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From: "Tarver Engineering"
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Thu, 26 Sep 2002 14:50:38 -0700
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Mike Poulton" wrote in message
> On 26 Sep 2002, Peter said:
> > "Peter Gottlieb" wrote:
> >>2) Reduces chances of damaging spikes getting to sensitive
> > What would happen if I start the engine by allowing the vehicle to run
> > downhill, when the ignition switch is on all the time? The engine is
> > started while all the appliances are connected. Lots of people do this
> > and one never hears of things getting damaged.
> > What would happen if I stop the engine by stalling it (while in gear)
> > with the ignition switch on? Lots of people do this, again no reports
> > of damage.
> > I think the real reason why the ignition switch disconnects the
> > appliances is that it reduces the battery loading - especially true
> > with fans, headlamps etc.
> > Can anyone can offer a proper reasoned electrical explanation of this
> > often claimed phenomenon?
> The current draw when starting is huge. Hundreds of amps. That's
> enough that the battery voltage sags well below the reset voltage for
> most all the electronics attached to it. Consequently, they would reset
> anyways -- why not shut them off completely to reduce drain? Also, the
> starter is an inductive load. When it turns off, it will produce a high
> voltage across the starter relay. Depending on how things are wired
> (and where other wires go) this could conceivably cause a spike in the
> 12V system, or in other circuitry (sensor wires, etc.). he starter
> motor is not involved when you pish start or stall your car, so that is
> not really an analogous situation. It's worth noting that the engine
> computer must not be turned off when you are starting, or it wouldn't
> work. --
The engine computer is of course switched into a special starting mode, for
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