From: John Woodgate
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Fri, 27 Sep 2002 11:29:44 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Fri, 27 Sep 2002 13:22:03 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Peter
wrote (in ) about
'(Avionics) How can this circuit produce an "inductive surge"?', on Fri,
27 Sep 2002:
>I've seen qualitative answers only so far, regurgitating stuff from
>the standard texts on the subject, but nothing that answers the
As is not unusual, the original question is probably long gone from the
minds of respondents.
Your diagram doesn't seem to show where the 'engine off' switch is. If
it's one of the three on the right, then it is in a very good position
to dump any inductive surge from the ignition circuit into the avionics
if they were switched on. Remember that for surges, every wire is a
significant inductor. Long conductors do not conduct well; short ones
You have been concentrating, I think, on relatively slow surges caused
by failure of the alternator voltage control to react quickly. It
probably doesn't need to react quickly when the field switch is closed,
because the inductance of the field winding slows down the rate of rise
of current in it.
A full discussion of the subject would require a very long post. A
summary would be:
Slow voltage surges due to varying current demand by the starter during
cranking. Possible induction of surges into low-current wiring by the
magnetic fields of the starter, its relay and the heavy-current wiring.
Fast surges due to the inductance of the ignition circuit on
interruption. Similar fast surges due to the opening of the alternator
field switch and perhaps of the master relay.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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