From: Jim Pennino
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Sat, 28 Sep 2002 06:55:24 +0000 (UTC)
Organization: Special Solutions, LLC
NNTP-Posting-Date: Sat, 28 Sep 2002 06:55:24 +0000 (UTC)
User-Agent: tin/1.4.5-20010409 ("One More Nightmare") (UNIX) (SunOS/5.9 (sun4u))
In rec.aviation.owning Dan Thomas wrote:
>> The inductive caused transients from things like turning off the starter
>> relay can easily be thousands of volts for a short period of time; i.e.
> I once measured the spike off the master solenoid at 600 volts. Some
> here are saying that the avionics are designed to withstand this; that
> is likely true. However, if the internal protection isn't working, the
> pilot is taking his chances.
>> To make matters worse, when you turn off an inductive load such as the
>> starter relay/solenoid, the polarity of the induced voltage is the opposite
>> polarity of the applied voltage.
> No, it's the same polarity. An inductor resists an increase in
> current, and resists any decrease by giving the flow a shove as the
> field collapses. The result is a spike of the same polarity. The diode
> across the Cessna solenoids doesn't allow any current flow during
> normal operation, but shorts the spike. Drawing it on paper makes it
Yes, drawing it on paper makes it quite clear the voltage reverses and
The cathode of the diode is connected to the (normally) positive side of
the inductor, the anode to the negative side. Under normal operation, the
diode is back biased and no current flows through it.
When the switch is opened, the voltage across the inductor reverses, the
diode is now forward biased and limits the voltage to one diode drop, or
about .7 Volts. The initial magnitude of the current through the diode will
equal the steady state current that was flowing before the switch was opened.
If the voltage didn't reverse, the diode would never do anything. The diode
across the inductor is an ordinary diode, not a zener diode or anything
Without the diode and with an ordinary switch, the voltage instantaneously
tries to go to infinity (which doesn't happen in the real world, but
it gets large fast) and at some point the switch contacts arc and you have
a very large negative voltage across your bus.
If you still don't believe it, get any basic electronics book and look up
inductive loads and diode protection. I suggest "The Art of Electronics"
by Horowitz and Hill.