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From: Jim Pennino
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Thu, 26 Sep 2002 22:16:19 +0000 (UTC)
Organization: Special Solutions, LLC
NNTP-Posting-Date: Thu, 26 Sep 2002 22:16:19 +0000 (UTC)
User-Agent: tin/1.4.5-20010409 ("One More Nightmare") (UNIX) (SunOS/5.9 (sun4u))
In rec.aviation.owning Peter wrote:
> "Dennis O'Connor" wrote:
>>Yes... Common knowledge is commonly wrong... The alternator cannot run away
>>during engine cranking and fry the electronics... Leaving the radios on
>>during engine start up and shut down involves no more risk of overvoltage
>>than at any other time or point in flight... >small battery at a critical time>
> Nothing I have seen so far contradicts what you say - especially as
> the operating input voltage range of e.g. 24V avionics is normally up
> to about 36V (e.g. Bendix King).
> So what is going on? I do know of several people who blew up their
> avionics, reportedly during engine start/stop operations.
> OK, all it takes is a dodgy connection to the voltage regulator, and
> you might get 50V or more...
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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.
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.
Back in the days of tube electronics this was not a problem. Tubes don't
even notice such transients.
Solid state electronics, however, can be fried in nanoseconds.
Least you think the problem is limited to GA, the Army years ago (when
solid state radios began to showing up) started placarding vehicles to
turn off the radios before starting because the troops were frying
their new radios.
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