Diode Question

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.

This is my first rough sketch of my plans. If you have critiques, I would be interested to hear.

The 87A NC Relay is Normally closed. So when there is power to the relay the contacts open

Just saw this and can help you out, but first, can you mark you relay pins in your schematic so I can see and know exactly which is 30, 85, 86, and 87 or 87A?

My guess, but want to verify, is:

Left = 30
Right = 87

Bottom and Top are 85 and 86.

Also what is the exact relay you are planning on using? Should be more of a solenoid or 60A panel mount style vs an automotive one.

Hey Air Male.

Thanks for the offer to help but I actually went in a different, simpler direction.

You want to use Schotkky diodes chosen for a low forward voltage drop at peak current (target .5 volts or less). In this application, the diode will dissipate significant power and will need some form of heat sink. You need to be concerned with forward current, voltage and power dissipated when selecting the device.

Aeroelectric Connection does make a single Schottkey diode for this purpose which takes into concern how to mount and connect to it: http://www.aeroelectric.com/Catalog/...M9001-700A.pdf

In your application, I think you need a second diode between the relay and the essential bus. They make dual Schotkky diode packages, but it would be up to you to connect and heat sink it. Example (TO-220 package): https://www.mouser.com/ds/2/389/stps40m60c-956723.pdf

What is the simpler direction you've chosen?