Date: Wed, 2 Aug 2000 10:20:03 -0400
From: James Frantz <LFrantz@compuserve.com>
Subject: Enough  with Diodes
Sender: James Frantz <LFrantz@compuserve.com>
To: "INTERNET:lancair.list@olsusa.com" <lancair.list@olsusa.com>
Message-ID: <200008021021_MC2-AE4B-B734@compuserve.com>
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
          <<  Lancair Builders' Mail List  >>
          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> 
>>
>1. An overvoltage transient from a failed alternator will arrive at the
anode of the diode(s), i.e., in the "forward direction", and will not
stress
the reverse capability, regardless of it's voltage rating.<

The overvoltage will produce an over current.  The current rating of the
proposed isolation diode was too small. The max forward voltage rating of
the diode was too small and the max peak reverse voltage rating may be too
small as well depending upon what is on the  isolation bus!  By the time
you properly size the diode, you may be back with the large voltage drop
across the diode which was earlier found to be unacceptable.

>2. As long as either battery is connected to the bus, most of any "failure
transient" will be absorbed by the battery.  If you had one of Bob
Nuckoll's overvoltage crow-bar circuits (or equivalent) installed, the
alternator breaker should trip off when the voltage rose to maybe 18 - 20
volts anyway and the surge wouldn't be excessive.<

"As long as" and "if" is a big statement.  Ever hear of poor battery
connections and poor grounds or a master relay failure?  Was the proposed
diode tested in this failure mode?  Did the system include overvoltage
protection?  You could be up there IFR with no instruments, no gyros, and
no engine assuming you opted to go with an all electric gyro system and an
all electric ignition system.  You may want to talk with George Stevenson
(651) 454-3745 who had an overvoltage which blew up his battery (the sink)
and fried his diode (the isolation).  The result was that  his backup
battery was powering everything but was only intended to power the
electronic ignition system.  Fortunately, George did an emergency gear
extendsion else the engine may have quit as he attempted to electrically
put the gear down.   This incident was reported in the LNN several years
ago and is a good lesson in failure mode analysis. Because of the fried
diode his little ignition battery actually powered down the flaps.  Lucky
this didn't cause to battery voltage to pull low and his engine electronic
system to quit.   

>3. Regardless, even without that device, your diode wouldn't see a reverse
voltage - alternators just don't fail where their output voltage goes
negative.<  I never stated that alternators fail with a reverse voltage. 
Lots of other devices do cause large reverse voltages such as inductive
loads when they are switched off.  And we have lots of inductive devices in
our aircraft.  I can list 22 in mine including one really big one the
hydraulic pump.

My advise is to forget diode isolation systems unless you are confidant
that you understand all the  issues that are important in designing a
isolation system and test it.  From what I saw on an earlier post, the
issue is not that well understood, the common failure modes had not been
considered nor could the system have been easily tested for those failure
modes that would be expected. 

I submitted the previous comments based upon an earlier Lancair pilot's
actual experience with an isolation diode system.  The over voltage and
subsequent events could have resulted in a deadly incident!  In this case,
the incident cost only $$$ about 3,000 of them.  See issue 22 page 271 of
the Lancair Network News. 

BTW, overvoltage protection is a must!  At least we argree on one point. 

Jim Frantz 
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
LML website:   http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>