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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 15 Dec 2003 21:41:24 -0800
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Would Snubbers Kill Ignition Coil Output? (fwd)

There are bipolar transistors (darlingtons I think) that are specifically
designed for this sort of application with a 500V+ BVceo.
----- Original Message -----
From: "High Voltage list" <hvlist@xxxxxxxxxx>
To: <hvlist@xxxxxxxxxx>
Sent: Monday, December 15, 2003 7:10 PM
Subject: Re: Would Snubbers Kill Ignition Coil Output? (fwd)


> Original poster: 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
> >
> >
> >
>
>
>