Ignition coil saturation time

[HENRY CRAIG STEVEN]

The Question has been:

> >>>Should you run the coil until the current saturates?  Slightly less, or
> ..
> >The answer for your coil can be found by trial and error. Build a variable
> ..
> >   |_____|-----|
> >A  B     C     D
> >
> >A is the before switch on time
> ..
> 
> 
> Does anyone remember the amps vs. time equation for an L-R coil load?  This 
> would help the thinking on the tradeoff between coil *energy* (as others 
> previously posted) and rise time for the current till saturation.
> 
> Isn't it something like I = V / R *(1-e^-((L/(R*t))) ?
> 

The equation is i(t) = V/R [1-exp(-(R/L)t)]

However, I may be just blowing in the breeze here, but if you want to 
quickly figure a proper charge time then draw a simple R-L circuit then 
find i(t) by arbitarily choosing the direction of the current around the 
loop and obtain an equation.  Note that the current in the inductor at t=0 
is ~0, since the current in the inductor cannot change from zero to some 
value instantaneously.

Take the Laplace transform and use the initial condition i(0)=0, then take 
the inverse Laplace transform and you should get the eq.:
  i(t) = V/R[1-exp(-(R/L)t)] 

Ploting i(t) vs time will show the circuit reaches aproximatly 98% 
steady-state at 4 time-constants (4 tau),  or aprox 92% at 3 tau.

Therfore, IMHO I would look at the available charge-time for the 
smallest engine period (Max rpm), then select a proper time constant between 
3-tau to 4-tau that does not exceed 4tau.

 That would be my $.02 if I had any. (money not sense, I mean cents).

 Craig Henry
 University of Texas @ Arlington
 Mechanical Engineering
 csh5742 at decster.uta.edu