The collapsing magnetic field in the coil also causes a high 'back EMF' to be
induced in the primary windings, which is what causes the sparking on the
points. The back EMF is generated by the coil, so it affects a transistorized
ignition as well, and if the designer did not put in enough of a snubber circuit
to keep the voltage rise under control, the coil drive transistor gets zorched.
In their coil driver app note, Motorola recommends 600V transistors, and I've
personally fried some transistors rated at 800V trying to run a high-energy coil
without a voltage clamping circuit. Now I know better :)
Theo Smit
tsmit@novatel.ca
B382002705
> -----Original Message-----
> From: Gregory Wells [SMTP:gwells@mindspring.com]
> Sent: Thursday, November 12, 1998 9:13 PM
> To: Tiger List
> Subject: Re: Electronic ignition
>
> Bob Palmer wrote:
> >
> > I can also believe that some form of electronic points would
> improve this capability over mechanical points, since the current that can
> flow through mechanical points is limited from a practical standpoint by
> arcing.
>
> Bob,
>
> In a standard coil and battery ignition the points simply complete the 12v
> circuit to the primary side of the coil. The spark at the plug is initiated
> when the points open, which causes the collapsing magnetic field in the coil
> to induce a high voltage current in the secondary coil which eventually jumps
> the plug gap. The problem with point type ignitions is that there is an arc
> produced across the newly-opened points as well that tends to shorten the life
> and timing accuracy of the points. (The purpose of the condensor in a standard
> ignition is to reduce this arcing.) Many coil and battery ignitions only run
> the full 12v through the points during starting and drop via a resistor to
> some lesser voltage (usually around 8v) during normal running. The dwell or
> period of time the primary coil is being charged through the closed point set
> decreases with rpm, which is not the ideal situation.
>
> Aftermarket ignitions can give alternative methods of triggering spark
> production (i.e. optical triggers, hall effect triggers), alternative methods
> of spark production less tied to the dwell time (i.e. capacitive discharge),
> or both. What you want to use depends on what problems with the c&b ignition
> you are trying to solve.
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