Hope there are some aircraft electric wizards on this forum. I am planning a backup system for a couple devices on my plane. In other words a single device powered by two separate power sources. I need to install a diode on the primary power source output lead to keep power from feeding back into the primary source when the backup power source is energized. My question is what diode should I be using? There are a million of them out there. The power sources are 12V and the devices are protected by 15amp breakers. Any help would be appreciated. Ron

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Ron, if you don't have a copy of Bob Nuckolls' book The AeroElectric Connection, you should pick up a copy. $23.50 and well worth it. Or, you can download the pdf free.

The AeroElectric Connection 12th Edition

For eliminating a bus tie switch and provide automatic isolation of the power distribution bus from the essential bus during failed alternator operation, Bob recommends a silicon diode array having a forward current rating of 20 amperes or more. For current of more than 3-4 amperes, you'll need a heat sink for the diode array.

B&C sells this essential bus diode which should work:

B&C D-25 Diode Array

B&C also offers the diode array mounted to a heat sink capable of handling a full 25 amperes.

B&C Essential Bus Diode w/ 15 Watt Heatsink

This is what I used.

I found it hard to take any other advice than Bob Nuckoll's when it comes to wiring. He has spent a lifetime designing aircraft electrical systems and has learned what works - and what is practical. He uses these exact diodes in most of his wiring diagrams. His book explains why. 

Great book! Simple electrical designs and well explained. Thank you for the link.

I would suggest taking a look at several other options, Look into a MOSFET or transistor switch with a current limit. Or possibly a solid state relay setup if you're not so comfortable with building circuits.  Lots of ways to do this simple power supply isolation.  One reason to look at other options is the Diodes' forward voltage drop (.6-.7).  At 15a that could produce 10w of heat at the diode. The low Rds of the MOSFET makes it a good choice for high current switching.  A relay could also be used to do the same job low cost easy install. Just a few thoughts.

Charles, reviewing Bob Nuckolls' 12th edition, it appears he now recommends a Schottky diode for isolation, which he says is more efficient and with less voltage drop than the diode array he suggested in the 10th edition which I've been using.

It's part number AEC9001-1 and doesn't need a big heatsink.

AEC9001-1 Schottky Diode

I like the simplicity of the diode, however i felt it should be known this does waste quite a chunk of power.  (unless its cold out in which case its just an electric heater)  There are a lot of modern boards that handle this sort of thing very well.  An example of an assembled Mosfet source selector switch would be http://www.linear.com/docs/35205  or just a simple IC http://www.linear.com/docs/17899  Just a couple of examples search digikey or mouser for "Source Selector" there should be a few hundred options out there.

Good info, Thanks

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