looking for power design

[humptybump]

OK, I am really struggling with my electrical system. I honestly can not figure out what a good answer is based on the other threads in this forum. Each one is helpful but missing some important details.

Let's assume the following:

1) There is a battery, an alternator, and a backup battery. For simplicity sake, lets not complicate the alternator with external VR or OVP. Let's just treat these as three 12V+ sources.

2) There is a minimum set of equipment and then there is everything else. These translate to the essential bus and the non-essential bus. Some designs I see have a hot bus or battery bus and if this is important, then lets add that too.

3) we want to minimize complexity.

4) For sake of design, the Alternator produces 60A, the essential bus requires 20A, and the non-essential bus requires 40A.

Given the above, can the experts here help me accomplish the following:

A) Wire the alternator and battery sources such the main battery starts the airplane, the alternator charges both batteries, and the essential bus will run off of the alternator, the primary battery, or the backup battery (in that order of failure).

B) Identify where the circuit breakers, fuses, or ANLs go and what size.

C1) If you use a switch, please let me know a source and part number.
C2) If you use a relay, please let me know a source and part number.
C3) If you use a diode, please let me know a source and part number.

(one of the problems I am having in understanding a lot of the related threads is they say "use a diode" or "you don't need a relay" but the information I can find does not tell me which diode or switch can handle the circuit and thus I am left no knowing if I will create smoke.)

If you have answers to the above questions or can point me to sources, I would be grateful for your assistance. (I also think I am not alone so I suspect this thread will prove useful to those who follow).

[jdeas]

Glen,
I see your a computer guy. PM me if you want and I will be happy to view what you have via meeting software and explain what each component does and where you can get valid information to help. Each system is different and you are going to get about a dozen valid different ways to wire your aircraft. Best to understand the schematic first so you don't get any surprises halfway down the line.

[humptybump]

Thanks JD. My problem at this point is my design schematic has been interrogated by a number of people and it just isn't working to meet the expectations I listed above. Here is my other thread that has my initial design.

It may be easiest to break my problem into pieces. here are just one example question ...

If I have a 60A 12V+ source and a 40A bus, how to I excite it ? Is there a switch that can do it directly or must I use a relay ? Where do any breakers / fuses need to go ?What parts are needed ?

[Ironflight]

Well Glenn, you already foudn my thread on the design we used for the RV-3. Using diodes to isolate and route allows you to have redudnant paths with few movign parts beyond a standard contactor. It is also pretty much free from pilot intervention in the case of failures - power of uneeded equipment if you lose the alternato,and if you smell smoke, power off the main bus.

I find it pretty simple and goof proof, and it is working well after a year.

I don't give "advice" - I'll simply share what has worked for me, and why. There are many ways to wire an airplane- you simply have to decide which best fits your requirements.

Paul

[humptybump]

Paul - I definitely did find both the RV-3B thread and an older thread on your GRT backup battery.

Where I am really struggling in are the specifics. I watch for additional details over on your reply in your RV-3B electrical thread.

thanks!

[jdeas]

Originally Posted by humptybump

If I have a 60A 12V+ source and a 40A bus, how to I excite it ? Is there a switch that can do it directly or must I use a relay ? Where do any breakers / fuses need to go ?What parts are needed ?

Not a direct or full answer but a fuse or circuit breaker goes as close to the power source as practical. Fuses and circuit breakers protect the wire from melting and causing smoke or fire. For the wire size:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2043.13-1B/$FILE/Chapter%2011.pdf

The chart on page 11-30 allows you to choose the proper wire size for the electrical loads. Once the wire size has been determined you choose the appropriate size fuse or breaker to protect it. From the phrasing of your questions I think 15min with an electrical engineer would give you all the background you need to move forward.

[Mike S]

I just went to Fry's and looked at the Shotkkey diodes until I found one that was rated at something like 40 amp and 30+ voltage.

This gave me a two to one (or better) safety margin for the planned usage.

I made a heat sink out of a piece of 3/4" x 3/4" angle stock, and mounted it with plastic standoffs.

[humptybump]

Originally Posted by jdeas

a fuse or circuit breaker goes as close to the power source as practical

So, the simple design is to just add a fuse upstream of each bus.

Originally Posted by jdeas

15 minutes with and electrical engineer

Good, bad or indifferent, all my EE friends are digital. An "amp" is a foreign concept to them

Originally Posted by Mike S

looked at the Shotkkey diodes until I found one that was rated at something like 40 amp and 30+ voltage ... and made a heat sink out of a piece of 3/4" x 3/4" angle stock, and mounted it with plastic standoffs.

Thanks Mike - no Fry's around here (or anything else) but you gave me enough info to order what I need !

[humptybump]

Above are answers to the diode question and at least one option for protecting the different buses.

The last part of my puzzle will be the places I need to switch substantial current.

Is a 40A switch a reality ? or is this where I fall back to a relay ? Which switch or relay ?

[jdeas]

Digikey.com

DSS60-0045B-ND is a good choice if you don't mind soldering and can make an isolated heat sink for up to 24 watts..

You could mount two of these devices to the same heat sink and use it as a distribution point. Cheaper diodes can be had for lower current but I prefer a smaller inventory of spare parts. This one device covers your worst case and is cheap enough to use everywhere.