[Gmecm] Big cap HEI on 7747

[bcroe at juno.com]

I agree it fires at the threshold.  But the point is, the 
threshold is very near the polarity crossover voltage 
(zero) relative to the signal amplitude.  So amplitude 
variations (due to temp, velocity, etc) have negligible 
effect on timing.  There are no perfect circuits out 
there, and I won't have time to measure how close 
that threshold is to zero for a while.  Maybe you know?  

Bruce Roe

13 Jun 2006  "Steve Ravet" <Steve.Ravet at arm.com> writes:
> > The module senses the crossing of voltage from one polarity 
> > to the other; the voltage it switches at is zero, position of 
> > which is not affected by amplitude.  The module is supposed 
> > to fire on the very rapid transition, which will be very accurate. 
>  
> > If the wires are reversed, the module will fire on the very 
> > slow changing part of the waveform crossing zero, which is 
> > way off in time and not so accurate.  My recollection is that 
> > proper firing is on the negative edge (relative to ground).  
> > 
> > Bruce Roe
> 
> My recollection is that the module fires on a threshold voltage, not 
> at
> zero.  The threshold voltage will vary in time from the zero 
> crossing
> depending on RPM, meaning that the module has some RPM based advance
> built into the triggering circuit.  Somewhere in the archives is a
> patent (and SAE article number) that describes this.  In fact I 
> think
> the datasheet of the triggering IC also has a good description.
> 
> For Robin's question, millions of TBI computers are triggered by 
> this
> HEI setup.
> 
> --steve
> 
> 
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