Message-ID: <000e01bffaca$e9d12980$6d6b1b3f@oemcomputer>
Reply-To: "dfs" <dfs@gateway.net>
Reply-To: lancair.list@olsusa.com
From: "dfs" <dfs@gateway.net>
To: "Lancair List" <lancair.list@olsusa.com>
Subject: Using diodes for battery isolation
Date: Mon, 31 Jul 2000 01:39:58 -0700
Mime-Version: 1.0

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Excuse me, Jim, but several things in your latest post don't jibe.

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.

2. As long as either battery is connected to the bus, most of any "failure
transient" will be absorbed by the battery. So even if the alternator was to
potentially generate a 50 volt spike, most of it would be squashed by the
battery. Sure, there would be a momentary current surge as the battery
absorbed the spike but as long as the diode was properly sized,
current-wise, the voltage wouldn't affect it. 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.

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.

Dan Schaefer


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