Re: Would Snubbers Kill Ignition Coil Output? (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 15 Dec 2003 18:10:37 -0800
From: Ed Phillips <evp@xxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Would Snubbers Kill Ignition Coil Output? (fwd)

High Voltage list wrote:
>
> Original poster: Steven Roys <sroys@xxxxxxxx>
>
> ---------- Forwarded message ----------
> Date: Sun, 14 Dec 2003 17:10:58 +1030
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Would Snubbers Kill Ignition Coil Output?
>
> Hi All
>
> Thinking of putting electronic ignition on my aged Massey Ferguson tractor -
> discretely of course, don't want to spoil the look of the thing.
>
> Just fiddling around with the components that I've got, I have some IRF748V
> HEXFETS, which look ideal for the job of switching, having a TINY R{DS(on)}.
>
> Only problem is that they are only rated up to 60V.  This is fine for the sort
> of voltages that I would expect to find around the tractor, but I'm rather
> concerned about the magnitude of the back-EMF from the ignition coil.
>
> My thought would be to put in a snubber with RC in parallel in series with a
> diode (I've got some nice hefty TO220-style SMPS diodes).
>
> However, would putting the snubber in kill the output of the coil and not leave
> me enough to fire the plug?  My documentation is all SMPS stuff and doesn't
> cover this type of core arrangement, so I'm not sure what's going on in there.
> Does it act like a flyback with the mother and father of all airgaps?
>
> The idea was to have the points pulse a 555 in monostable mode, putting out a
> 900ns pulse, driving the MOSFET via a totem-pole arrangement to speed up the
> switching.  (The idea being to cut down losses and stop the thing getting too
> darned hot.)
>
> Cheers
>
> M

	The "snubber" will definitely reduce the maximum voltage.  I have done
quite a bit of experimenting with transistor drivers for ignition coils
and the least breakdown voltage I've used is 200V.  The best driver uses
an IGBFT with 1500 volts breakdown.

Ed