Would Snubbers Kill Ignition Coil Output? (fwd)

[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


-- 
Matthew Smith
Kadina Business Consultancy
South Australia
http://www.kbc.net.au