts engage, disengage
there is a
"wiping" action to clean the contacts. Also the arcing may be concentrated at
one end of
the contact, leaving the remainder of the contact clean and useable. But then,
this is
a Lucas switch, isn't it?
-Roger
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