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From: "Tarver Engineering"
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Thu, 26 Sep 2002 08:40:36 -0700
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"Jim Weir" wrote in message
> "Sir Charles W. Shults III"
> shared these priceless pearls of wisdom:
> -> Charging an inductor produces a magnetic field where energy is
> ->power is removed from the inductor, the field collapses and the energy
> ->somewhere. Usually, this energy creates a high voltage spike or pulse.
> ->common DC motor that runs on 3v (like many toys contain) can easily
> ->volts or more in spikes.
> That is so.
> -> This spike can occur most readily when the power is removed, not
> ->applied, because that is when the magnetic field collapses.
> That also is true. On engine startup, that magnetic field collapses in a
> way when the starter motor is de-energized after the engine starts. On
> shutdown, the master switch relay dumps a hell of a spike onto a system
> just had the massive spike absorber called a battery removed from the
Letting the system stabilize before energising the avionics bus contactor is
a good plan.
> The solution is
> ->usually pretty simple- a large rectifier placed backwards in parallel
> ->field winding will snub almost all of this spike, and the use of a
> ->resistor/capacitor snubber will usually handle the rest of it.
> Neither master switch relay nor starter motor have a snubber diode, and I
> roundly jeered in this ng for a design I published showing transorbs used
> inrush current limiters on a light bulb. You really can't afford the I*R
> of an RC network on most avionics busses.
It is the choke that is common, for avionics busses; as it can remove noise
from the audio panel.
> -> It seems odd that aircraft would not have this most basic sort of
> ->suppression built in.
> They didn't have these devices in the 1930s, which is when most of our
> electrical systems were designed.
They didn't have modern capacitors in the 30s, but inductors were the way.
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