Ignition questions & the Nat semi Injector driver

William Sarkozy mymove at serv01.net-link.net
Tue Apr 16 00:31:31 GMT 1996


At 05:44 AM 4/14/96 GMT, you wrote:

>   General: When the field in the primary collapses (the +12V side of the
coil) 
>there is a sudden induced voltage, duh... But is it true that the direction of 
>the voltage is the reverse of it's initial condition? Also these back
conditions
>can have voltages which can be of several hundered volts--if anyone has done 
>anything in this area don't be shy!!!!! I know that the stupid point system
can 
>take it so why can't a transistor and some diodes?

Sometimes its convenient to think of mechanically equivilant systems.....The
inductance of the ignition coil ("L"...a physical constant based on geometry
and phisical characteristics) is similar to mass (M) in a mechanical system.
Current (I) is similar to velocity and voltage (V) is similar to force.  Now
then, when you have a mass moving at a finite velocity (a finite current
passing through an inductor) and you attempt to abruptly stop that mass (the
"brick wall" syndrome), what have you?  You guessed it: a force proportional
to the time rate of change of velocity (F=m*dv/dt).  So it goes with an
inductor: V=L*di/dt.  The polarity requires a little thought.  When you
"push" on a mass to accelerate it, it resists.  We tend to think of this
resistance as a "positive" force, although it really makes no difference.
What is important is that once the mass is moving and we attempt to slow it
down, it wants to "pull" us along (negative or opposite of "push").  Again
so it goes with the inductor.....when you first close the points (or turn on
the transistor), the current through the primary of the coil rises over some
finite time.  When you open the points....wham...small dt, finite di, finite
L all add up to a large voltage.  Note that since the current is FALLING
(rising in the negative direction), the voltage is opposite of that when the
inductor is being charged. 

How do you manage those small displacement, large forces in a mechanical
system?  You add springs at strategic locations.  So it goes with the
electrical system (note the capacitor across the ignition points).  It is
similar to a spring between you and the mass in the mechanical system.  Note
that the spring can't be too "soft" (capacitor value too large) or you'll
not get those high di/dt values which are responsible for the induced
secondary voltage.  You want high di/dt, but not so high that you destroy
the driver circuits.  All is a trade-off.   

This is a simple explanation.  Modern CD (Capacitor Discharge) circuits add
subtle refinements which lead to ultimate performance increases.  There once
was a book by Markus called ELECTRONIC CIRCUITS MANUAL which has a dozen or
so ignition schematics.  They're probably all out of date by now, but are
always good for reference.  E-mail me and I'll fax them to you if you like.

                                                                            
                 Bill




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