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nuckolls.bob(at)aeroelect
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 Posted: Fri Jun 20, 2014 9:51 am Post subject: P-leads |
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At 12:37 PM 3/24/2014, you wrote:
| Quote: |
The concept of a "fuse" to keep faults in a tachometer from
disabling an ignition needs to be stamped out, it is a dangerous
misconception. I know of at least one incident where this almost
caused a fatal crash. It takes very little current through a P lead
from a magneto or points wire in a Kettering ignition to disrupt
performance of the ignition. Can you depend on this amount of
current blowing a fuse? Is the resistor going to open if there is a
fault? Is the engine going to keep running well if the far end of
the resistor is shorted to ground or +12? Not unless it has been
tested to do so.
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Yes . . . these techniques have been tested many times . . .
except for the fuse . . . I don't recall seeing fuses
recommended for this application but agreed . . . they're
a bad idea.
Having said that, let us consider the use of resistors to
sample the really trashy signal that appears across the points
of either a magneto or Kettering system.
The points are already paralleled by a capacitor on the
order of .1 to .3 uF. The capacitor is necessary in
both systems to 'resonate' with the primary winding such
that rate of rise for voltage across spreading points
is slower than that level which establishes an arc
in the gap.
The source impedance of this 'ringing' energy source
is pretty low in the Kettering system . . . on the order
of 10 ohms, lower still in a magneto. Yes, the voltages
at the points can exceed 200 volts during the spark
interval . . . but the idea that a 1,000 ohms or larger
isolation resistor presents a meaningful load simply
doesn't compute.
| Quote: | There are many tach designs that work without an ignition pickup.
VDO and other companies make programmable tachs that can be set to
take a signal from one phase of an alternator output, or an
inductive pickup from a flywheel or magneto case. Proper design of a
tach could also yield failsafe operation but that's still putting a
lot of faith in something you haven't tested. The "proof by
assertion" that I've seen from a few instrument vendors isn't really
confidence-inspiring.
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I've never heard of this being an issue. I've sampled
p-lead and Kettering points through resistive attenuators
dozens of times over the years with nary a concern
for any ill effects being reflected back to the source.
Magneto p-lead signals are really trashy, I have
found it useful to run the attenuated signal
through 4 poles of band pass filtering before squaring
it up to drive the digital interpreters.
A fuse doesn't even make sense. The signal being
observed is WAAYyyyy to big as a digital signal
source . . . attenuation on the order of 30:1
is the first thing you need to do with it.
Bob . . .
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raymondj(at)frontiernet.n
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| Posted: Fri Jun 20, 2014 10:42 am Post subject: P-leads |
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Please give a quick definition of the ignition designs and techniques
that are being discussed. I need some background to follow the discussion.
Thanks
Raymond Julian
Kettle River, MN
The things we admire in men, kindness and generosity, openness, honesty,
understanding and feeling are the concomitants of failure in our system.
And those traits we detest, sharpness, greed, acquisitiveness, meanness,
egotism and self-interest are the traits of success. And while men
admire the quality of the first they love the produce of the second.
-John Steinbeck, novelist, Nobel laureate (1902-1968)
On 06/20/2014 12:50 PM, Robert L. Nuckolls, III wrote:
| Quote: |
<nuckolls.bob(at)aeroelectric.com>
At 12:37 PM 3/24/2014, you wrote:
>
> <dlj04(at)josephson.com>
>
> The concept of a "fuse" to keep faults in a tachometer from disabling
> an ignition needs to be stamped out, it is a dangerous misconception.
> I know of at least one incident where this almost caused a fatal
> crash. It takes very little current through a P lead from a magneto or
> points wire in a Kettering ignition to disrupt performance of the
> ignition. Can you depend on this amount of current blowing a fuse? Is
> the resistor going to open if there is a fault? Is the engine going to
> keep running well if the far end of the resistor is shorted to ground
> or +12? Not unless it has been tested to do so.
Yes . . . these techniques have been tested many times . . .
except for the fuse . . . I don't recall seeing fuses
recommended for this application but agreed . . . they're
a bad idea.
Having said that, let us consider the use of resistors to
sample the really trashy signal that appears across the points
of either a magneto or Kettering system.
========================== SNIP =====================================
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