Re: ignition condenser - revisited


TeGGeR® wrote:
> jim beam <nospam@xxxxxxxxxxx> wrote in
> news:7rudnY5d1a-dCfzeRVn-pw@xxxxxxxxxxxxx:
>
> > TeGGeR® wrote:
> >> jim beam <nospam@xxxxxxxxxxx> wrote in
> >> news:HpSdnczef4l1EvzenZ2dnUVZ_sudnZ2d@xxxxxxxxxxxxx:
> >>
> >>
> >>>TeGGeR® wrote:
> >>
> >>
> >>>>And less effective capacitance would result in what? A reduced
> >>>>spark?
> >>>>
> >>>
> >>>yes, lower spark energy. this affects output - to a degree.
> >>>
> >>>
> >>
> >>
> >>
> >> Would this condenser perform the same sort of role a Kettering
> >> condenser would?
> >
> > yes, identical function.
>
>
> Are you sure? It's in the line from ignition switch to coil, which suggests
> to me that it's there to prevent back-voltage from getting to the ignition
> switch.
>
> Kettering condensers absorb primary voltage for a time to keep the coil
> field from collapsing until the points are far apart, to prevent flashover.
> Transistorized systems don't need that, do they?
>
snip
+++++++++++++++++++
I checked my ancient engineering textbooks, from 1973 when solid
state ignitions were just begining to become viable, and they all
mention the capacitor is to prevent premature wear due to arcing
of the actual points. I do know that in the early 1960s I was given
an automotive ignition coil andwas disapointed to find out that
you had to constantly intrupt the current to get a continious spark.
My dad showed me how to wire a 12V relay as a very crude vibrator.
I burned up the contacts in less then a week so we placed his
capactiro substitution box across the "new" relay and tuned for
minimum contact spark. The output voltage was uneffected, or
the effect was so slight as to be unnoticable.

I have designed and built "snubbers", a capacitor and a resistor
that much be matched to reduce switch arcs under haeavy loads,.
There are some very nice formulae that don't produce acceptable
results.

Your comment,"> Kettering condensers absorb primary voltage for a
time to keep the coil > field from collapsing until the points are
far apart, to prevent flashover.", could mean the capacitor keeps
the filed form colasping by taking time to charge mimicking a
temporary "short" across the switch.

> Transistorized systems don't need that, do they?"
Transistors are almost certain generate much higher transients,
and transostors desinged for heavy indudctive switching have
a special internal structure to enabel them to cope without
being destroyed. A common solution is to place a zener diode
across the collector emitter junction to limit the peak
voltage spike. I am old enough that I have seen early transistor
switches had a resistor capactor snubber across them.
I would rate the modern, specialty transistors that are designed
to switch inductive loads a 100, a hefty transitor protected by a
zener a 80, and a "plain oold transistor" a 50. Now if we had
24V or 48V as our power rail, MOSFETs would be very usefull,
and very robust. I designed a door latch using an early power
MSOFET in ~1982. It operated from a 48V supply and it is still
working. Newer MOSFETs are much better from every charactoristic.

My favorite ignition system was the CDI, "Capacitor Dischage
Iignition",
system. I designed and built one for a marine application that was
killer. It tended to eat spark plugs, but you couldn't foul the thing
with 50% oil premix. If I was on a budget and my ignitor died I would
be very tempted to design and build a CDI for my Civic. My main
complant
about CDI systems is the nasty RFI they produce. Which I why I suspect
Honda and everyone else went to active "ignitors" A nice hot spark,
but not too hot.

Terry

.

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