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From: "Pat Ford"
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Mon, 30 Sep 2002 10:40:47 -0400
Organization: National Research Council of Canada
NNTP-Posting-Date: 30 Sep 2002 14:40:41 GMT
X-Newsreader: Microsoft Outlook Express 5.50.4807.1700
Why do they isolate the battery on shutdown?
"Jim Weir" wrote in message
> "Sir Charles W. Shults III"
> shared these priceless pearls of wisdom:
> -> 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
> 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 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.
> Jim Weir (A&P/IA, CFI, & other good alphabet soup)
> VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
> http://www.rst-engr.com firstname.lastname@example.org
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