From: Jim Weir
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Thu, 26 Sep 2002 08:27:04 -0700
Organization: RST Engineering
X-Newsreader: Forte Agent 1.91/32.564
"Sir Charles W. Shults III"
shared these priceless pearls of wisdom:
-> Charging an inductor produces a magnetic field where energy is stored.
->power is removed from the inductor, the field collapses and the energy must go
->somewhere. Usually, this energy creates a high voltage spike or pulse. A
->common DC motor that runs on 3v (like many toys contain) can easily create 200
->volts or more in spikes.
That is so.
-> This spike can occur most readily when the power is removed, not when it
->applied, because that is when the magnetic field collapses.
That also is true. On engine startup, that magnetic field collapses in a big
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 that has
just had the massive spike absorber called a battery removed from the circuit.
The solution is
->usually pretty simple- a large rectifier placed backwards in parallel with the
->field winding will snub almost all of this spike, and the use of a transorb or
->resistor/capacitor snubber will usually handle the rest of it.
Neither master switch relay nor starter motor have a snubber diode, and I was
roundly jeered in this ng for a design I published showing transorbs used as
inrush current limiters on a light bulb. You really can't afford the I*R loss
of an RC network on most avionics busses.
-> It seems odd that aircraft would not have this most basic sort of noise
->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