From: Mike Poulton
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: 26 Sep 2002 21:45:51 GMT
Organization: MTP Technologies
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
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