Tach input from spark voltage (idea)

[Johnny]

Don't let what I said discourage you. Most of the applications you are
probably going to be targeting use the "standard" 12 volt coil. It is only
the CD ignitions, whether electronic or not, that give the big kick to the
primary side of the coil, and a lot of them use the standard 12 volt coil
anyway. I would think that you would be covering the vast majority of apps
if you designed around that model. Or, if you couldn't do a one fits all
solution, do one that fits what you need or the majority of the market
place first, then cover the rest later with a different model.

I can see the usefulness of the inductive opto-isolated primary side tach.
No matter what happens to the tach or the interface, it won't affect the
ignition itself (aircraft thinking here). Most electronic ignitions have a
tach output, but what happens when the tach that is connected to them
shorts out? I think you have a great idea by doing inductive pickup on the
primary side.

-j-

----------
> From: Grant Beattie <grantb at nait.ab.ca>
> To: diy_efi at coulomb.eng.ohio-state.edu
> Subject: Re: Tach input from spark voltage (idea)
> Date: Sunday, October 20, 1996 12:34 PM
> 
> On Sun, 20 Oct 1996, Johnny wrote:
> 
> > one box that uses coils that see about 400 volts on the primary side.
Some
> > of these systems use a standard 12 volt coil, some do not. I know that
this
> > answer isn't as cut and dried as you had hoped, but they are not all
the
> > same. If you are looking to do a "standard" ignition, the primary
voltage
> > is less than 12 volts. Are your concerns for an inductive pickup about
> > saturation or calibration or?
> 
> The inductive approach works fine for single-cylinder (kart, lawnmower
> <g>, ...) engines, but in a car, there are too many stray signals from
the
> other plug wires and such.  I started playing around with the
sensitivity,
> but that means I would need different pickups for different engines.  So
I
> thought maybe if I went back to the source... 
> 
> The range 12v to 400v is mildly discouraging.  I know national makes 
> devices that might fit this application.  I was hoping to use an 
> opto-isolator though, and the limited current range (1mA to 16mA) is 
> tough to match to 12v to 400v.   Hmmm.
> 
> Thanks for the info,
> GB
> 
> http://ourworld.compuserve.com/homepages/grantb
> (Data acquisition for racing)