Monitor Automotive Spark Plug?

eeboarder wrote:
> What kind of electrical signal is involved with the spark plug? I know the
> voltage is amplified with a induction coil to thousands of volts, but is
> the signal just a simple pulse? What kind of current is there? Does the
> current create the spark and then return through the same cable?
> 
> I just hooked a split core current transformer with 3000 windings around a
> spark plug cable and got nothing. I'm trying to re-create a signal similar
> to that which a timing light would use. 
> 
> Really, I would like to just know when this engine is on and off without
> any lag. I also don't want to shutdown the engine to install it.
> 
> Any ideas or help would be much appreciated.
forget inductive coupling: what you have is a fast-edge, high voltage, 
infinitesimal current signal. Capacitive coupling works fine. Try this: 
bring your oscilloscope probe _near_ a spark plug wire. One pF of 
coupling will give you a booming signal into a high impedance.

Bob

On May 18, 5:49=A0pm, Bob <SkiBoy...@excite.com> wrote:
> eeboarder wrote:
> > What kind of electrical signal is involved with the spark plug? I know =
the
> > voltage is amplified with a induction coil to thousands of volts, but i=
s
> > the signal just a simple pulse? What kind of current is there? Does the
> > current create the spark and then return through the same cable?
>
> > I just hooked a split core current transformer with 3000 windings aroun=
d a
> > spark plug cable and got nothing. I'm trying to re-create a signal simi=
lar
> > to that which a timing light would use.
>
> > Really, I would like to just know when this engine is on and off withou=
t
> > any lag. I also don't want to shutdown the engine to install it.
>
> > Any ideas or help would be much appreciated.
>
> forget inductive coupling: what you have is a fast-edge, high voltage,
> infinitesimal current signal. Capacitive coupling works fine. Try this:
> bring your oscilloscope probe _near_ a spark plug wire. One pF of
> coupling will give you a booming signal into a high impedance.
>
In fact 1pF of coupling into 10nF of load will produce 10000:1
attenuation which will convert 10kV to 1volt. Also, if a high
impedance is used a very wide frequency response will be obtained. The
data logger on my racing kart uses a wire that is clipped onto the
outside of the plug lead and the other end is clamped to the outside
of the plastic housing. The unit comes out of sleep mode when the
motor is started.