Message-ID: <19980928230014.26127.qmail@www08.netaddress.usa.net>
Date: Mon, 28 Sep 1998 23:00:14
From: Dan Schaefer <dfschaefer@usa.net>
To: lancair.list@olsusa.com
Subject: arc suppression, longeron strength, acronyms
Mime-Version: 1.0

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WOW! It seems that there is more structural wonks out there (hey! I'm 
just repeating what my EE profs used to call 'em) than pilots. Now I'm
getting a little paranoid when I go fly, since I've been reading the 
discussions re: longeron strength, carbon over glass, termination of 
layups, etc., etc. because I have such an early S/N that I have the 
holes drilled in the longeron to accomodate the canopy mechanism.
Well, I do slow down in the bumps, try to land as gently as I can, 
and I don't dork with the stick much when IAS is above 160-170 MPH.
It's an interesting discussion, though, but I keep coming away with 
this helpless feeling. Let's face it guys, you ain't going to get out
of this world alive, no matter who you know. (Unless Lance's customer
list is REALLY impressive!)

An interesting comment by Brent Regan that "..capacitors reduce brush
arcing in motors...". I believe him to mean that caps reduce noise 
CAUSED by brush arcing in motors by keeping it localized or grounded.
Reducing arcing is a bad idea since the brushes rely on the arc-
generated plasma formed between the brush and commutator (or slip-
ring) to conduct the required current without mutually destroying
each other through friction.

Ed Armstrong said using MOV's are safer and handle higher currents.
Not terribly important since the diode's current rating only needs to
be equivalent to current going thru the relay coil since that's the 
current that is transferred to the diode when you open the switch.
If you want to do like we do for military airplanes, you can hedge 
your bets and double the current rating for just pennies. If you 
ascribe to the maxim "Keep it simple, stupid" as I do, (some say , for 
obvious reasons, too) diodes are simple, limit noise voltages to just
about the lowest possible, impose no inrush and create no ringing, and
modern technology almost off-handedly produces extremely reliable
devices by the Billions. Even the ones you get at your local RS store,
if they survive the first hit, they'll likely survive a long time. 
Bad ones are usually bad when you buy them so it's smart to check 
each one before you use it.

For my relay coils, the highest current I found was the starter 
relay at about 3 Amps - so naturally, I put a 6 Amp diode across that
one. Any voltage rating over about 18 volts (36 for a 28 volt system) would have worked just fine since the highest voltage it sees is the 
voltage you apply to the coil when you turn on the switch. When you 
turn the switch off, the current flowing in the coil transfers to the
diode, in the forward direction, so the un-grounded end (cathode) 
goes to about negative 0.7 volts. Worst case is bus voltage (+14 or 
28, to about -0.7 vdc - not counting the tiny spike caused by wiring
inductance)- so of course, I used a 100 volt diode - Hey! these habits
die hard!! And, because I had them around, I used 1N4001 diodes for 
the little relays - if I remember, those are rated at 1 Amp, 100 
volts. I don't worry about them shorting as that's a fairly low 
probability failure mode - but even if it happens, hitting the switch
a couple of times will most likely burn it open (they are really 
quite fragile, internally) and then you're back to the original, 
non-arc-suppressed state, which you need to fix though it's not a 
critical failure.

Marv, you've probably got this info a number of times but RS has a 
neat little booklet that gives the specs and pin-outs for the 555
(both single and double) as well as a few hobbyist circuits. I think
they also have the same kind of booklets providing info on various 
transistor pin-outs too.

And thanks to the guys good enough to include a glossary of the more
common acronyms they like to use. IHDLM! (It Helps Dummies Like Me!)

Cheers,

Dan Schaefer


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