Re: Maximum input voltage for "can" style (Lucas/Bosch) ignition coil (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Wed, 17 Aug 2005 14:22:22 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Maximum input voltage for "can" style (Lucas/Bosch) ignition coil
    (fwd)

On 16 Aug 2005, at 18:00, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Wed, 17 Aug 2005 07:46:00 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: hvlist@xxxxxxxxxx
> Subject: Re: Maximum input voltage for "can" style (Lucas/Bosch)
> ignition coil
> 
> Malcolm wrote:
> "Have you measured the primary inductance of the coil and calculated
> the peak current that will be reached?"
> 
> I know how to calculate the reactance of an inductor in an AC circuit,
> but have to confess that I wouldn't know how to do this.
> 
> Would you care to share the formula?

If you apply a step function to it (i.e. switch some voltage V across 
the primary, the current is time related by:  i(t) = V.t/Lp
If you dump a capacitor into it, the surge impedance is SQRT(Lp/CP)
and the peak current assuming no losses and no energy transfer to the 
secondary will be i = V.SQRT(Cp/Lp)
 
> Malcolm:
> "The ignition coil is basically a resonant device
> and that means that the caps will have to withstand high amplitude
> reversals, something they are not designed to do."
> 
> No, they are not!  To avoid electrolytic pyrotechnics, maybe I should
> have some sort of (large) diode/resistor arrangement between across
> the output of the capacitor bank to sink anything nasty coming back at
> me. Something like:
> 
> ...--+----|>|----+------+
>      |     D     |      |
>     ___          >      )
>     ___ C        <      )
>      |           > R    ) L
>      |           |      )
>      |          ___     |
>      |           ^ D    |
>      |           |      |
> ...--+-----------+------+
> 
> ...where C is the last capacitor and L is the primary of the coil.

I'm not sure how well that would work. When the voltage across L 
changes sign, the R will absorb energy you want to use. On the next 
half cycle, L will be looking at something like an open circuit 
(maybe a few hundred pF of capacitance if there is no load and the 
voltage across the coil would probably skyrocket. I'd be tempted to 
model it in SPICE but you may just want to try it. I doubt whether 
the capacitors would explode on a single discharge.

Malcolm