Ignition questions & the Nat semi Injector driver

[Jim Staff]

    I got some of them as samples, and was wondering if anyone had used them 
before? They look real easy to set up if fact I've put the schematic together 
for it, I'll got to Radio Shack, and I'll be good to go with that part.

    But to business, It warns that if the current to the sense pin is below a 
certain value then it doesn't stop saturating the coil until a timer times out, 
what would be the best way to set up the timer if you know your injector is 
going to be a bad ass load, like 8 amps?

    2nd, could I use this driver to control a ignition circuit?

    3. What is a good design for an electronic igniton system. Particularly how 
do I stop the Back EMF from blowing everything apart after ignition? I though 
about placing several (4) tip120 darlingtons to switch the current, and then a 
couple (2) 6A10ct 1000V @ 400Amps max reverse conditions. That way I can switch 
on a max of 32 amps, for Å1ms, and after the field collapse I can take a 2000V 
reverse spike.

   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?

                     +14V from Battery
                            |
                    --------+----------|
                    |       ---|       -------O  <-- Ignition Event signal
                    |          |       |
                   / \          -----<-|   < 2n222A Low signal 
Protection Diode-> | |         |
                   ---      ---| < Tip120 5amps & 65 watts
                    |       |
                    |       |
                    ----+----
                        |
                        |
                        |
                       GND          Note : Only 1/4 of total circuit


Standard Wave as seen from the ignition circuit
                                V---- Ignition event
    +14V- - - - - - - - - - - * - - - - - - - - -
                         *     *
                     *         *
                  *             *
                *               *
    0V- - - -  * - - - - - - - - * - -  * - - - - -
                                 *     *
                                 *    *
                                 *    *
                                 *   *
   -xV- - - - - - - - - - - - - - * * - - - - - - -
                                   *


Over X time units the coil saturates, then when the circuit is disconnected the 
primary collapses this causes the spark energy to be produced, but also it 
produces a backrush that finds its peak as the circuit is switched. then it 
homes back in on 0V. This large negative voltage spike is what I have to stop 
from destroying my transistors. I believe that the diode will allow this voltage
to reach the positive terminal and be dispanded, but as a side effect will be 
super saturated, but I have very limited ignition requirments even though I 
would like a MSD from this I'll live..


                  Thanks everyone for the help!!!!
                                   Jim Staff
                  P.S. : Sorry about the ASCII Art, I used to do graphics for 
BBSes (Which use ansi) but I'm rusty!!