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 12:21:28 -0500
From: D.C. Cox <resonance@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Maximum input voltage for "can" style (Lucas/Bosch) ignition coil
    (fwd)



I have used 650 VDC with a 1.5 uF cap in a single shot pulse mode ---  
repeated once every 5 seconds.

Dr. Resonance
>
>
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> ---------- 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
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