Most (all?) unmodified, internally regulated alternators would need an overvoltage-controlled relay to disconnect the B lead in order to protect the electronics from overvoltage in the case of a regulator failure. Some failure modes of the regulator could cause continued (unregulated) power to the field winding & therefore unregulated output from the alternator.

Was that as hard to read as it was to write?

Charlie
(flying without protection)

Finn Lassen wrote:

Yes, you'll need a relay in series with the "fat" wire, IF you have an internally regulated alternator that does NOT stop putting out voltage when the "thin" wire is disconnected. Looks like Bernie's alternator will not need that relay. No need to introduce an unneeded additional component - another point of possible failure.

Finn

Jim Sower wrote:

Finn Lassen wrote:

The small wire goes to its separate circuit breaker. The crowbar is connected to this wire (and ground).
When the crowbar circuit senses an over-voltage, it clamps that wire to ground (and pops the circuit breaker).
Only works on your kind of alternator that stops putting out voltage on the fat wire when it doesn't get voltage in on the "thin" wire.  I have an internal regulator, so all I have is a "fat" (B+) wire.  I jump the B+ terminal of the Alt to the "F" terminal with a couple inches of 17 ga wire.  I connect and disconnect the B+ to the rest of the airplane with a "master" relay powered by the Alt switch on the panel and protected by Nuckolls' crowbar.  In an over voltage situation, the relay opens.  The Alt is disconnected from the airplane and I could care less what the voltage is. As someone else asked, does it really stop putting out voltage on the fat wire - easily measured on the fat wire with a voltmeter - voltage should drop to maybe 13.2 V as opposed to the 15 - 16 Volts when alternator is delivering power.

Finn

 Homepage:  http://www.flyrotary.com/
 Archive:   http://lancaironline.net/lists/flyrotary/List.html