>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Sascha
>Sent: 03 November 2004 06:08
>To: 'Microcontroller discussion list - Public.'
>Subject: RE: [EE]:Coil core saturation
>
>
>Hi guys,
>
>I am designing a multispark ignition for my car and I 
>encountered a problem. I whant to know how much time the 
>ignition coil needs to be driven so it stores the maximum 
>magnetic field in it's core. Unfortunately I can not find the 
>specs of the induction coil so I can compute it. Is there any 
>way I can find this time? The induction coil has a primary 
>winding of about 200 turns and a secondary winding of 20.000 
>turns. That's all I could find about it. If I keep the driving 
>transistor saturated too much I loose power and if I turn it 
>off too soon I will loose energy in the spark. None of this is 
>acceptable. 
>First I thought of adding a emitter resistor and measure the 
>voltage drop across it. But this again means power loss 1.5V 
>across the transistor, 0.5 across the resistor and if the 
>battery is not very strong when I try to start the car it will 
>drop to 9-10V so the remaining voltage across the induction 
>coil will be as low as 7V. And this will translate as a weak 
>spark right at the moment where I need the strongest. I want 
>to drive the coil with a BUT12 transistor which is driven by a 
>PNP or a PMOS.
>
>Any suggestions?
>
>TIA,
>Sascha

You cannot practicly use an inductive system for a multi-spark ignition.
Even with the low inductance coils used for electronic systems, the time
required to charge the coil means you will only get multiple sparks at very
low RPM.  Inductive systems give a long duration spark which means that
multiple sparks are not generaly required anyway.

The proper multispark systems use a capacitive discharge system with a high
power inverter to charge the capacitor quickly.  This results in multiple,
short duration sparks.

Regards

Mike

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