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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 16 Dec 2003 10:47:18 +1300
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Would Snubbers Kill Ignition Coil Output? (fwd)

On 15 Dec 2003, at 9:41, 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.

Which is hundreds of volts.

> 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?

Yes and yes. In fact, the capacitor one normally finds across the
points (or switching element) does limit the voltage rise somewhat
but also causes the coil to ring like a TC. You are probably better
off using a high voltage switching bipolar transistor.

> 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.)

What about dwell time? It takes time for the core to store energy.

Malcolm


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