So I have tried googling this but for such a basic question I can’t really find a definitive answer. Not in aero electric connection either.

Do diodes need to be sized for the current that will be flowing through them or the current they will be blocking? I’m assuming it’s the current flowing through.

Basically I have two alternators, an aux. buss, and an essential buss for all my efi stuff.

After understanding that the internal alternator on the Rotax once excited produces power until the motor stops and sheds power if it isn’t being used. Because of this, I want to put my ECU, TCU, injectors, and one fuel pump on this Alt. so that if the Master gets inadvertently hit in flight, the engine won’t stop. I am planning on having two independent electrical systems that are can be connected by a normally closed relay (My internal alternator quits, the relay loses power, closes, and the battery and 40A alternator are instantly connected to my ess. buss.) The problem is once the power is returned to the ess buss, it would excite the relay again and open it so I think I need a diode leaving the 18A alt so that when the 40A alt is charging the ess buss it can’t give power to the relay. So in a long winded way, would a 25A diode work? And would a bridge rectifier as described in Aero Electric Connection be better in this application? Or would a Schottky diode be better?

I hope I was being clear to what I mean. Let me know if you have questions. Maybe I can make a diagram so it’s easier to understand.